Chapter 3 - Geometrics
3.01 REFERENCES
- A Policy on Design Standards - Interstate System, AASHTO, 2005
- A Policy on Geometric Design of Highways and Streets, AASHTO, 2011, 6th Edition
- Highway Capacity Manual, 2000, published by Transportation Research Board, National Research Council.
- MDOT Geometric Design Guides
- Michigan Manual of Uniform Traffic Control Devices, current edition, by the Michigan Department of Transportation
- Roadside Design Guide, AASHTO, 2006
- Standard Plan R-107-Series, Superelevation and Pavement Crowns
- MDOT Sight Distance Guidelines
3.02 DEFINITION OF TERMS
Acceleration Lane - An auxiliary lane, including tapers, for the acceleration of vehicles entering another roadway.
Arterial Road – A roadway which provides a high speed, high volume, network for travel between major points.
Auxiliary Lane – Portion of the roadway adjoining the traveled way for speed change, turning, storage for turning, weaving, truck climbing, passing and other purposes supplementary to through-traffic movement.
Average Daily Traffic (ADT) - The average 24 hour traffic volume, based on a yearly total.
Broken Back Curve - Two curves in the same direction joined by a short tangent distance.
Compound Curve - Two connecting horizontal curves in the same direction having different radii.
Collector Road – Roadway linking a Local Road to an Arterial Road, usually serving moderate traffic volumes.
Crash Analysis - A site specific safety review of crash data performed to identify whether or not a specific geometric design element has either caused, or contributed, to a pattern or concentration of crashes at the location in question. The analysis is a critical component used in determining the appropriate application of geometric design criteria and in the evaluation of design exception approval requests.
Critical Grade - The grade and length that causes a typical truck or other heavy vehicle to have a speed reduction of 10 mph or greater.
Cross Slope – Transverse slope rate of traveled lane or shoulder.
Cross Slope Break - Algebraic difference in rate of adjacent lane cross slopes having slopes in same direction (eg., between thru lanes or thru and auxiliary lanes).
Crown Line Crossover – The algebraic difference in rate of adjacent lane cross slopes at the crown point.
Crown Runout - (also called Tangent Runout) - The distance necessary to remove adverse crown before transitioning into superelevation on curves. (Referred to as “C†distance in Standard Plan R-107-Series.)
Deceleration Lane - An auxiliary lane that enables a vehicle to slow down and exit the highway with minimum interference from through traffic.
Design Hour Volume (DHV) - The hourly volume used to design a particular segment of highway.
Directional Design Hour Volume (DDHV) - The directional distribution of traffic during the DHV.
Free Access Highway - A highway, with no control of access, usually having at-grade intersections, which may or may not be divided.
Freeway - A divided arterial highway with full control of access and grade separations at intersections. Gore Area - The "V" area immediately beyond the convergence or divergence of two roadways bounded by the edges of those roadways.
Grade Separation - A structure that provides for highway traffic to pass over or under another highway or the tracks of a railway.
Horizontal Clearance – An operational offset providing clearance for external vehicle components such as mirrors on trucks and buses and for opening curbside doors of parked vehicles. A minimum 1’-6" horizontal clearance from the face of curb to an obstruction is required on curbed roadways. If the roadway and curb are separated by a shoulder, the shoulder width is included as part of the clearance.
Interchange - A system of interconnecting roadways in conjunction with grade separations providing for the interchange of traffic between two or more intersecting roadways.
Level of Service - A qualitative measure describing operational conditions within a traffic stream; generally described in terms of such factors as speed and travel time, freedom to maneuver, traffic interruptions, comfort and convenience, and safety. Levels of service are given letter designations, from A to F, with LOS A representing the best operating conditions and LOS F the worst.
Local Road – A road which serves primarily to provide access to farms, residences, business, or other abutting properties.
Passing Lane Section (PLS) - Extra lane(s) to provide additional capacity and reduce delay caused by slow moving vehicles, such as recreational vehicles, during peak periods. These are often desirable in areas where slower vehicles are not necessarily the result of long steep grades.
Passing Relief Lane (PRL) - Common, all-inclusive reference to a traffic lane provided for increased passing opportunities along a route, can be a Truck Climbing Lane (TCL) or a Passing Lane Section (PLS).
Ramp - A connecting roadway between two intersecting roadways, usually at grade separations.
Reverse Curve – Horizontal curves in the opposite direction joined by a short tangent distance or common point.
Roll-over - Algebraic difference in rate of cross slope between traveled lane and shoulder.
Safety Review - A general safety review of a project performed to identify potential safety enhancements within the limits of a proposed 3R or 4R project.
Service Road (also Frontage Road) - A roadway usually parallel and adjacent to a highway which provides access to abutting properties by separating local and through traffic.
Sight Distance - The unobstructed distance that can be viewed along a roadway - usually referenced to decision points for drivers.
Spiral Curve Transition - A variable radii curve between a circular curve and the tangent. The radii of the transition and the curve are the same at the curve and increase to infinity at the tangent end of the transition.
Superelevation – The banking of the roadway in the direction of the curve to help counter balance the centrifugal force on the vehicle.
Superelevation Transition (sometimes referred to as superelevation runoff) – The distance needed to change the pavement cross slope in the direction of the curve from a section with adverse crown removed to a fully superelevated section, or vice versa. (Referred to as “L†distance in Standard Plan R-107-Series.)
Truck Climbing Lane (TCL) - An extra lane for heavy vehicles slowed by the presence of a long steep “critical gradeâ€, that provides passing opportunities for non-slowed vehicles.
Vehicles Per Hour (vph) - A measurement of traffic flow.
3.03 ALIGNMENT-GENERAL
The geometric design of a roadway consists of horizontal alignment, vertical alignment, and a combination of the two. A properly designed alignment (horizontal and vertical) leads to the safe and efficient movement of traffic.
3.03A Horizontal Alignment
Horizontal alignment is a major factor in determining safety, driving comfort, and capacity of a highway.
Some important factors to consider when designing for horizontal alignment:
- Passing sight distance on two-lane, two-way roadways should be maximized.
- Curves should be as flat as physical conditions permit. Abrupt changes in alignment introduce the element of surprise to the driver and should be avoided.
- Broken back curves should be avoided because they are unsightly and drivers do not expect succeeding curves to be in the same direction.
- If possible, the minimum distance between reverse curves should be the sum of the superelevation transitions, outside the curves, plus the crown runout lengths. The crown runout can be eliminated in some situations. See the Geometrics Unit (Design Division) for additional guidance. When it isn't possible to obtain the desired distance between reverse curves, up to 40% of the transition may be placed in the curves.
3.03B Vertical Alignment
Vertical alignment establishes the profile grade of a proposed road construction project. The grade can be over virgin land as in the case of a relocation project or along an existing roadway, as in the case of a resurfacing project. In either case and in most proposed construction projects, a profile grade should be established.
Obviously a profile grade must always be established for new construction or relocation projects. Most reconstruction and rehabilitation projects will require new profile grades if improvements for sight distance, superelevation, and drainage are included. A simple resurfacing project can usually be constructed without establishing a new vertical alignment.
Establishing the vertical alignment is based on many factors, including terrain, existing conditions, soils, drainage, coordination with the horizontal alignment, location of bridges, culverts, crossroads, design speed, earthwork balance, etc. The Designer must work with available resources such as the Geometrics Unit of the Design Division to provide the best possible vertical alignment. The final product should be safe, functional, aesthetically pleasing, and economical.
3.03C Combined
Horizontal and vertical alignments are permanent design elements. It is extremely difficult and costly to correct alignment deficiencies after the highway is constructed.
A proper combination of horizontal and vertical alignment is obtained by engineering study using the following general controls.
- Vertical curvature superimposed on horizontal curvature, generally results in a more pleasing appearance. Successive changes in profile not in combination with horizontal curvature may result in a series of humps visible to the driver for some distance.
- Sharp horizontal curvature should not be introduced at or near the top of a pronounced crest vertical curve. This condition may make it difficult for the driver to perceive the horizontal change in alignment. This can be avoided if the horizontal curvature leads the vertical curvature, i.e., the horizontal curve is made longer than the vertical curve.
- Sharp horizontal curvature should not be introduced at or near the low point of a pronounced sag vertical curve. Because the road ahead would appear to be fore- shortened, a relatively "flat" horizontal curve should be used to avoid this undesirable phenomenon.
- Horizontal curvature and profile should be made as flat as possible at intersections where sight distance along both roads or streets is important.
See Chapter 3 of A Policy on Geometric Design of Highways and Streets, AASHTO, 2004 for elements of design.
3.03.01 Horizontal Alignment - Design Controls
3.03.01A Minimum Radius
The minimum radius is a limiting value of curvature for a given design speed and is determined from the maximum rate of superelevation and the maximum side friction factor. The minimum radius of curvature should be avoided wherever practical. Attempt to use flatter curves, saving the minimum radius for the most critical conditions. The minimum radius (Rmin) is shown in the Standard Plan R-107-Series superelevation tabulation at the bottom of each column for each design speed. Values for Rmin are also tabulated for the straight line superelevation table in Section 3.04.03.
3.03.01B Minimum Curve Lengths
Curves should be sufficiently long for small deflection angles to avoid the appearance of a kink.
Curves on rural free access trunklines should be at least 500 feet long for a central angle of 5° and the minimum length should be increased 100 feet for each 1° decrease in the central angle. The minimum should be approximately 15 times the design speed with a desirable length of at least 30 times the design speed. For example a design speed of 60 mph multiplied by 15 gives a minimum curve length of 900’.
