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Section 7: Minimum Designs for Truck and Bus Turns

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Overview

This section contains the following information on minimum designs for truck and bus turns:

  • application
  • channelization
  • alternatives to simple curvature
  • urban Intersections
  • rural Intersections.
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Application

There are no firm guidelines governing the selection of the type of large vehicle to be used as a design vehicle. Factors that influence design vehicle selection are as follows:

  • the type and frequency of use by large vehicles
  • consequences of encroachment into other lanes or the roadside
  • availability of right-of-way
  • 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.

Minimum turning path templates for single unit trucks or buses, semi‑trailer combinations with wheelbases of 40, 50 and 62 ft [12.2, 15.24 and 18.9 m], and double-trailer combination with wheelbase of 67 ft [20.43 m] are shown in Figures 7-1, 7-2, 7-3, 7-4, 7-5, and 7-6 respectively. The AASHTO publication A Policy on Geometric Design of Highways and Streets provides additional information on turning paths and turning radii of these and other vehicles.

Turning Template for Single Unit Trucks
or Buses, (not to scale). Click here to see a PDF of the
image. (click in image to see full-size image) Anchor: #i1001418grtop

Figure 7-1. Turning Template for Single Unit Trucks or Buses, (not to scale). Click here to see a PDF of the image.

Turning Template for Semi-Trailer with
40 ft [12.20 m] Wheelbase, (not to scale). Click here to
see a PDF of the image. (click in image to see full-size image) Anchor: #i1001420grtop

Figure 7-2. Turning Template for Semi-Trailer with 40 ft [12.20 m] Wheelbase, (not to scale). Click here to see a PDF of the image.

Turning Template for Semi-Trailer with
50 ft [15.24 m] Wheelbase, (not to scale). Click here to
see a PDF of the image. (click in image to see full-size image) Anchor: #i1001422grtop

Figure 7-3. Turning Template for Semi-Trailer with 50 ft [15.24 m] Wheelbase, (not to scale). Click here to see a PDF of the image.

Turning Template for Semi-Trailer with
62 ft [18.9 m] Wheelbase, (not to scale). Click here to
see a PDF of the image. (click in image to see full-size image) Anchor: #i1001424grtop

Figure 7-4. Turning Template for Semi-Trailer with 62 ft [18.9 m] Wheelbase, (not to scale). Click here to see a PDF of the image.

Turning Template for Semi-Trailer with
62 ft [18.9 m] Wheelbase (Radius = 75 ft [22.9 m], (not to scale).
Click here to
see a PDF of the image. (click in image to see full-size image) Anchor: #i1001426grtop

Figure 7-5. Turning Template for Semi-Trailer with 62 ft [18.9 m] Wheelbase (Radius = 75 ft [22.9 m], (not to scale). Click here to see a PDF of the image.

Turning Template for Double-Trailer Combination
with 67 ft [20.41 m] Wheelbase, (figure not to scale). Click here to
see a PDF of the image (click in image to see full-size image) Anchor: #i1001428grtop

Figure 7-6. Turning Template for Double-Trailer Combination with 67 ft [20.41 m] Wheelbase, (figure not to scale). Click here to see a PDF of the image

(US). Example of Pavement Edge Geometry
(US Customary). (click in image to see full-size image) Anchor: #i1001430grtop

Figure 7-7. (US). Example of Pavement Edge Geometry (US Customary).

 (M). Example of Pavement Edge Geometry
(Metric). (click in image to see full-size image) Anchor: #HKNBRHHHgrtop

Figure 7-8. (M). Example of Pavement Edge Geometry (Metric).

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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 [20 km/h] or more (i.e., 50 ft [15 m] 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 [4.5 m2 ] in urban and 75 ft2 [7.0 m2] for rural conditions (100 ft2 [9.0 m2] preferable for both) in size and may range from a painted to a curbed area.

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Alternatives to Simple Curvature

To accommodate the longest vehicles, off-tracking characteristics in combination with the large (simple curve) radius that must be used 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.

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Angle of Turn1 (degrees)

Design Vehicle

Simple Curve Radius

Simple Curve Radius with Taper

3-Centered Compound Curve, Symmetric

3-Centered Compound Curve, Asymmetric

-

-

(ft)

Radius (ft)

Offset (ft)

Taper

Radii (ft)

Offset (ft)

Radii (ft)

Offset (ft)

60

P

40

-

-

-

-

-

-

-

-

SU

60

-

-

-

-

-

-

-

-

WB-40

90

-

-

-

-

-

-

-

-

WB-50

150

120

3.0

15:1

200-75-200

5.5

200-75-275

2.0-7.0

75

P

35

25

2.0

10:1

100-75-100

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-50

-

65

3.0

15:1

150-50-150

6.5

150-50-225

2.0-10.0

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-50

-

60

4.0

15:1

180-60-180

6.5

120-40-200

2.0-10.0

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-50

-

55

4.0

15:1

180-45-180

8.0

150-40-210

2.0-10.0

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-50

-

45

4.0

15:1

180-40-180

8.5

150-35-220

2.0-12.0

1 “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.



