2.4 Physical Dimensions and Weights
AASHTO’s “A Policy on Geometric Design of Highways and Streets” (AASHTO (2018)), commonly referred to as the “Green Book” includes guidelines for design vehicle dimensions. Table 2.1 lists those values for the commercial truck classifications.
| Design Vehicle Type | Symbol | Overall Height | Overall Width | Overall Length | Overhang-Front | Overhang-Rear |
|---|---|---|---|---|---|---|
| Single Unit Truck | SU | 11-13.5 | 8.0 | 30.0 | 4.0 | 6.0 |
| Intermediate Semitrailer | WB-40 | 13.5 | 8.0 | 45.5 | 3.0 | 4.5# |
| Interstate Semitrailer | WB-62* | 13.5 | 8.5 | 69.0 | 4.0 | 4.5# |
| Interstate Semitrailer | WB-67** | 13.5 | 8.5 | 73.5 | 4.0 | 4.5# |
| “Double Bottom” Semitrailer / Trailer | WB-67D | 13.5 | 8.5 | 72.3 | 2.3 | 3.0 |
| Rocky Mountain Double-Semitrailer / Trailer | WB-92D | 13.5 | 8.5 | 97.3 | 2.3 | 3.0 |
| Triple-Semitrailer / Trailer | WB-100T | 13.5 | 8.5 | 104.8 | 2.3 | 3.0 |
| Turnpike Double Semitrailer / Trailer | WB-109D | 13.5 | 8.5 | 114.0 | 2.3 | 4.5# |
* Design vehicle with 48.0-ft trailer as adopted in 1982 STAA (Surface Transportation Assistance Act)
** Design vehicle with 53.0-ft trailer as grandfathered in 1982 STAA
# Length of the overhang from the back axle of the tandem axle assembly.
Source: AASHTO (2018), (Table 2-4a. Design Vehicle Dimensions (U.S. Customary Units))
Figure 2.9: Illustration of length and height dimensions for WB-62 design vehicle
Figure 2.10: Illustration of length and height dimensions for WB-67 design vehicle
2.4.1 Regulatory Limits
Per the Federal Highway Administration (https://ops.fhwa.dot.gov/freight/sw/overview/index.htm), the following weight and dimension regulations are specified.
On the National Network of highways (the interstate highway system and highways, formerly classified as Primary System routes, capable of safely handling larger commercial motor vehicles, the following dimension limits apply:
- overall vehicle length:
- No federal limit is imposed on most truck tractor-semitrailers operation on the National Network.
- Exception: Combination vehicles (truck tractor plus semitrailer or trailer) designed and used specifically to carry automobiles or boats in specially designed racks may not exceed a maximum overall vehicle length of 65 feet, or 75 feet, depending on the type of connection between the tractor and trailer.
- trailer length:
- Federal law provides that no state may impose a length limitation of less than 48 feet (or longer if provided for by grandfather rights) on a semitrailer operating in any truck tractor-semitrailer combination on the National Network.
- Federal law also provides that no state may impose a length limitation of less than 28 feet on a semitrailer or trailer operating in a truck tractor-semitrailer-trailer (twin-trailer) combination on the National Network.
- vehicle width: On the National Network, no state may impose a width limitation of more or less than 102 inches. Safety devices (e.g., mirrors, handholds) necessary for the safe and efficient operation of motor vehicles may not be included in the calculation of width.
- vehicle height: No federal limit. State limits range from 13.6 feet to 14.6 feet.
Additional information about federal size and weight limits can be found at Compilation of Existing State Truck Size and Weight Limit Laws.