3.03.01C Compound Curves
Compound curves should be used with caution. Although compound curves give flexibility to fitting the highway to the terrain and other controls, designers should avoid them whenever possible. When curves with considerably different radii are located too close together, the alignment will not have a pleasing appearance. On one-way roads such as ramps, the difference in radii of compound curves is not so important if the second curve is flatter than the first. On compound curves for open highways, the ratio of the flatter radius to the sharper radius should not exceed 1.5 to 1. On ramps the ratio of the flatter radius to the sharper radius may be increased to a 2 to 1 ratio.
3.03.01D Sight Distances
Both stopping sight distance and passing sight distance must be considered for two-way roadways. On one-way roadways only stopping sight distance is required. The designer must be aware that both horizontal and vertical alignments need to be considered when designing for sight distance.
From Exhibit 3-1 of A Policy on Geometric Design of Highways and Streets, AASHTO, 2004 stopping sight distance can be determined from design speed.
| Design Speed | Stopping Sight Distance (Design) |
|---|---|
| 25 | 155 |
| 30 | 200 |
| 35 | 250 |
| 40 | 305 |
| 45 | 360 |
| 50 | 425 |
| 55 | 495 |
| 60 | 570 |
| 65 | 645 |
| 70 | 730 |
| 75 | 820 |
For general use in the design of a horizontal curve, the sight line is a chord of the curve and the stopping sight distance is measured along centerline of the inside lane around the curve
Knowing the stopping sight distance (SSD) and the radius of curve (R) the horizontal sightline offset (HSO) can be calculated from:
These equations are exact only when the vehicle and sight obstruction are within the limits of a circular curve.
When determining sight distances, use A Policy on Geometric Design of Highways and Streets, AASHTO. The MDOT Sight Distance Guidelines also provide detailed information on sight distance calculation.
The four types of sight distances given are stopping, passing, decision, and intersection.
- Stopping Sight Distance is defined as the sight distance available on a roadway that is sufficiently long to enable a vehicle traveling at or near the design speed to stop before reaching a stationary object in its path.
- Passing Sight Distance is defined as the length needed to complete a passing maneuver as described in A Policy on Geometric Design of Highways and Streets, AASHTO.
- Decision Sight Distance is the distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source or condition in a roadway environment that may be visually cluttered, recognize the situation or its potential threat, select an appropriate speed and path, and initiate and complete the required maneuver safely and effectively.
- Intersection Sight Distance is the distance that allows drivers sufficient view from a minor road to safely cross or turn on a major road.
- Generally 7.5 seconds of entering sight distance is used for passenger vehicles stopped on a minor road grade of 3% or less to turn left onto a two-lane roadway. An additional 0.5 seconds is added for each additional lane.
- Adjustments for other varying conditions that may increase or decrease the time gap are provided in A Policy on Geometric Design of Highways and Streets, AASHTO.
- The designer is cautioned that the element of Clear vision for at-grade intersections is very important, for safety reasons, particularly on high speed trunklines.
3.03.01E Horizontal Curve Computations
∆ = Deflection or Central Angle (Delta), degrees
R = Radius of Curve, ft
T = Length of Tangent (P.C. to P.I. or P.I. to P.T.) = R Tan (∆/2), ft
E = External Distance = R [Sec (∆/2) - 1] or T Tan (∆/4), ft
M = Middle Ordinate Distance = R Versine (∆/2) or E Cos (∆/2), ft
L = Length of Curve = ∆ x R ÷ 57.2958, ft
P.C. = Point of Curvature
P.I. = Point of Intersection of Tangents
P.O.C.T. = Point on Curve Tangent
P.T. = Point of Tangency
D = Degree of Curvature =
3.03.01F Spirals
Spiral curves are used to transition into circular curves and should be used on new alignments based on the design speed and radius of the curve, as shown on the table in Standard Plan R-107-Series. Spiral curve lengths are normally equal to the superelevation transition length. The relationship between the various elements of spiral curves and their methods of computation are shown below and on the following pages.
Usually the P.I. station and the deflection angle (∆) are established. The spiral length (Ls) equals the length of superelevation runoff; appropriate values for (Ls) can be obtained using Standard Plan R-107-Series. The remaining curve data can then be computed or read from the tables of spiral curve functions found in the Construction Manual.
T.S. Sta. = P.I. Sta. - T (Sta.)
S.C. Sta. = T.S. Sta. + Ls (Sta.)
C.S. Sta. = T.S. Sta. + Ls (Sta.)+ Lcc (Sta.)
S.T. Sta. = T.S. Sta. + 2Ls (Sta.) + Lcc (Sta.)
The radius of Central Angle (R) should be specified to the nearest 15 feet; all other curve data will be calculated and shown to the nearest one-hundredth of a foot or to the nearest second, whichever is applicable.
P.I. = Point of Intersection
T.S. = Tangent to Spiral
S.C. = Spiral to Curve
C.S. = Curve to Spiral
S.T. = Spiral to Tangent
3.03.02 Vertical Alignment – Design Controls
Vertical curves are in the shape of a parabola. The basic equation for determining the minimum vertical curve length is:
L = KA
WHERE:
L = length of vertical curve, feet
K = horizontal distance to produce 1% change in gradient, feet
A = Algebraic difference between the two tangent grades, percent
(Refer to AASHTO's A Policy of Geometric Design for Roads and Streets for additional Vertical Curve Formulas). Also refer to the MDOT Sight Distance Guidelines for more detailed information on sight distance calculation.
3.03.02A Minimum / Maximum Grades
See the “Grade†section of Appendix 3A, the Geometric Design Elements table.
3.03.02B Minimum Vertical Curve Lengths
Minimum length (in feet) of a vertical curve should be three times the design speed in mph.
3.03.02C Stopping Sight Distance
Stopping Sight Distance (SSD) is the principal control of the design of both crest and sag vertical curves. AASHTO's A Policy on Geometric Design of Highways and Streets gives values for K and lengths of vertical curves for various operational conditions. Values based on reduced design speeds may be used on non-freeway 3R projects. Minimum design guidelines for non-freeway 3R projects are presented in Section 3.09.02. The design speed used for a ramp vertical alignment should meet or exceed the design speed used for the ramp horizontal alignment. See MDOT Sight Distance Guidelines for more detailed information on sight distance calculation.
3.03.02D Drainage
Minimum grades correlate with adequate drainage. A desirable minimum grade is typically 0.5%, but grades of 0.3% may be used for paved roadways. On curbed roadways, when it is necessary to use grades that are flatter than 0.3%, provide enclosed drainage with compensating decreased inlet spacing. In addition, close attention to inlet spacing is critical for sag and crest vertical curves when the K value (rate of grade change) is greater than 167.
Uncurbed roads with ditch drainage can have a level longitudinal grade if the crown adequately drains the pavement. Independent ditches should be used when the grade is less than 0.3%. However, efforts to achieve minimum roadway grades of 0.5% would be of great benefit in the event that future curb and gutter or concrete barrier may be installed.
3.03.02E Other Considerations
Comfort criteria is sometimes a consideration for sag vertical curves. The equation for length of curve for comfort is:
WHERE:
L = length of vertical curve, feet
A = algebraic difference of tangent grades, percent
V = design speed, mph
Passing sight distance must be considered on two way roadways. Passing sight distance is the distance required for a motorist to safely perform a passing maneuver as described in AASHTO.
Intersection Sight Distance is the distance that allows drivers sufficient view from a minor road to safely cross or turn on a major road. See Section 3.03.01.D4.
3.03.02F Computations
The following pages show mathematical details used in the design of vertical curves. This section includes definitions, formulas, and examples.
| ELEMENT | ABBREVIATION | DEFINITION |
|---|---|---|
| Vertical Point of Curvature | The point at which a tangent grade ends and the vertical curve begins. | |
| Vertical Point of Tangency | The point at which the vertical curve ends and the tangent begins. | |
| Vertical Point of Intersection | The point where the extension of two tangent grades intersect. | |
| Grade | The rate of slope between two adjacent VPI’s expressed as a percent. The numerical value for percent of grade is the vertical rise or fall in feet for each 100 feet of horizontal distance. Upgrades in the direction of stationing are identified as plus (+). Downgrades are identified as minus (-). | |
| External Distance | The vertical distance (offset) between the VPI and the roadway surface along the vertical curve. | |
| Algebraic Difference in Grade | The value A is determined by the deflection in percent between two tangent grades. | |
| Length of Vertical Curve | The horizontal distance in feet from the VPC to the VPT. |
3.04 SUPERELEVATION AND CROSS SLOPES
The maximum rate of superelevation is determined from the design speed, curve radii, and the maximum allowable side friction factor.
Michigan, because of its climate, limits superelevation to 7% maximum on rural freeways, free access trunklines, and rural ramps. For maximum superelevation on urban freeways and urban ramps see Standard Plan R-107-Series.
Standard Plan R-107-Series (7% Emax) is the preferred method for obtaining superelevation rates. Please note that interpolating between the AASHTO 6% and 8% Emax charts to obtain an estimated value for 7% Emax criteria is not appropriate. Standard Plan R-107-Series should be used. When it is not possible to use the rates provided in Standard Plan R-107-Series, the straight line method may be used on a curve by curve basis as needed. See Section 3.04.03. This method employs a distribution that generally produces more moderate superelevation rates and uses a maximum rate of superelevation of 6%. If, as a maximum the straight line method cannot be met, a design exception will be required.
