4.3.3 Control Vehicles

Geometric design may also incorporate a control vehicle. The control vehicle is an infrequent but necessary user of the facility and informs what operations need to be accommodated in the design of the facility. The control vehicle is generally larger than a design vehicle. As an example, a facility may be designed for WB-67’s but may also need to account for oversized tractor trailers carrying large materials such as wind turbines. Adopting both a design vehicle and a control vehicle encourages a safer design that can accommodate frequent and infrequent users. To prevent the overdesign of a facility, the control vehicle should only dictate how the design might accommodate a larger vehicle’s needs when using the facility.
Typically, a design vehicle can turn using one incoming and one receiving lane, while the control vehicle can turn using multiple lane spaces. Engineering judgement should be exercised when determining how many lane spaces a control vehicle can use.
Figure 4-1
shows an example of how the turning movements can differ between a design vehicle and control vehicle.
Table 4-1: Minimum Turning Radii of Design Vehicles
Design Vehicle Type
Passenger Car
Single-Unit Truck
Single-Unit Truck (Three Axle)
Intercity Bus (Motor Coach)
City Transit Bus
Conventional School Bus (65 pass.)
Large School Bus (84 pass.)
1
Articulated Bus
Intermediate Semi-trailer
Symbol
P
SU-30
SU-40
BUS-40
BUS-45
CITY-BUS
S-BUS36
S-BUS40
A-BUS
WB-40
Minimum Design Turn Radius (ft)
23.8
41.8
51.2
41.7
44.0
41.6
38.6
39.1
39.4
39.9
Centerline Turning Radius (CTR) (ft)
4
21.0
38.0
47.4
37.8
40.2
37.8
34.9
35.4
35.5
36.0
Minimum Inside Radius (ft)
14.4
28.4
36.4
24.3
24.7
24.5
23.8
25.3
21.3
19.3
Design Vehicle Type
Interstate Semi Trailer
"Double Bottom" Combinati on
Rocky Mtn Double
Triple Semi-trailer/ Trailers
Turnpike Double Semi- trailer / Trailer
Motor Home
Car and Camper Trailer
Car and Boat Trailer
Motor Home and Boat Trailer
Symbol
WB-62
2
 & WB-62TX
WB-67
3
WB-67D
WB-92D
WB-100T
WB-109D
2
MH
P/T
P/B
MH/B
Minimum Design Turn Radius (ft)
44.8
44.8
44.8
82.0
44.8
59.9
39.7
32.9
23.8
49.8
Centerline Turning Radius (CTR) (ft)
4
41.0
41.0
40.9
78.0
40.9
55.9
36.0
30.0
21.0
46.0
Minimum Inside Radius (ft)
7.4
1.9
19.1
55.6
9.7
13.8
26.0
18.3
8.0
35
Notes:
  1. The turning radius assumed by a designer when investigating possible turning paths is a set of the centerline of the front axle of a vehicle. If the minimum turning path is assumed, the CTR approximately equals the minimum design turning radius minus one-half the front width of the vehicle.
  2. Design vehicle with 48-ft trailer as adopted in 1982 Surface Transportation Assistance Act (STAA)
  3. Design vehicle with 53-ft trailer as grandfathered in with 1982 Surface Transportation Assistance Act (STAA)
  4. School buses are manufactured from 42-passenger to 84-passenger sizes. This corresponds to wheelbase lengths of 11.0- to 20.0-ft, respectively. For these different sizes, the minimum design turning radii vary from 28.1- to 39.1-ft and the minimum inside radii vary from 17.7- to 25.3-ft.
  5. This table is not inclusive of all design vehicles. For specific design vehicles that are not included but are frequent users of a facility refer to NCHRP 505.
Lane usage for Design and Control Vehicle Turning Movements ( click in image to see full-size image)
Figure 4-1: Lane usage for Design and Control Vehicle Turning Movements