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Angle of Turn1 (degrees)

Design Vehicle

Simple Curve Radius

Simple Curve Radius with Taper

3-Centered Compound Curve, Symmetric

3-Centered Compound Curve, Asymmetric

-

-

(m)

Radius (m)

Offset (m)

Taper

Radii (m)

Offset (m)

Radii (m)

Offset (m)

60

P

12

-

-

-

-

-

-

-

-

SU

18

-

-

-

-

-

-

--

-

WB-12

28

-

-

-

-

-

-

-

-

WB-15

45

29

1.0

15:1

60-23-60

1.7

60-23-84

0.6-2.0

75

P

11

8

0.6

10:1

30-8-30

0.6

-

-

-

SU

17

14

0.6

10:1

36-14-36

0.6

-

-

-

WB-12

-

18

0.6

15:1

36-14-36

1.5

36-14-60

0.6-2.0

-

WB-15

-

20

1.0

15:1

45-15-45

2.0

45-15-69

0.6-3.0

90

P

9

6

0.8

10:1

30-6-30

0.8

-

-

-

SU

15

12

0.6

10:1

36-12-36

0.6

-

-

-

WB-12

-

14

1.2

10:1

36-12-36

1.5

36-12-60

0.6-2.0

-

WB-15

-

18

1.2

15:1

55-18-55

2.0

36-12-60

0.6-3.0

105

P

-

6

0.8

8:1

30-6-30

0.8

-

-

-

SU

-

11

1.0

10:1

30-11-30

1.0

-

-

-

WB-12

-

12

1.2

10:1

30-11-30

1.5

30-17-60

0.6-2.5

-

WB-15

-

17

1.2

15:1

55-14-55

2.5

45-12-64

0.6-3.0

120

P

-

6

0.6

10:1

30-6-30

0.6

-

-

-

SU

-

9

1.0

10:1

30-9-30

1.0

-

-

-

WB-12

-

11

1.5

8:1

36-9-36

2.0

30-9-55

0.6-2.7

-

WB-15

-

14

1.2

15:1

55-12-55

2.6

45-11-67

0.6-3.6

1“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 7-7 shows sample alternative (to simple curvature) edge of pavement geometry for a 90 degree turn using a WB 50 [WB‑15] design vehicle. Although not shown in this figure, a radius of 80 ft [25 m] without channelizing island would be necessary to accommodate the wide, off-tracking path of a WB-50 [WB‑15] without undesirable encroachment. A geometric design of this sort is undesirable, however, since there would be a confusing, wide expanse of surfaced area; furthermore, there is no convenient, effective location for traffic control devices.

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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:

  • Radii of 15 ft [4.5 m] to 25 ft [7.5 m] 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.
  • Radii of 25 ft [7.5 m] or more at minor cross streets should be provided on new construction and on reconstruction where space permits.
  • Radii of 30 ft [9 m] or more at major cross streets should be provided where feasible so that an occasional truck can turn without too much encroachment.
  • Radii of 40 ft [12 m] or more, and preferably three-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.
  • Radii dimensions should be coordinated with crosswalk distances or special designs to make crosswalks safe for all pedestrians.

For arterial-arterial urban intersections, turning radii of 75 ft [23 m] or more are desirable if frequent use is anticipated by the WB-62 [WB-19] design vehicle. Where other types of truck combinations are used as the design vehicle, pavement edge geometry as shown in Table 7-2: Minimum Edge of Pavement Designs at Intersections and Figure 7-7 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 [WB-12], 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 [WB-12] design vehicle is generally appropriate and the WB-50 [WB‑15] should be used where specific circumstances warrant.

For arterial‑arterial intersections, use by the WB-62 [WB-19] design vehicle should be anticipated within project life. Two template layouts, Figure 7-4 and Figure 7-5, are shown with radii of 45 ft [13.7 m] and 75 ft [ 23 m] respectively. For turning roadway widths to be reasonable in width, a design radius of 75 ft [23 m] or more is required. Where circumstances at a particular rural arterial‑arterial intersection precludes the use of the WB-62 [WB-19] design vehicle, the WB-50 [WB-15] should be used.

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