The department uses a standard cross slope of 2% as shown on Standard Plan R-107-Series and Appendix 3A Geometric Design Elements. See Section 6.09 for more information on pavement crowns and cross slope. Also refer to Section 3.09.02 for cross slopes allowable on 3R Projects. A design exception is required when minimum cross slopes are not met and/or when pavement cross slopes exceed 2% except as stated below.
Cross slopes up to and including 2% are barely perceptible in terms of vehicle steering. A maximum cross slope of 2% should be used on the two lanes adjacent to the crownline. This will translate to crownline crossover of 4%.
When three or more lanes are inclined in the same direction on free access curbed highways, each successive lane or portions thereof, outward from the first two lanes adjacent to the crown line, may have an increased slope. The cross slope rate may be increased up to 1%. This helps facilitate parabolic crown modifications when existing side conditions do not allow the preferred uniform standard crown rate. This use of multiple crown rates requires additional transition in superelevated sections. See sketch below.
3.04.01 Point of Rotation
Superelevation may be obtained by rotating about the center or about inside or outside pavement edge profiles. Currently our crowned two-way and two-lane roadways are rotated about the pavement centerline per Standard Plan R-107-Series. This method reduces the edge distortion because the required change in elevation is distributed along both pavement edges rather than all on one edge. Uncrowned or straight cross slope pavements, such as ramps, are rotated about the alignment edge. Special consideration should be given to superelevating wider pavements (i.e., three-lane or five-lane sections) as the point of rotation should be determined by site conditions. See Standard Plan R-107-Series.
3.04.02 Superelevation Transitions
The superelevation transition consists of the superelevation runoff (or transition (L) ) and tangent runout (or crown runout (C) ). The superelevation runoff section consists of the length of roadway needed to accomplish a change in outside-lane cross slope from zero (flat) to full superelevation, or vice versa. The superelevation runoff is determined by the width of pavement (W), superelevation rate (e), and the relative gradient along the edges of pavement (Δ%). As indicated in Standard Plan R-107-Series, one third of the superelevation runoff length is located in the curve. When this cannot be achieved, the runoff length can be adjusted to a 30% minimum and 40% maximum inside the curve. The tangent runout section consists of the length of roadway needed to accomplish a change in outside-lane cross slope from the normal cross slope rate to zero (flat), or vice versa. The tangent runout is determined by the width of pavement (W), normal cross slope/normal crown (N.C.), and the relative gradient. Relative gradient values correspond to the superelevation rates. The gradient may be increased as needed up to the maximum relative gradient for the design speed. A design exception is required for values exceeding the maximum relative gradient.
3.04.03 Superelevation Using a Straight Line Method
| STRAIGHT LINE SUPERELEVATION | ||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| RADIUS Feet |
30 mph | 35 mph | 40 mph | 45 mph | 50 mph | 55 mph | 60 mph | 65 mph | Freeways | Urban Freeways and Urban Ramps | ||||||||||||
| 70 mph | 75 mph | 60 mph | ||||||||||||||||||||
| e | ∆% | e | ∆% | e | ∆% | e | ∆% | e | ∆% | e | ∆% | e | ∆% | e | ∆% | e | ∆% | e | ∆% | e | ∆% | |
| 20000 | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- |
| 17000 | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | 2.0 | 0.30 | N.C. | -- |
| 14000 | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | 2.0 | 0.32 | 2.0 | 0.31 | 2.0 | 0.30 | N.C. | -- |
| 12000 | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | 2.0 | 0.36 | 2.0 | 0.32 | 2.0 | 0.31 | 2.0 | 0.30 | 2.0 | 0.36 |
| 10000 | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | 2.0 | 0.38 | 2.0 | 0.36 | 2.0 | 0.32 | 2.0 | 0.31 | 2.0 | 0.30 | 2.0 | 0.36 |
| 8000 | N.C. | -- | N.C. | -- | N.C. | -- | N.C. | -- | 2.0 | 0.40 | 2.0 | 0.38 | 2.0 | 0.36 | 2.0 | 0.32 | 2.0 | 0.31 | 2.0 | 0.30 | 2.0 | 0.36 |
| 6000 | N.C. | -- | N.C. | -- | N.C. | -- | 2.0 | 0.40 | 2.0 | 0.40 | 2.0 | 0.38 | 2.0 | 0.36 | 2.0 | 0.32 | 2.0 | 0.31 | 2.5 | 0.31 | 2.0 | 0.36 |
| 5000 | N.C. | -- | N.C. | -- | 2.0 | 0.40 | 2.0 | 0.40 | 2.0 | 0.40 | 2.0 | 0.38 | 2.0 | 0.36 | 2.0 | 0.32 | 2.5 | 0.32 | 3.0 | 0.32 | 2.0 | 0.36 |
| 4000 | N.C. | -- | 2.0 | 0.45 | 2.0 | 0.40 | 2.0 | 0.40 | 2.0 | 0.40 | 2.0 | 0.38 | 2.0 | 0.36 | 2.5 | 0.33 | 3.1 | 0.33 | 3.8 | 0.34 | 2.0 | 0.36 |
| 3500 | N.C. | -- | 2.0 | 0.45 | 2.0 | 0.40 | 2.0 | 0.40 | 2.0 | 0.40 | 2.0 | 0.38 | 2.3 | 0.37 | 2.8 | 0.34 | 3.5 | 0.34 | 4.3 | 0.35 | 2.0 | 0.36 |
| 3000 | 2.0 | 0.50 | 2.0 | 0.45 | 2.0 | 0.40 | 2.0 | 0.40 | 2.0 | 0.40 | 2.1 | 0.38 | 2.7 | 3.8 | 3.3 | 0.36 | 4.1 | 0.36 | 5.0 | 0.36 | 2.4 | 0.37 |
| 2500 | 2.0 | 0.50 | 2.0 | 0.45 | 2.0 | 0.40 | 2.0 | 0.40 | 2.0 | 0.40 | 2.5 | 0.39 | 3.2 | 0.39 | 4.0 | 0.37 | 4.9 | 0.38 | 6.0 | 0.38 | 2.8 | 0.38 |
| 2050 | 2.0 | 0.50 | 2.0 | 0.45 | 2.0 | 0.40 | 2.0 | 0.40 | 2.4 | 0.41 | 3.1 | 0.40 | 3.9 | 0.40 | 4.8 | 0.40 | 6.0 | 0.40 | 3.4 | 0.39 | ||
| 1800 | 2.0 | 0.50 | 2.0 | 0.45 | 2.0 | 0.40 | 2.1 | 0.41 | 2.8 | 0.42 | 3.5 | 0.41 | 4.4 | 0.42 | 5.5 | 0.42 | 3.9 | 0.40 | ||||
| 1675 | 2.0 | 0.50 | 2.0 | 0.45 | 2.0 | 0.40 | 2.3 | 0.41 | 3.0 | 0.42 | 3.8 | 0.42 | 4.8 | 0.42 | 5.9 | 0.43 | 4.2 | 0.41 | ||||
| 1425 | 2.0 | 0.50 | 2.0 | 0.45 | 2.0 | 0.40 | 2.7 | 0.42 | 3.5 | 0.44 | 4.5 | 0.44 | 5.6 | 0.44 | 5.0 | 0.43 | ||||||
| 1350 | 2.0 | 0.50 | 2.0 | 0.45 | 2.2 | 0.41 | 2.9 | 0.43 | 3.7 | 0.44 | 4.7 | 0.44 | 5.9 | 0.45 | ||||||||
| 1150 | 2.0 | 0.50 | 2.0 | 0.45 | 2.5 | 0.42 | 3.4 | 0.45 | 4.3 | 0.46 | 5.5 | 0.46 | ||||||||||
| 1075 | 2.0 | 0.50 | 2.0 | 0.45 | 2.7 | 0.43 | 3.6 | 0.46 | 4.7 | 0.47 | 5.9 | 0.47 | ||||||||||
| 850 | 2.0 | 0.50 | 2.4 | 0.47 | 3.4 | 0.46 | 4.5 | 0.49 | 5.9 | 0.50 | ||||||||||||
| 820 | 2.0 | 0.50 | 2.5 | 0.47 | 3.5 | 0.47 | 4.7 | 0.49 | ||||||||||||||
| 800 | 2.0 | 0.50 | 2.6 | 0.47 | 3.6 | 0.47 | 4.8 | 0.50 | ||||||||||||||
| 720 | 2.0 | 0.50 | 2.8 | 0.49 | 4.0 | 0.49 | 5.4 | 0.52 | ||||||||||||||
| 650 | 2.1 | 0.51 | 3.1 | 0.50 | 4.5 | 0.51 | 5.9 | 0.54 | ||||||||||||||
| 600 | 2.3 | 0.51 | 3.4 | 0.51 | 4.8 | 0.53 | ||||||||||||||||
| 500 | 2.8 | 0.53 | 4.1 | 0.54 | 5.8 | 0.57 | ||||||||||||||||
| 450 | 3.1 | 0.54 | 4.5 | 0.56 | ||||||||||||||||||
| 400 | 3.5 | 0.56 | 5.1 | 0.58 | ||||||||||||||||||
| 345 | 4.0 | 0.58 | 5.9 | 0.62 | ||||||||||||||||||
| 300 | 4.6 | 0.61 | ||||||||||||||||||||
| 232 | 6.0 | 0.66 | ||||||||||||||||||||
| ∆% max | 0.66 | 0.62 | 0.58 | 0.54 | 0.50 | 0.47 | 0.45 | 0.43 | 0.40 | 0.38 | 0.45 | |||||||||||
| Rmin | 232 | 340 | 485 | 643 | 833 | 1061 | 1333 | 1657 | 2042 | 2500 | 1412 | |||||||||||
Use 7% superelevation for loop ramps (see Standard Plan R-107-Series). However, special consideration should be given to curves which approach a ramp terminal (stopping condition). If relative gradient (∆%) values from the tables cannot be obtained for the design radius, use ∆% max for the corresponding design speed.
