Comprehensive Truck Size and Weight Limits Study - Modal Shift Comparative Analysis Technical Report
Chapter 2: Scenario Definitions
The only explicit direction provided in Section 32801 of MAP-21 regarding vehicles to be studied in the 2014 CTSW Study was that the study should consider "six-axle and other alternative configurations of tractor-trailers."
USDOT determined that up to six alternative truck configurations could be examined as part of this comparative analysis in the timeframe established in MAP-21. Also, to be selected for study, USDOT stipulated that an alternative truck configuration needed to be currently in use in the United States, Canada, or elsewhere, and practical for use in the United States. USDOT then proposed three specific truck configurations and solicited input from stakeholders regarding the selection of the additional configurations to include in the mix.
After extensive public and stakeholder input, USDOT identified the six alternative truck configurations to compare with control or baseline vehicles meeting current federal size and weight limitations. In addition, two truck configurations that now meet Federal size and weight limitations were selected to serve as “baseline” or “control” vehicles. The comparisons would be conducted over six illustrative network scenarios, using data analysis, modeling, and other state of the art methods to derive technical results in each of the five study focus areas. All but one of the vehicles selected for analysis are currently in use on some highways in the United States so that there is some experience with these vehicles in this country. The box on the following page shows the reasons why each alternative configuration was selected for inclusion in this Study. Table 1 describes key attributes of each configuration.
USDOT developed details of analytical scenarios based on these vehicles to serve as the basis for estimating potential impacts associated with widespread use of each vehicle configuration.
First, each scenario assumes an increase in size and weight for only one of the vehicle types identified above. In other words, none of the scenarios involve increases in weight for more than a single vehicle configuration. The impacts presented in this study cannot be added or subtracted from each other – for example one cannot add the impact of the higher gross vehicle weight five-axle tractor semitrailer to that of the 6-axle tractor semitrailer. To understand the impact of multiple changes to truck size and weight, a new analysis would be necessary.
Second, maximum weight limits for the scenario vehicles are assumed to extend beyond the Interstate System. Except for the triple trailer combinations, States are assumed to allow the scenario vehicles to operate on the same networks on which tractor-semitrailers and twin trailer combinations with 28.5-foot trailing units currently operate and to have the same access to terminals and facilities for food, fuel, rest, and repairs. Wherever tractor-semitrailers currently operate, the scenario tractor-semitrailers would also operate. Wherever twins with 28.5-foot trailers operate, the scenario twin-trailer combination with 33-foot trailers would operate. Triple trailer combinations are assumed for analytical purposes to be limited to a much more restricted network of 74,500 miles of Interstate and other principal arterial highways. Figure 2 shows the network assumed to be available for triples in the modal shift analysis. Because of their length and challenge in maneuvering, access by triples to points of loading and unloading off the network is assumed to be limited to 2 miles.
Alternative Truck Configurations and Control Truck Configurations
Control Vehicle for Comparison with One Trailer Combinations
- Five-axle, tractor-semitrailer combination (3-S2), 80,000 lbs.: This is the "standard" configuration of a three-axle tractor with a 53-foot long, two-axle semitrailer and a GVW of 80,000 pounds that operates on U.S. Interstates and other National Highways. This combination is used in the study to compare with alternative truck configurations 1 through 3 below. It is a STAA vehicle meeting current Federal size and weight limitations.
Alternative Truck Configurations with One 53-Foot Semitrailer
- Five-Axle, Tractor-Semitrailer Combination (3-S2), 88,000 pounds: The same vehicle as the Control but loaded to the Gross Manufacturers Weight Rating (GMWR) of 88,000 lbs. This configuration was identified for inclusion at the outset of the 2014 CTSW Study to understand the performance implications of trucks operating at the manufacturers’ gross vehicle weight rating.
