Roadway Design Manual: Minimum Designs for Truck and Bus Turns

Although there are no firm guidelines governing the selection of the type of large vehicle to be used as a design vehicle, there is suggested guidance provided for urban and rural intersections near the end of this section. Factors that influence design vehicle selection are as follows:

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Type and frequency of use by large vehicles;

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Consequences of encroachment into other lanes or the roadside;

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Availability of right-of-way; and

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Functional class of intersecting routes and location (urban versus rural) affect this selection in a general sense. Project-specific traffic data, specifically the frequency of use by the various design vehicle classes, is often the most important consideration in the selection process. The Transportation Planning and Programming Division (TPP) may be contacted to obtain volume data for the various vehicle classes.

Refer to AASHTO’s A Policy on Geometric Design of Highways and Streets for information on turning paths and turning radii of design vehicles.

Additionally, the use of AutoTURN, AutoTrack, or a similar software should be used to determine the extent of design vehicle encroachment.

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Channelization

Where the inner edges of pavement for right turns at intersections are designed to accommodate semi‑trailer combinations or where the design permits passenger vehicles to turn at 15-mph or more (i.e., 50-ft or more radius), the pavement area at the intersection may become excessively large for proper control of traffic. In these cases, channelizing islands should be used to more effectively control, direct, and/or divide traffic paths. Physically, islands should be at least 50-ft² in urban and 75-ft² for rural conditions (100-ft² preferable for both) and may be painted or curbed.

Additional guidance for channelization of a right turn slip lane is provided in Appendix D, Right-Turn Slip Lane Design Guidelines.

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Alternatives to Simple Curvature

To accommodate the longest vehicles, off-tracking characteristics in combination with the large (simple curve) radius, results in a wide pavement area. In this regard, three‑centered compound curves, or offset simple curves in combination with tapers, are preferred since they more closely fit the paths of vehicles. Table 7-2 shows minimum edge of pavement designs for right turns to accommodate various design vehicles for turn angles varying from 60 to 120 degrees.

Anchor: #i1009736Table 7-2: Minimum Edge of Pavement Designs for Right Turns for Various Design Vehicles for Turn Angle Varying from 60 to 120 Degrees
Angle of Turn¹ (degrees)	Design Vehicle	Simple Curve Radius (ft.)	Simple Curve Radius with Taper			3-Centered Compound Curve, Symmetric		3-Centered Compound Curve, Asymmetric
Angle of Turn¹ (degrees)	Design Vehicle	Simple Curve Radius (ft.)	Radius (ft.)	Offset (ft.)	Taper	Radii (ft.)	Offset(ft.)	Radii (ft.)	Offset (ft.)
60	P	40	-	-	-	-	-	-	-
-	SU	60	-	-	-	-	-	-	-
-	WB-40	90	-	-	-	-	-	-	-
-	WB-62	170	140	4.0	15:1	400-100-400	15.0	110-100-220	10.0-12.5
-	WB-67	200	140	4.5	15:1	400-100-400	8.0	250-125-600	1.0-6.0
75	P	35	25	2.0	10:1	100-25-100	2.0	-	-
-	SU	55	45	2.0	10:1	120-45-120	2.0	-	-
-	WB-40	-	60	2.0	15:1	120-45-120	5.0	120-45-195	2.0-6.5
-	WB-62	-	145	4.0	20:1	440-75-440	15.0	140-100-540	5.0-12.0
-	WB-67	-	145	4.5	20:1	420-75-420	10.0	200-80-600	1.0-10.0
90	P	30	20	2.5	10:1	100-20-100	2.5	-	-
-	SU	50	40	2.0	10:1	120-40-120	2.0	-	-
-	WB-40	-	45	4.0	10:1	120-40-120	5.0	120-40-200	2.0-6.5
-	WB-62	-	120	4.5	30:1	400-70-400	10.0	160-70-360	6.0-10.0
-	WB-67	-	125	4.5	30:1	440-65-440	10.0	200-70-600	1.0-11.0
105	P	-	20	2.5	-	100-20-100	2.5	-	-
-	SU	-	35	3.0	-	100-35-100	3.0	-	-
-	WB-40	-	40	4.0	-	100-35-100	5.0	100-55-200	2.0-8.0
-	WB-62	-	115	3.0	15:1	520-50-520	15.0	360-75-600	4.0-10.5
-	WB-67	-	115	3.0	15:1	500-50-500	13.0	200-65-600	1.0-11.0
120	P	-	20	2.0	-	100-20-100	2.0	-	-
-	SU	-	30	3.0	-	100-30-100	3.0	-	-
-	WB-40	-	35	5.0	-	120-30-120	6.0	100-30-180	2.0-9.0
-	WB-62	-	100	5.0	15:1	520-70-520	10.0	80-55-520	24.0-17.0
-	WB-67	-	105	5.2	15:1	550-45-550	15.0	200-60-600	2.0-12.5
Notes: “Angle of Turn” is the angle through which a vehicle travels in making a turn. It is measured from the extension of the tangent on which a vehicle approaches to the corresponding tangent on the intersecting road to which a vehicle turns. It is the same angle that is commonly called the delta angle in surveying terminology.