For radii less than those tabulated (but not less than Rmin) use e max and ∆% max.
Maximum superelevation for urban freeways and urban ramps (with 60 mph design speed) is 5%, otherwise e max = 6%.
3.05 Section deleted
3.06 DESIGN SPEED
Design speed is a selected speed used to determine the various geometric design features of the roadway. Once selected, all of the pertinent design features of the highway should be related to the design speed to obtain a balanced design.
Design speeds shown in Appendix 3A are applicable for 4R projects. The design speed used for freeway 3R projects (interstate and non-interstate) may be the design speed approved at the time of original construction or reconstruction, whichever is most recent. The design speeds used for non-freeway 3R projects are shown in 3.09.02. See Section 3.08.01C for information on combined 3R and 4R work.
It is MDOT practice under most circumstances to design roadway geometrics based on a recommended project design speed 5 mph greater than the posted speed (See Appendix 3A, Geometric Design Elements and Section 3.09.02A, 3R Minimum Guidelines, Non-Freeway NHS). This practice is founded in research that shows actual operating speeds are typically greater than the posted speeds. Posted speed is generally used as project design speed only for non-freeway, non-NHS 3R design (Section 3.09.02B). The designer should strive to meet the standards for all geometric elements based on the prescribed MDOT recommended design speeds. This is the project design speed to be recorded on the title sheet.
Design elements not meeting standards based on MDOT recommended design speeds require a Safety Review, Crash Analysis and documented justification (use Design Exception Form FC26). If the highest attainable design corresponds to criteria for speeds less than the posted speed (except where permitted in Section 3.09.02B for non-NHS 3R), a design exception must be submitted to the Engineer of Design Programs for approval. For additional information see Section 14.11.
For designs meeting criteria for a speed greater than or equal to the posted speed but less than MDOT recommended design speed, the project manager must still document the basis for not meeting standards for MDOT recommended design speed. This internal documentation (including Form FC26 signed by the Project Manager, Safety Review and Crash Analysis) must be retained in the project files and in the final document retention when closing out the project files. A copy of the documentation must also be sent to the Design Division’s Geometrics Unit.
It should be indicated on Form FC26 if subsequent approval by the Engineer of Design Programs is required or if the documentation is for file retention only.
Documentation must be for each geometric element and not a blanket statement applying to all geometric elements. A design speed reduction for individual geometric elements does not redefine the overall “project design speedâ€.
3.07 GEOMETRICS
3.07.01 Lane Width, Capacity and Vehicle Characteristics
3.07.01A Lane Width and Capacity
The lane width of a roadway greatly influences the safety and comfort of driving. See Appendix 3A for lane width information for 4R work and Section 3.09.02 for 3R work.
3.07.01B Vehicle Characteristics
There are two general classes of vehicles: passenger and commercial (trucks). The geometric design requirements for trucks and buses are much more severe than they are for passenger vehicles. Consult the Geometrics Unit in the Design Division for the appropriate design vehicle to be used on the job. Intersection radii for various types of commercial vehicles are given on Geometric Design Guide GEO-650-Series, "Flares and Intersection Details". Also, for short radii loops, additional ramp width may be needed to accommodate these vehicles. Generally, the Michigan WB-62 is the design vehicle to be used in determining the radii to be used in turning movements at trunkline to trunkline intersections and interchanges.
Acceleration and deceleration rates of vehicles are often critical in determining highway design. These rates often govern the dimensions of design features such as intersections, freeway ramps, speed change lanes, and climbing or passing lanes.
3.07.02 Interchange Geometrics
General: Contact the Geometrics Unit in the Design Division for the recommendations / requirements of all geometric features of highway facilities.
3.07.02A Rural and Urban
Geometric Design Guides, developed by the Geometrics Unit in the Design Division show approved criteria for ramp and interchange design. See Geometric Design Guides.
3.07.02B Interchange Layout
The following items should be considered in conjunction with the Geometric Design Guides for interchange design.
- Exit ramps should be designed for adequate visibility for the motorist exiting the freeway. Sight distance along a ramp should be at least as great as the design stopping sight distance. There should be a clear view of the entire exit terminal, including the exit nose and a section of the ramp roadway beyond the gore.
- Exit ramps should begin where the freeway is on a tangent, when possible.
- Drivers prefer and expect to exit in advance of the structure. Loop ramps that are located beyond the structure, usually need a parallel deceleration lane.
- Left-hand entrances and exits are contrary to the concept of driver expectancy. Therefore, extreme care should be exercised to avoid left-hand entrances and exits in the designing of interchanges.
- The geometric layout of the gore area of exit ramps should be clearly seen and understood by the approaching drivers.
- The cross slope in the gore area between the 2’ point and the 22’ point should not exceed 8%, with a 6% maximum algebraic difference in grades between the gore and the adjacent lane. The 6% algebraic difference also applies within crowned gores. However, these “rollovers†should desirably be 5% or less to minimize the effect on vehicles inadvertently crossing the gore area. It is recommended that detail grades for the above paved portion of the gore area be provided to verify both cross slopes and algebraic differences. The unpaved portion beyond the 22’ point (to the extent the clear zone from each roadway overlaps) should be graded as level with the roadway as practical and be clear of major obstructions. See sketch below.
- In order to identify the location of ramps at interchanges, the following system of lettering ramps should be used on projects insofar as possible:.
The northeasterly quadrant is designated ramp A. Ramps B, C, & D follow in sequence in a clockwise direction. Interchange interior loops would similarly follow with clockwise designations E, F, G, & H.
3.07.02C Crossroads Over Freeways
Local or county roads over freeways should be designed for stopping sight distance based on the project design speed.
In interchange areas, the intersection sight distance and clear vision areas at diamond ramp terminals must be according to current Department practice. See MDOT Sight Distance Guidelines, Section 3.03.02E and the Geometric Design Guide GEO-370-Series. The driver's eye position, for a vehicle on a ramp, is assumed to be between 14.5 feet minimum and 18 feet desirable from the edge of the crossroad.
3.07.02D Ramp Radii
The speeds at which ramps may be driven, if they are free flowing, is determined primarily by the sharpest curve on the ramp proper. Loop ramps, because of their design restrictions, have the sharpest curvature, and if possible the designer should not use a radius of less than 260 feet. For radii less than 260 feet, contact the Geometrics Unit of the Design Division.
3.07.02E Single Lane Ramp Widths
Single lane ramp widths are normally 16'-0". The total paved width including paved shoulders should not exceed 28’. Wider paved widths invite undesirable passing of slow-moving vehicles or invite two-lane operation.
Current ramp widths are shown in Chapter 6, Appendix 6-A.
3.07.03 Speed Change Lanes and Transitions
The change in vehicle speed between highways and ramps is usually substantial, and provision should be made for acceleration and deceleration. Therefore, in order to minimize interference with through traffic on highways, speed change lanes (deceleration and acceleration lanes) are added at turning roadways.
The Geometric Design Guides allow for either parallel or tapered deceleration lanes for exit ramps. Parallel deceleration lanes should be used where the ramp exit is on a freeway curve.
3.07.04 Intersections
An intersection is defined as the general area where two or more highways join or cross, including the roadway and roadside facilities for traffic movements within the area.
Intersections are an important part of a highway facility because, to a great extent, the efficiency, safety, speed, cost of operation, and capacity of the facility depends on their design. Although many of the intersections are located in urban areas, the principles involved apply equally to design in rural areas.
The angle of intersection between the approach road and the trunkline should not be less than 60° or more than 120°, with desirable values between 75° and 105°.
The gradient of the intersecting roads should be as flat as practical on those sections that are to be used for storage of stopped vehicles. If possible, side roads should have a "landing" of no more than 3 percent grade. Even though stopping and accelerating distances for passenger cars, on grades of 3 percent or less differ little from the distances on the level, larger vehicles need the flatter landing area.
Where two roadways intersect, crown manipulation of both roadways can be used to improve the drivability of both roadways. In this case, to insure proper drainage, detail grades should be provided. See Geometric Design Guide GEO-650-Series for allowable approach road grades.
Intersection sight distance should be provided on all intersections legs. Clear vision corners should be provided when it is practical. See Section 5.24.
Center lanes for left turns or passing flares may be required at certain intersections. See Geometric Design Guide GEO-650-Series.
Ramp terminals should be according to Geometric Design Guide GEO-370-Series.
3.08 3R, 4R AND OTHER PROJECTS
3.08.01 General
3.08.01A (3R) Resurfacing Restoration and Rehabilitation
This work is defined in 23 CFR (Code of Federal Regulations) as "work undertaken to extend the service life of an existing highway and enhance highway safety. This includes placement of additional surface material and/or other work necessary to return an existing roadway, including shoulders, bridges, the roadside and appurtenances to a condition of structural or functional adequacy. This work may include upgrading of geometric features, such as widening, flattening curves or improving sight distances." Examples of this type of work include:
- Resurfacing, milling or profiling, concrete overlays and inlays (without removing subbase).