- Six-axle, Tractor-Semitrailer Combination (3-S3), 91,000 pounds: This six-axle, 91,000 lb. configuration was selected to evaluate a six-axle truck that complies with the Federal Bridge Formula.*
- Six axle, Tractor-Semitrailer Combination (3-S3), 97,000 lbs.: A tractor-semitrailer configuration with a 3-axle tractor and a 3-axle semitrailer (hence 3-S3) and a GVW of 97,000 lbs. This configuration was selected because of the reference to analyzing the impacts of a six-axle truck in Section 32801 and the weight of 97,000 lbs. was identified due to Congressional interest (HR 612, as introduced in the 113th Congress in 2013).
Control Vehicle for Combinations with More Than One Trailer
- Twin 28.5-foot, 80,000 lbs.: This "standard" configuration is in wide use. Like the Control Vehicle for One Trailer Combinations above, this vehicle is used to provide “baseline” data in the comparative analyses, and is defined as a STAA vehicle that meets current Federal size and weight limitations. (Note: While the control double has an authorized GVW of 80,000 lbs., the actual study is based on a GVW of 71,700 lbs. This GVW is based on actual data collected from weigh-in motion (WIM) equipped weight and inspection facilities and is a more accurate representation of actual vehicle weights than the STAA authorized GVW.)
Alternative Configurations with More than One Semitrailer/Trailer
- Twin 33 foot, 80,000 lbs. (2-S1-2): A configuration with two twin trailers, each 33-foot long and a GVW of 80,000 lbs. This combination was selected because of the strong interest expressed by carriers specializing in Less-Than-Truckload (LTL) shipments. This is the only alternative configuration not currently used in the United States.
- Triple 28.5-foot, 105,000 lbs. (2-S1-2-2): A triple-trailer configuration with three 28.5-foot trailers, seven axles, and a GVW of 105,000 lbs. This combination was selected because of the high level of interest from diverse Stakeholders.
- Triple 28.5-foot, 129,000 lbs. (3-S2-2-2): The triple-trailer configuration with three 28.5-foot trailers and a GVW of 129,000 lbs. It was selected to evaluate the upper GVW limit allowed to operate under the ISTEA Freeze.
* The Bridge Formula established weight limits on vehicle axle groups for different distances between axles and set a maximum GVW of 80,000 pounds. Congress enacted the Bridge Formula to limit the weight-to-length ratio of a vehicle crossing a bridge. This is accomplished either by spreading weight over additional axles or by increasing the distance between axles. return to note
| Scenario | Configuration | Depiction of Vehicle | # Trailers or Semi-trailers | # Axles | Gross Vehicle Weight (pounds) | Roadway Networks |
|---|---|---|---|---|---|---|
| Control Single | 5-axle vehicle tractor,53 foot semitrailer (3-S2) |  |
1 | 5 | 80,000 | STAA 1 vehicle; has broad mobility rights on entire Interstate System and National Network including a significant portion of the NHS |
| 1 | 5-axle vehicle tractor, 53 foot semitrailer (3-S2) |  |
1 | 5 | 88,000 | Same as Above |
| 2 | 6-axle vehicle tractor, 53 foot semitrailer (3-S3) |  |
1 | 6 | 91,000 | Same as Above |
| 3 | 6-axle vehicle tractor, 53 foot semitrailer (3-S3) |  |
1 | 6 | 97,000 | Same as Above |
| Control Double | Tractor plus two 28 or 28 ½ foot trailers (2-S1-2) |  |
2 | 5 | 80,000 maximum allowable weight 71,700 actual weight used for analysis 2 | Same as Above |
| 4 | Tractor plus twin 33 foot trailers (2-S1-2) |  |
2 | 5 | 80,000 | Same as Above |
| 5 | Tractor plus three 28 or 28 ½ foot trailers (2-S1-2-2) |  |
3 | 7 | 105,500 | 74,500 mile roadway system made up of the Interstate System, approved routes in 17 western states allowing triples under ISTEA Freeze and certain four-lane PAS roads on east coast 3 |
| 6 | Tractor plus three 28 or 28 ½ foot trailers (3-S2-2-2) |  |
3 | 9 | 129,000 | Same as Scenario 5 3 |
1 The network is the 1982 Surface Transportation Assistance Act (STAA) Network (National Network or NN) for the 3-S2, semitrailer (53’), 80,000 pound gross vehicle weight (GVW) and the 2-S1-2, semitrailer/trailer (28.5’), 80,000 pound. GVW vehicles. The alternative truck configurations have the same access off the network as its control vehicle. return to Footnote 1