Figure 7-46 shows sample alternatives to simple curvature edge of pavement geometry for a 90-degree turn using a WB-62 design vehicle. Although not shown in this figure, a radius of 100-ft without channelizing island would be necessary to accommodate the wide, off-tracking path of a WB-62 without undesirable encroachment. A geometric design of this sort is undesirable because such a wide pavement may confuse drivers and limit effective space for traffic control devices.

Vehicle swept path analysis software (e.g. AutoTURN or similar software) can also be used to analyze turning movements through intersections to optimize edge of pavement geometry and ensure the design vehicle can adequately maneuver the intersection.

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Figure 7-46. Example of Pavement Edge Geometry

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Urban Intersections

Corner radii at intersections on arterial streets should satisfy the requirements of the drivers using them to the extent practical and in consideration of the amount of right-of-way available, the angle of the intersection, numbers of and space for pedestrians, width and number of lanes on the intersecting streets, and amounts of speed reductions. The following summary is offered as a guide:

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Radii of 15-ft to 25-ft are adequate for passenger vehicles. These radii may be provided at minor cross streets where there is little occasion for trucks to turn or at major intersections where there are parking lanes. Where the street has sufficient capacity to retain the curb lane as a parking lane for the foreseeable future, parking should be restricted for appropriate distances from the crossing.

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Radii of 25-ft or more at minor cross streets should be provided on new construction and on reconstruction where space permits.

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Radii of 30-ft or more at major cross streets should be provided where feasible so that an occasional truck can turn without too much encroachment.

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Radii of 40-ft or more, and preferably 3-centered compound curves or simple curves with tapers to fit the paths of appropriate design vehicles, should be provided where large truck combinations and buses turn frequently. Larger radii are also desirable where speed reductions would cause problems.

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Radii of 75-ft or more should be provided for arterial-arterial urban intersections where frequent use a WB-62 or larger design vehicle is anticipated.

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Radii dimensions should be coordinated with crosswalk distances or special designs to make crosswalks safe for all pedestrians.

Where other types of truck combinations are used as the design vehicle, pavement edge geometry as shown in Table 7-2 and Figure 7-46 permit the use of lesser radii. An operational measure that appears promising is to provide guidance in the form of edge lines to accommodate the turning paths of passenger cars, while providing sufficient paved area beyond the edge lines to accommodate the turning path of an occasional large vehicle.

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Rural Intersections

In rural areas space is generally more available and speeds higher. These factors suggest more liberal designs for truck turning even when the frequency of long vehicles may not be as great as in urban areas.

In the design of highway intersections with other (non-highway system) public roads, long vehicles are generally infrequent users. Minimally, the SU, or on some occasions the WB-40, design vehicle is appropriate for use unless special circumstances (location of a truck stop or terminal) influence the frequency of use by certain vehicle classes.

For arterial intersections with collectors, the WB-40 design vehicle is generally appropriate and the WB-62 should be used where specific circumstances warrant.

For arterial-arterial intersections, use by the WB-62 or larger design vehicle should be anticipated within project life. For turning roadway widths to be reasonable, a design radius of 75-ft or more is required. Where circumstances at a particular rural arterial-arterial intersection precludes the use of the WB-62 or larger design vehicle, the WB-40 may be used. Refer to Table 7-2 for radii information with respect to the design vehicle.

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Median U-Turn Movements

In some instances, median openings can be used to facilitate U-turns. Figure 7-47 provides minimum median widths “m” (ft) to complete a U-turn movement. For instances where the U-turn can't be completed, a loon may be used to provide additional space. Refer to Appendix E, Section 4, Median U-Turn Intersection (MUT) for additional information on loon design.

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Figure 7-47. AASHTO Minimum Median Widths for U-Turn Crossovers