- Lane and/or shoulder widening (no increase in number of through lanes).
- Roadway base correction.
- Minor alignment improvements.
- Roadside safety improvements.
- Signing, pavement marking and traffic signals.
- Intersection and railroad crossing upgrades.
- Pavement joint repair.
- Crush and shape and resurfacing.
- Rubblize and resurface.
- Intermittent grade modifications (used to correct deficiencies in the vertical alignment by changing the paving profile for short distances) that leave the existing pavement in service for more than 50% of the total project length.
- Passing relief lanes.
See Chapter 12 of the Bridge Design Manual for examples of “bridge†3R work.
3.08.01B (4R) New Construction/Reconstruction
Projects that are mainly comprised of the following types of work are not considered 3R.
- Complete removal and replacement of pavement (including subbase).
- Major alignment improvements.
- Adding lanes for through traffic.
- New roadways and /or bridges.
- Complete bridge deck or superstructure replacement.
- Intermittent grade modifications (used to correct deficiencies in the vertical alignment by changing the paving profile for short distances) that leave the existing pavement in service for less than 50% of the total project length.
The above lists are not all inclusive, but are intended to give typical examples of 3R and 4R work.
3.08.01C Combined 3R and 4R Work
If a project includes 3R and 4R work, the applicable standards are governed by the standards that correspond individually to each work type (3R or 4R). Identify the logical limits of each work type on the project information sheet to distinguish where 3R guidelines and 4R standards are separately applied. Work type overlap between separation limits may cause a default to 4R standards within the overlap.
When other work types are combined with 3R or 4R projects, they are also governed separately and identified as such on the project information sheet. See Section 3.08.01D.
3.08.01D Other Work Categories
Projects categorized by other work types such as CPM, M-Funded Non-Freeway Resurfacing, Signal Corridor and Signing Corridor projects are governed by guidelines that differ from 3R and 4R Guidelines. For information related to specific requirements for these categories of work, use other appropriate references. When other work types are packaged with a 3R or 4R project, the portion of the project that is outside the 3R or 4R work limits is governed by the guidelines that pertain to the other work type. When describing the work type in the request for Plan Review Meeting, identify the logical limits of work type separation so that the appropriate requirements are considered within those limits. Work type overlap within these separation limits may cause a default to 3R or 4R requirements.
Note that the applicability of CPM minimum design requirements is contingent on the program eligibility of the roadway. Regardless of funding source used to design and construct CPM work, CPM minimum design requirements can only be applied to work done on roadways that would otherwise be eligible for funding under the CPM program.
3.08.01E Design Exceptions
The sections to follow include standards for geometric design elements for the various classifications of roadways and work types. For specific controlling geometric design elements, a formal design exception must be submitted and approved when the standards can not be met. Design exceptions should be addressed as early in the life of a design as possible. Designers should strive to process design exceptions during the scoping process. Along with the justification for not meeting MDOT and/or AASHTO standards the design exception includes a crash analysis and the estimated total cost required to attain full standards compliance. See Section 14.11 for design exception submittal procedures and required forms.
The controlling elements for which design exceptions are required include:
- Design Speed
- Lane Width
- Shoulder Width
- Bridge Width
- Structural Capacity
- Horizontal Alignment
- Vertical Alignment
- Grade
- Stopping Sight Distance (Horizontal Sightline Offset (HSO) and K-value)
- Cross Slope
- Superelevation
- Vertical Clearance
- Horizontal Clearance (not including clear zone, see definition of terms)
- Ramps (See Section 3.11.03B)
When a bridge falls within a road project and no work is planned for the bridge, AASHTO “bridges to remain in place†criteria apply to the bridges. See AASHTO publication, A Policy on Design Standards-Interstate System, or A Policy on Geometric Design of Highways and Streets. If the bridge does not meet the criteria to “remain in place†the Road Designer shall be responsible for submitting any necessary design exceptions for the bridge.
3.08.01F Safety Review / Crash Analysis
A safety review is required for all 3R and 4R projects. The Project Manager should contact the TSC Traffic Engineer during scoping, so that a safety review can be performed throughout the project limits. On corridor projects only one analysis that includes roadways and bridges is required. This review should consist of an analysis of available crash data to determine where safety enhancements are warranted. Safety reviews more than 3 years old shall be updated to verify the original safety review.
A site specific crash analysis is required as justification for any design exception. It is also required in determining appropriate 3R design criteria according to Section 3.09.02A and Section 3.09.02B. Site specific crash analyses more than 3 years old shall be updated to verify the original crash analysis.
3.09 NON-FREEWAY RESURFACING, RESTORATION AND REHABILITATION (3R) MINIMUM DESIGN GUIDELINES
3.09.01 General
The intent of the 3R guidelines is to extend the useful life of existing roadways and enhance safety while incurring minimal aesthetic and environmental disturbance and economic burden. Often, design guidelines used for new and major reconstruction are not cost effective on 3R projects. Where economically and physically practical, design guidelines should be according to AASHTO requirements to insure the greatest traffic service. The ultimate goal is to improve operating conditions and provide highways that are reasonably safe and fit for travel.
3R guidelines are divided into three categories that are addressed in subsequent sections of this chapter. These are NHS, Non-NHS and 3R Safety Considerations. They apply strictly to non-freeway applications. Guidelines for freeway 3R and 4R type work are addressed separately in Section 3.11.
3.09.02 3R Minimum Guidelines
Minimum guidelines for controlling design elements shall be according to the following:
3.09.02A Non-Freeway, NHS
| Geometric Elements | Non-Freeway, NHS 3R Minimum Guidelines | |||
|---|---|---|---|---|
(see Section 3.06) |
||||
Two-Way |
Shoulder |
Shoulder | ||
(and three lane when the center lane is a left turn lane) |
750 - 5000 >5000 - 10,000 >10,000 |
6'-0" (3'-0" Paved) 8'-0" (3'-0" Paved) 8'-0" (7'-0" Paved) | ||
Undivided |
> 10,000 |
8'-0" (3'-0" Paved) | ||
Divided |
> 10,000 |
3'-0" Paved |
8'-0" (3'-0" Paved) | |
>750 |
11'-0" 10'-0" lanes may be considered in urban areas for multi-lane un-divided (regardless of ADT) and multi-lane divided (ADT < 10,000). 12'-0" lanes are desirable on the Priority Commercial Network (PCN). 12'-0" lanes are required on the National Network (also known as the National Truck Network). Design exceptions to maintain existing narrower lanes generally receive favorable consideration but a high burden of justification is placed on requests to reduce lane widths to less than 12’-0". | |||
Structural Capacity & Horizontal Clearances |
||||
| Traveled way width plus 2'-0" each side. Minimum Design Loading HS20. |
Curb to curb approach width. Minimum Design Loading HS20. | |||
| (See Bridge Design Manual Appendix 12.02 for other trunkline classifications) | ||||
Alignment and Stopping Sight Distance |
Existing alignment and stopping sight distance may be retained if the design speed of the existing curve is not more than 15 mph below the project design speed and there is no crash concentration. Otherwise standards for new construction apply. See current AASHTO Green Book or MDOT Sight Distance Guidelines. | |||
| Review crash data. Existing grade may be retained without crash concentration. | ||||
| Traveled way 1.5% - 2%, Shoulder see Section 6.05.05 | ||||
| Standard Plan R-107-Series or reduced maximum (6%) Straight Line Superelevation Chart using the project design speed. | ||||
| See Section 3.12. | ||||
3.09.02B Non-Freeway, Non-NHS
| Geometric Elements | Non-Freeway, Non-NHS 3R Minimum Guidelines | |||
|---|---|---|---|---|
Two-Way |
||||
750 - 2000 >2000 |
3'-0" (Paved) 6'-0" (3'-0" Paved) | |||
(Divided & Undivided) |
(Divided) |
(Both sides for un-divided) | ||
>750 |
11'-0" 10'-0" lanes may be considered in urban areas for multi-lane un-divided (regardless of ADT) and multi-lane divided (ADT < 10,000). 12'-0" lanes are desirable on the Priority Commercial Network (PCN) and the National Network (also known as the National Truck Network). Existing narrower lanes may be retained without design exceptions. Reduction of existing lane widths on the National Network to less than 12-0" require a design exception request having a high burden of justification. | |||
Structural Capacity & Horizontal Clearances (Existing Bridges to remain in place) |
(Design Year) |
Design Loading |
||
Alignment and Stopping Sight Distance |
Existing alignment and stopping sight distance may be retained if the design speed of the existing curve is not more than 15 mph (horizontal alignment) or 20 mph (vertical alignment) below the project design speed and there is no crash concentration. Otherwise standards for new construction apply. . See current AASHTO Green Book or MDOT Sight Distance Guidelines. | |||
| Review crash data. Existing grade may be retained without crash concentration. | ||||
| Traveled way 1.5% - 2%, Shoulder see Section 6.05.05 | ||||
| Standard Plan R-107-Series or reduced maximum (6%) Straight Line Superelevation Chart using the project design speed. | ||||
| See Section 3.12. | ||||
3.09.02C Design Exceptions
Non-freeway 3R minimum guidelines should be followed on all non-freeway 3R projects, including Heritage Routes. When non-freeway 3R guidelines are not met for any one or more of controlling design elements (See Section 3.08.01E.), a formal request for an exception should be prepared.