2 The 80,000 pound weight reflects the applicable Federal gross vehicle weight limit; a 71,700 gross vehicle weight was used in the study based on empirical findings generated through an inspection of the weigh-in-motion data used in the study. return to Footnote 2
3 The triple network starts with the network used in the 2000 Comprehensive Truck Size and Weight (CTSW) Study and overlays the 2004 Western Uniformity Scenario Analysis. The LCV frozen network for triples in the Western States was then added to the network. The triple configurations would not have the same off network access as its control vehicle, the 2-S1-2, semitrailer/trailer (28.5’), 80,000 pound GVW. Use of the triple configurations beyond the triple network would be limited to that necessary to reach terminals that are immediately adjacent to the triple network. It is assumed that the triple configurations would be used in Less-Than-Truck Load (LTL) line-haul operations (terminal to terminal). As a result, the 74,454 mile triple network used in this Study includes: 23,993 mile network in the Western States (per the 2004 Western Uniformity Scenario Analysis, Triple Network), 34,802 miles in the Eastern States, and 15,659 miles in Western States that were not on the 2004 Western Uniformity Scenario Analysis, and the Triple Network used in the 2000 Comprehensive Truck Size and Weight Study (2000 CTSW Study). return to Footnote 3
Third, it is assumed that current typical weight/horsepower ratios would be maintained so long as tractors are widely available with sufficient horsepower to maintain the weight/horsepower ratio. A typical over-the-road tractor currently has about 485 horsepower, which results in a weight/horsepower ratio of about 165 for an 80,000 pound vehicle. The largest tractors commonly available have approximately 588 horsepower. Thus weight/horsepower ratios can be maintained or nearly maintained for all scenario vehicles except the triple trailer combinations. The weight/horsepower ratio for a 129,000 pound triple with a 588 horsepower tractor is 219. This could adversely affect triples’ performance relative to the standard twin trailer combinations and is one reason that triples are assumed to be limited to a smaller network of roads than the other scenario vehicles.
Figure 2: Analytical Network for Triples Operation
Fourth, diversion of freight traffic from one truck configuration to another or from one operating weight to another will be limited for the various scenario vehicles. It is assumed that the tractor-semitrailers analyzed in Scenarios 1-3 will attract freight traffic only from five- and six-axle tractor semitrailers. Numerous other truck configurations, including tractor-semitrailers with seven axles or more and various truck-trailer combinations (a straight truck pulling a full trailer behind it), operate within the weight range that might shift to one of the heavier scenario tractor-semitrailers, but many of those vehicles already operate above 80,000 pounds under special permits, and the cargo they carry in those specialized vehicle configurations generally would not be suitable for loading on one of the scenario configurations—otherwise they would have been using that equipment in the first place.
Cargo shifting to one of the multi-trailer combinations analyzed in Scenarios 4-6 is limited to less-than-truckload traffic currently being hauled in five- or six-axle twin trailer combinations or in a five-axle tractor semitrailer. While the additional cubic capacity of the twin 33-foot trailers and the triple trailer combination would be attractive to many carriers, logistical issues with loading, unloading, and maneuvering multi-trailer combinations at origins and destinations are difficult for most shippers to manage and more than offset the benefits of increased cubic capacity. There certainly could be exceptions for certain types of shippers and carriers, but these exceptions are believed to represent a small share of freight that otherwise might be attracted to the multi-trailer combinations.
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