When requesting exceptions to design elements on Heritage Routes, it is important to address the fact that the requested exception is based on historic, economic, or environmental concerns for the preservation of the natural beauty or historic nature of the facility.
3.09.02D Section deleted
3.09.02E Vertical Curves
Without crash concentrations and/or other geometric features such as intersections, driveways, lane drops, and horizontal curves warranting consideration, existing vertical curves corresponding to a speed 0 to 15 mph (0 to 20 mph for Non-NHS) less than the project design speed may be retained with a supporting site specific crash analysis. However, consideration should be given to re-grading vertical curves where economically and geometrically feasible. A design exception will not be required when an existing vertical curve is improved to meet 3R guidelines on 3R non-freeway projects, with verification that there is no crash concentration attributed to the curve. However, in the presence of a crash concentration, the curve shall be improved to meet 4R guidelines. When entering sight distance is restricted, an appropriate sign warning of the intersection may be installed, including advisory speed panels as needed.
3.09.02F Horizontal Curves
Without crash concentrations that warrant revision, existing horizontal curves corresponding to a speed 0 to 15 mph less than the project design speed may be retained without further documentation.
If the existing horizontal alignment is retained, the operation and safety should be improved to the extent feasible through other elements such as superelevation modifications, removing adverse crown, and removal of sight obstructions to improve stopping sight distance. When the horizontal alignment does not meet the design speed, applicable traffic control devices should be installed according to the Michigan Manual of Uniform Traffic Control Devices.
3.09.03 3R Non-Freeway Safety Considerations
The following additional information serves as guidance for the review of existing and proposed roadside features. Policies on roadside features are not standards and therefore do not require formal design exceptions. When deviations are necessary, a note should be written for the project file. This would not be subject to formal review or approval, however, a note to the design file shall provide the rationale for appropriate alternatives to these guidelines.
3.09.03A Signing
Consideration should be given to upgrading sign reflectivity, supports, and locations.
3.09.03B Evaluation of Guardrail and Bridge Rail
- An onsite inspection of height, length, and overall condition should be done to determine guardrail upgrading needs
- Existing Type A guardrail will be upgraded to current standards (see Chapter 7) at all locations, except as follows. Type A guardrail which is in good condition may be retained at cul-de-sacs, "T" intersections, and in front of the opening between twin overpassing structures.
- Blunt ends and turned down endings shall be upgraded to current standard terminals.
- Unconnected guardrail to bridge rail transitions shall be connected or upgraded to current standards.
- Existing bridge rail may remain in place if it meets AASHTO static load requirements and has an acceptable crash history. Otherwise, the bridge rail shall be upgraded or retrofitted with thrie beam guardrail. Note that new rail or complete rail replacement shall meet current standards. See Bridge Design Manual Section 12.05.
- By Federal mandate, existing Breakaway Cable Terminals (BCT) must be removed on 3R projects on the National Highway System (NHS). See Section 7.01.41B for upgrading guardrail terminal guidelines.
3.09.03C Tree Removal
Tree removal will be selective and generally "fit" conditions within the existing right-of-way and character of the road. The 2002 AASHTO Roadside Design Guide presents ideal clear zone distance criteria, however, these distances are not always practical in Michigan. Consequently, trees within the clear zone should be considered for removal subject to the following criteria:
1. Crash Frequency
Where there is evidence of vehicle-tree crashes either from actual crash reports or scarring of the trees.
2. Outside of Horizontal Curves
Trees in target position on the outside of curves with a radius of 3000 feet or less.
3. Intersections and Railroad
Trees that are obstructing adequate sight distance or are particularly vulnerable to being hit.
4. Volunteer Tree Growth
Consider removal of volunteer trees within the originally intended tree line. Volunteer trees are those that have naturally occurred since original construction of the road.
5. Maintain Consistent Tree Line
Where a generally established tree line exists, consider removing trees that break the continuity of this line within the clear zone.
6. Clear Zone
See Section 7.01.11B for Treatment / consideration of obstacles inside the calculated project clear zone. Review crash history for need for spot improvements.
3.09.03D Roadside Obstacles
Roadside improvements should be considered to enhance safety. Improvements may include removal, relocation, redesign, or shielding of obstacles such as culvert headwalls, utility poles, and bridge supports that are within the clear zone as referenced in Section 3.09.03C.
A review of crash history will provide guidance for possible treatments. However, treatment of some obstacles, such as large culverts, can add significantly, perhaps prohibitively, to the cost of a project. This means that in most instances only those obstacles that can be cited as specifically related to crashes or can be improved at low-cost should be included in the project. Ends of culverts that are within the clear zone should be considered for blending into the slope. See MDOT Drainage Manual, Section 5.3.5 and Table 5-1.
3.09.03E Cross Section Elements
1. Crown Location
Existing pavement crown point location may be retained on a project where the rate of resurfacing is less than 4" in thickness. Otherwise, standard crown location should be used.
2. Side Slopes
Use the following chart for side slope rates:
| Side Slopes | Review crash history for improvement needs. |
Current ADT Two-Way | Foreslope | |
| Two-Lane | ≤ 750 > 750 |
1:3 1:4 | ||
| Multi-Lane Undivided | ≤ 10,000 > 10,000 |
1:3 1:4 | ||
| Multi-Lane Divided | All | 1:4 |
3. Shoulder Cross Slopes
See Section 6.05.05.
3.09.04 Bridges
In most cases, bridge improvements will include upgrading approach guardrail, guardrail connections, and bridge rails to current Department practices. See Chapter 12 of the Bridge Design Manual.
3.09.05 Guidelines for Passing Relief Lanes
3.09.05A General
A passing relief lane, which is either a Truck Climbing Lane (TCL) or a Passing Lane Section (PLS), is intended to reduce congestion and improve operations along two-way, two-lane, rural highways. The congestion (platoon forming) being addressed is the result of: (1) speed reduction caused by heavy vehicles on prolonged vertical grades (TCL), and/or (2) slow moving motorists in combination with high traffic volumes or roadway alignment limiting passing opportunities (PLS). Platoons forming behind slow moving vehicles can be reduced or dispersed by increasing the speed or by increasing the opportunities to pass them. The conditions that cause the forming of platoons also restrict the passing opportunities needed to dissipate platoons, thereby increasing congestion.
The construction of Passing Relief Lanes (PRL) is not intended to connect existing multilane sections, but to provide a safe opportunity to pass slower vehicles.
The Geometrics Unit in the Design Division should be contacted to provide assistance in project selection, location, and design based on these guidelines.
3.09.05B Truck Climbing Lanes
The presence of heavy vehicles, as defined by the 2000 Highway Capacity Manual (HCM), on two-way, two-lane highway grades cause a problem because traffic is slowed and platoons form simultaneously as passing restrictions increase.
Warranting Criteria (TCL) (For Information Only)
Initially, design hour volumes (DHV) will be used in identifying candidate locations. Specific classification counts will be requested when required for more comprehensive analysis. FHWA requests that they be advised on any Federal Aid Project in which the 30th high hour is not used as the DHV in warranting a PRL. A combination of the following should be considered in identifying the need for a TCL.
1. Upgrade traffic flow rate exceeds 200 vph
2. Upgrade truck flow rate exceeds 20 vph.
3. One of the following conditions exists:
- Level-of-Service E or F exists on the grade.
- A reduction of two or more levels-of-service is experienced when moving from the approach segment to the grade.
- A typical heavy truck experiences a speed reduction of 10 mph or greater on the grade.
Location Consideration (TCL)
1. TCL’s should be:
- On the upgrade side of “critical gradesâ€.
- Along sections relatively free from commercial or residential development (driveways) and away from major intersections.
Design Consideration (TCL)
1. The TCL may normally be introduced on the grade some distance beyond the beginning of the upgrade because truck speed will not be reduced enough to create intolerable conditions for following drivers until it has traveled a certain distance up the grade.
2. TCL’s should extend beyond the crest to the point where a truck can attain a speed that is within 10 mph of the speed of other traffic and where decision sight distance when approaching the transition (taper) area is available.
3. The taper beginning a TCL should be at least 500 feet long.
4. The taper length L (feet) is approximately W×S, where W is the shift in feet and S is the posted speed in mph.
5. TCL’s should normally be 12'-0" wide.
6. TCL shoulders should be as wide as the shoulders on the adjacent two-lane sections but no less than 4'-0" (3'-0" paved). 4'-0" shoulders shall be limited to areas where wider shoulders are not feasible or environmental concerns prohibit wider shoulders.
3.09.05C Passing Lane Sections
Passing Lane Sections (PLS) along two-way, two-lane rural routes are often desirable even in the absence of “critical grades†required for TCL’s. PLS’s are particularly advantageous where passing opportunities are limited because of traffic volumes with a mix of recreational vehicles and/or roadway alignment. It is preferable to have a four-lane cross section for a PLS, but that is not always feasible because of right-of-way or environmental concerns.
Warranting Criteria (PLS) (For Information Only)
Initially, design hour volumes (DHV) will be used in identifying candidate locations. Specific classification counts will be requested when required for comprehensive analysis.
FHWA requests that they be advised on any Federal Aid Project in which the 30th high hour is not used as the DHV in warranting a PRL. A combination of the following should be considered in identifying the need for a PLS.
1. Combined recreational and commercial volumes exceed five percent of total traffic.
2. The level-of-service drops at least one level and is below Level B during seasonal, high directional splits.
3. The two-way DHV does not exceed 1200 vph. In situations where volumes exceed 1200 vph, other congestion mitigating measures should be investigated.
3.09.05C Passing Lane Sections
Passing Lane Sections (PLS) along two-way, two-lane rural routes are often desirable even in the absence of “critical grades†required for TCL’s. PLS’s are particularly advantageous where passing opportunities are limited because of traffic volumes with a mix of recreational vehicles and/or roadway alignment. It is preferable to have a four-lane cross section for a PLS, but that is not always feasible because of right-of-way or environmental concerns.
Warranting Criteria (PLS) (For Information Only)
Initially, design hour volumes (DHV) will be used in identifying candidate locations. Specific classification counts will be requested when required for comprehensive analysis.
FHWA requests that they be advised on any Federal Aid Project in which the 30th high hour is not used as the DHV in warranting a PRL. A combination of the following should be considered in identifying the need for a PLS.
1. Combined recreational and commercial volumes exceed five percent of total traffic.
2. The level-of-service drops at least one level and is below Level B during seasonal, high directional splits.
3. The two-way DHV does not exceed 1200 vph. In situations where volumes exceed 1200 vph, other congestion mitigating measures should be investigated.
Location Considerations (PLS)
Desirably, PLS should be located in areas:
1. That can accommodate four lanes (PLS for each direction of traffic) so that the amount of three-lane sections is minimized.
2. With rolling terrain where vertical grades (even though not considered “critical gradesâ€) are present to enhance:
- Visibility to readily perceive both a lane addition and lane drop.
- Differential in speed between slow and fast traffic. This occurs on upgrade locations and produces increased passing opportunities.
- Slower vehicles regaining some speed before merging by continuing the PLS beyond the crest of any grade.
3. Relatively free of commercial and/or residential development (driveways) and away from major intersections.
4. Where radius of the horizontal curve is greater than or equal to 1900 feet.
5. With no restrictions in width resulting from bridges or major culverts, unless structure widening is done in conjunction with PLS construction.
6. That are farther than 750 feet from a railroad crossing.
7. Where directional spacing of approximately 5 miles can be maintained.
Design Considerations (PLS)
1. The beginning and ending transition (tapers) areas of a PLS should be located where adequate decision sight distance is available in advance.
2. The added lanes should continue over the crest of any grade so that slower traffic can regain some speed before merging.
3. The beginning or approach taper should be at least 500 feet long.
4. The taper length L (feet) is approximately W×S, where W is the shift in feet and S is the posted speed in mph.
5. The lane widths on any PLS should normally be 12'-0" wide.
6. PLS shoulders should be as wide as the shoulders on adjacent two-lane sections but no less than 4'-0" (3'-0" paved). 4'-0" shoulders shall be limited to areas where wider shoulders are not feasible or environmental concerns prohibit wider shoulders.
7. The desirable minimum length of any PLS is 1 mile with an upper limit of about 1½ miles.
3.10 NON-FREEWAY RECONSTRUCTION / NEW CONSTRUCTION (4R)
3.10.01 General
“4R†projects are those that require complete reconstruction, new alignment, or the addition of lanes for through traffic.
3.10.02 Design Criteria
These projects are to be designed to the Geometric Design Elements. See Appendix 3A.
3.10.03 Design Exceptions
Design Exceptions are required whenever the design criteria given above (Section 3.10.02) cannot be met for controlling design elements (See Section 3.08.01E.)
3.11 FREEWAY RESURFACING, RESTORATION, REHABILITATION AND RECONSTRUCTION / NEW CONSTRUCTION (3R/4R) DESIGN CRITERIA
3.11.01 General
The 3R/4R program applies to freeways, which are defined as divided arterial highways with grade separated intersections and full control of access. Design criteria for Interstate freeways are established in the AASHTO publication, A Policy on Design Standards-Interstate System. Design criteria for non interstate freeways are established in the AASHTO publication A Policy on Geometric Design of Highways and Streets.
Current freeway standards are shown in Appendix 3A. The standards used for design speed, horizontal alignment, vertical alignment, and widths of median, traveled way and shoulders for freeway 3R projects may be the standards that were approved at the time of original construction or reconstruction, whichever is most recent. See Section 3.08.01C for information on combined 3R and 4R work.
3R/4R freeway projects should be reviewed to determine need for safety improvements such as: alignment modifications, superelevation modifications, sight distance improvements, lengthening ramps, widening shoulders, flattening slopes, increasing underclearances, upgrading guardrail and bridge railings, shielding of obstacles, and removing or relocating obstacles to provide a traversable roadside. (Also see Section 3.08.01F.)
3.11.02 Freeway 3R/4R Checklist
3.11.02A Section deleted
3.11.02B Geometrics and Signing
The Project Manager should also contact the Geometrics Unit in the Design Division and the Region Traffic and Safety Engineer to identify desirable enhancements prior to refining the project cost estimate. The Design Division – Traffic Sign Unit should be consulted to identify and coordinate plan preparation for sign upgrading needs.
3.11.02C Section deleted
3.11.02D Design Exceptions
Design Exceptions are required whenever the design criteria given in Section 3.11.01 cannot be met for controlling design elements (See Section 3.08.01E.)
3.11.03 Safety Considerations
3.11.03A Section deleted
3.11.03B Ramp Geometrics and Taper Lengths
When existing acceleration lanes, deceleration lanes and tapers are shorter than those shown in current MDOT guides, they should be lengthened to conform with the latest Geometric Design Guides. If these distances cannot be achieved, design exceptions are required. Normally, it is more cost effective to use the parallel design for on and off ramps. The need for additional lanes on the off ramp terminals should be analyzed for capacity improvements. Radii should be checked for adequacy. Gore areas should be flattened where desirable.
3.11.03C Vertical Curbs
Vertical curb should be entirely removed on freeway mainlines, high speed turning roadways and collector distributor roads. It should also be removed on other ramps for a minimum distance of 200 feet from the bifurcation or ramp nose.
3.11.03D Sight Distances
Vertical and horizontal sight distances along the mainline and within the entire interchange area, including ramp terminals, should be reviewed for conformance with current AASHTO guides. See MDOT Sight Distance Guidelines for more detailed discussion on sight distances.
3.11.03E Crown Location/Pavement Cross Slope
Where resurfacing is less than 4", the crown point will be retained in its existing location, but the 2.0% cross slope should be established or maintained.
Where resurfacing is 4" or more, the crown point should be moved to meet current standards by shifting it to the left edge of the outside lane. The 2.0% cross slope should be attained with the total yield kept close to 440 lbs/syd for 4", 550 lbs/syd for 5", etc. by reducing thickness on the median lane. However, this concept of relocating the crownline may not be feasible when the entire pavement is sloped in one direction. The desirable roll-over or algebraic difference between the pavement and shoulder cross slopes is six percent or less. A design exception is required when an existing parabolic crown is retained. Also, see Section 6.03.04B(1) “Crown and Superelevation Modification.â€
3.11.03F Superelevation
Current Standard Plan R-107-Series should be used to upgrade rural freeway projects, when feasible. When it is not possible to use the current Standard Plan R-107-Series, the straight line method may be considered on a curve by curve basis as needed. See Superelevation Using A Straight Line Method in Section 3.04.03. A design exception is required if neither of these options can be met.
3.11.03G Guardrail and Concrete Barrier
When designing barrier systems for rural freeways, use the 70 mph runout lengths from the runout chart in Section 7.01.19.
Piers and other obstacles near the center of medians, that are 70'-0" or less in width (edge to edge), will always be shielded from both sides, see Standard Plan R-56-Series. Obstacles in the median, near the edge of pavements, will be shielded from the near side. Shielding the far side will be determined on a project-by-project basis.
When it is not possible to maintain current guardrail offsets and still retain the 2'-0" distance from shoulder hinge line to the front face of guardrail post, it is generally more desirable to provide the additional offset between the guardrail and pavement edge than it is to reduce the offset in order to maintain the 2'-0" distance. The shoulder width can be maximized by using 8'-0" posts and relocating the guardrail to the shoulder hinge line. See 8'-0" post in Section 7.01.41D.
When entire runs of guardrail are replaced, the types of rails for upgrading freeways are the same as those specified in Section 7.01.12F for new freeways. The term "freeway" includes ramps. However, Type T guardrail on ramps should be transitioned to Type B when near a ramp terminal to avoid obstructing sight distance. See Section 7.01.41A.
The need for median barrier will be reviewed with the Geometric Design Unit.
The elimination of guardrail should be considered when economically feasible to flatten slopes, or where fixed objects can be removed or relocated outside the clear zone.
3.11.03H Attenuation
Where physical conditions prohibit the use of barriers, but shielding is needed, attenuation devices should be used. Contact the Geometric Design Unit for attenuation design.
3.11.03I Shoulder Cross Slopes
See Section 6.05.05.
3.11.03J Section deleted
Underclearance information in this section was moved to Section 3.12.
3.11.03K Clear Zones & Fixed Objects
The current clear zone criteria specified in Section 7.01.11 should be used when upgrading freeways. Obstacles within these limits should be shielded or removed. Obstacles beyond these limits, but within the recovery area, should be reviewed by the Geometrics Unit in the Design Division.
3.11.03L Culvert End Treatments
The ends of culverts located within the clear zone on projects programmed for upgrading shall be according to MDOT Drainage Manual, Section 5.3.5.
3.11.03M Bridges
See the Bridge Management Unit, Construction Field Services Division for FHWA conformance requirements.
3.12 UNDERCLEARANCES
3.12A 4R Freeway
Roadway 4R projects on the Freeway System must be designed to meet the current AASHTO vertical clearance requirement of 16'-0" (16'-3" is desired for future overlay of the road). Scoping of projects must include a determination of the most effective means of obtaining the vertical clearance standard. A cost/benefit analysis to determine how best to achieve the standard, either in full or with incremental progress needs to be prepared. The analysis should include the alternatives of obtaining all vertical clearances with the road project, a bridge project, or some combination of road and bridge work to meet the clearance requirements. In many cases it may not be possible to achieve the complete vertical clearance with the proposed road project. If the most efficient plan for meeting the vertical clearance requirement is incremental progress, a design exception will be required. The design exception should be submitted as soon as possible, preferably prior to the submittal of the call for projects. This assures that design is not started on projects that may not be approved. The following is the minimum information required to prepare a vertical clearance analysis. This information is also required if a design exception is submitted.
- Preliminary grades for the bridge and approaches, the route under the structure, and ramps if appropriate.
- Location of existing structure foundations related to the proposed grade changes.
- Impact evaluation on existing drainage.
- Evaluation of any other deficient geometric feature.
- Determination of ROW needs.
- Impacts on Environment.
- Cost estimates for alternatives to meet vertical clearances.
- Proposed time frame when the remainder of vertical clearance will be achieved (rough estimate)
- Accident analysis where appropriate.
- Soils (cut and fill information) and ground water information.
- Impact on local businesses and residences.
- User costs, constructability, maintaining traffic scheme and maintenance cost.
3.12B 4R Arterials
On Roadway 4R projects on Arterial systems, where no work is scheduled for the bridges, the bridges are considered existing structures and can be retained if they meet the 14'-6" vertical clearance standard, therefore no design exception is required. The existing clearance must be retained. It must not be reduced. Although not required, an evaluation should be performed to determine how best to achieve the standard, either in full or with incremental progress. Obtaining incremental progress toward the vertical clearance requirement with the road 4R project could prevent other more costly construction with the next major bridge rehabilitation or replacement project.
3.12C 4R Collectors and Local Routes
Maintain existing vertical clearance and a minimum of 14'-6" (14'-9" is desired on 4R projects if possible.)
3.12D 3R Freeway
Roadway 3R freeway projects must be designed to meet the current AASHTO vertical clearance requirement of 16'-0" (16'-3" is desired for future overlay of the road). A design exception is required if the vertical clearance requirement is not met. The format for the design exception does not require a detailed evaluation but should include the basis for the request and review of the accident history and high load hits for the structures in the immediate vicinity of the structure.
3.12E 3R Arterials
On roadway 3R projects on Arterial systems, the bridges are considered existing structures and can be retained if they meet the 14'-0" vertical clearance standard, therefore no design exception is required. The existing vertical underclearance must be retained. Although not required, an evaluation should be performed to determine how best to achieve the standard, either in full or with incremental progress. Obtaining incremental progress toward the vertical clearance requirement with the road 3R project could prevent other more costly construction with the next major bridge rehabilitation or replacement project.
A design exception is required to maintain the vertical clearance below 14'-0". The likelihood of obtaining design exceptions for reducing vertical clearance is extremely remote.
3.12F Preventive Maintenance
Project scope of work includes but is not limited to road work consisting of thin HMA overlays, pavement grinding, concrete joint repair, slurry seal (shoulders only), and seal coat (shoulders only). Maintain existing vertical clearance. No design exception is required.
3.12G Vertical Clearance Requirement Table
The desired vertical bridge underclearance should be provided as indicated below. If the desired underclearance cannot be provided, then the minimum underclearance shall be met. Where it is considered not feasible to meet these minimums, a design exception shall be requested. See the vertical underclearance design exception matrix in this section and Section 12.02 of the Bridge Design Manual. Requests to further reduce the underclearance of structures with existing vertical clearance less than indicated in the chart below should be made only in exceptional cases.
| VERTICAL CLEARANCE REQUIREMENT TABLE | |||||
|---|---|---|---|---|---|
| Route Classification Under the Structure |
All Construction (Desired) |
New Construction (Min*) |
Road 4R Construction (Min*) |
Bridge 4R Construction (Min*) |
3R Construction (Min*) |
| Freeways | 16'-3" | 16'-0" | 16'-0" | 16'-0" | 16'-0" |
| NHS Arterials (Local & Trunkline) |
16'-3" | 16'-0" | Maintain Existing** and 14'-6" Min |
16'-0" | Maintain Existing** and 14'-0" Min |
| Non NHS Arterials (Local & Trunkline) |
16'-3" | 14'-6" | Maintain Existing** and 14'-6" Min |
Maintain Existing** and 14'-6" Min |
Maintain Existing** and 14'-0" Min |
| Collectors, Local Roads & Special Routes(1) |
14'-9" | 14'-6" | Maintain Existing** and 14'-6" Min |
Maintain Existing** and 14'-6" Min |
Maintain Existing** and 14'-0" Min |
* Minimum vertical clearance must be maintained over complete usable shoulder width.
** Existing vertical clearances greater than or equal to the minimums shown may be retained without a design exception. Vertical clearance reductions that fall below the minimums for new construction require a design exception.
Information on the NHS system can be obtained by contacting the Statewide Planning Section, Bureau of Transportation Planning or found on the MDOT Web site at: http://www.michigan.gov/mdot-nfc
(1) Special Routes are in highly urbanized areas (where little if any undeveloped land exist adjacent to the roadway) where an alternate route of 16'-0" is available or has been designated. Bridges located on Special Routes in Highly Urbanized Areas can be found on the MDOT website at: http://mdotcf.state.mi.us/public/design/files/englishbridgemanual/Exempt_Structures.pdf
Ramps and roadways connecting a Special Route and a 16’-0†route require a vertical clearance minimum of 14’-6†(14’-9†desired). Ramps and roadways connecting two 16’-0†routes require a vertical clearance minimum of 16’-0†(16’-3†desired).
Pedestrian bridges are to provide 1’-0†more underclearance than that required for a vehicular bridge. For freeways (Interstate and non Interstate), including Special Route Freeways, the desired underclearance shall be 17’-3†(17’-0†minimum).
A vertical clearance of 23’-0†is required for highway grade separations over railroads. Clearance signs are to be present for structures with underclearance 16’-0†or less (show dimensions 2†less than actual). See http://mdotcf.state.mi.us/public/tands/plans.cfm for additional information and guidelines.
For shared use paths (pedestrian and bicycle) the vertical clearance to obstructions, including overhead fencing, shall be a minimum of 8’-6†(10’-0†desired). However, vertical clearance may need to be greater to permit passage of maintenance and emergency vehicles. In undercrossings and tunnels, 10’-0†is desirable for vertical clearance. See AASHTO’s Guide for the Development of Bicycle Facilities.
3.12H Design Exception Requirements for Vertical Clearances
| Design Exception Requirements Vertical Clearance | ||||
|---|---|---|---|---|
Exception Required |
with MTMCTEA Required |
Required by Engineer of Design Programs |
Approval | |
| New and 4R reconstruction work on interstate greater than $1 million. |
||||
| New and 4R reconstruction work on Interstate freeways less than $1 million. |
||||
| New and 4R reconstruction work on Non Interstate freeways greater than $1 million. |
||||
| New and 4R reconstruction work on Non Interstate freeways less than $1 million. |
||||
| New and 4R reconstruction work on NHS routes other than freeways greater than $1 million. |
||||
| New and 4R reconstruction work on NHS routes other than freeways less than $1 million. |
||||
| New and 4R reconstruction on Non-NHS Routes. |
||||
| 3R work on Interstate System. | oversight is assigned to FHWA on NHS projects > $5 million. | |||
| 3R work on Non-Interstate System. | ||||
| 3R work on Non-freeway Routes. | ||||
| Preventative Maintenance Work | ||||
MTMCTEA - Military Traffic Management Command Transportation Engineering Agency
In addition to normal processing of design exceptions, all proposed design exceptions pertaining to vertical clearance on Interstate routes including shoulders, and all ramps and collector distributor roadways of Interstate to Interstate interchanges will be coordinated with the Surface Deployment and Distribution Command Transportation Engineering Agency (SDDCTEA). The only Interstate routes the SDDCTEA is interested in are the routes that require a 16'-0" vertical clearance. These routes include all the Interstate system including US-131 between I-196 and I-96 (this roadway is technically I-296 but not signed as such). In addition to the Interstate route requirements listed above, US¬-23 between Ohio state line and I-75 south of Flint shall require coordination with SDDCTEA. This requirement does not apply to Special Routes (1).
MDOT (or its Consultant) is responsible for coordinating exceptions on all projects regardless of oversight responsibilities. MDOT will send a copy of all requests, and responses, to the FHWA. Michigan Interstate Vertical Clearance Exception Coordination, MDOT Form # 0333, is available from MDOT web site.
Requests for coordination shall be E-mailed or sent to:
Jason W. Cowin, P.E.
- Senior Engineer, Highway Systems
- Highways for National Defense
ATTN: SDDCTEA
709 Ward Drive, Building 1990
Scott AFB, IL 62225
Telephone: 618-220-5229, Fax: 618-220-5125
Email: jason.cowin@us.army.mil
MDOT (or its consultant) shall verify SDDCTEA receipt of the request. If no comments are received within ten working days, it may be assumed that the SDDCTEA does not have any concerns with the proposed design exception.
