Comprehensive Truck Size and Weight Limits Study: Comparison of Results Report

Chapter 2: Modal Shift Comparative Analysis

2.1 Purpose

In this section, results from the 2014 CTSW Study are compared with results from past truck size and weight studies. The desk scan conducted for the 2014 CTSW Study contain summaries of the studies included in this analysis as well as many other studies that do not lend themselves to a direct comparison of 2014 CTSW Study findings.

Impacts of truck size and weight changes are compared in Modal Shift Comparative Analysis for three areas - shifts in traffic across different truck configurations and between truck and rail; changes in energy consumption and environmental emissions; and potential impacts on highway traffic operations. Differences in findings across the various studies are interpreted in terms of differences in study scope, purpose, geographic scale, scenario vehicles analyzed, networks available to scenario vehicles, and other relevant factors.

2.2 Comparison of Modal Shift Study Findings

The Table 2-1 compares estimates of mode shifts from eight past studies to estimates from the 2014 CTSW Study. Five past studies estimated percentage changes in VMT that can be compared with percentage changes in VMT for the various scenarios examined in the 2014 CTSW Study. The largest percentage change in VMT was estimated by Bienkowski in a Texas study. The study examined three vehicle configurations, a 97,000 pound tractor-semitrailer; a 90,000 pound turnpike double; and a 148,000 pound turnpike double. The study was limited to several corridors in Texas. Interviews with trucking company officials and other trucking industry experts were conducted which led to assumptions that, within the study corridors,

• LCV approval would affect primarily standard 5-axle tractor-semitrailers;
• 15% of current truck cargo currently hauled by 5-axle tractor-semitrailers would remain in this vehicle class;
• 35% would be transferred to the 97-kip tridem axle tractor-semitrailers;
• 20% would be transferred to the light doubles; and,
• The remaining 30% would become the 138-kip double 53s.

Results for each vehicle were not reported separately, but base case VMT in 5-axle tractor-semitrailers in the study corridors was estimated to be reduced by 31 percent due to shifts to the larger, heavier vehicles.

The next highest traffic shift was estimated in the USDOT's 2004 Western Uniformity Scenario Analysis. This study examined impacts of allowing uniform LCV weights and dimensions in the western States that currently allow LCVs. Providing such uniformity within the region was estimated to reduce heavy truck traffic by 25 percent. The various vehicle configurations were generally limited to the Interstate System, but in states that currently allow one or more of those configurations to travel off the Interstate System, that same access was assumed to be retained. There was no breakdown in VMT reductions by vehicle class.

Studies in Wisconsin and Montana estimated much more modest reductions in heavy truck VMT associated with changes in truck size and weight limits. An interesting aspect of the Wisconsin study was that it estimated impacts assuming heavier vehicles were restricted from using the Interstate System as well as impacts if that restriction were removed and the heavy vehicles were allowed to use Interstate highways. As can be seen, restricting the heavier trucks from using the Interstate System had a significant impact on estimate traffic shifts. But, even when allowed to use Interstate highways, the greatest reduction in heavy truck traffic was only slightly more than 1 percent for the 98,000 pound tractor semitrailer. A major reason that traffic shifts were lower than for the Western Uniformity Scenario study is that the Western Uniformity Scenario covered a very large region and changes in weights and dimensions thus could benefit interstate moves as well as intrastate moves. The scenario vehicles in the Western Uniformity Scenario were larger as well.

The final past study that estimated changes in truck VMT is dissimilar from the other studies. McCullough examined the issue of diversion of traffic from rail to truck if truck size and weight limits were increased. He did not assume any specific truck configurations, but rather analyzed impacts associated with a hypothetical 10 percent reduction in trucking costs. This was consistent with the reduction is trucking costs estimated for the LCVs Nationwide Scenario in the 2000 CTSW Study. McCullough estimates that diversion of freight from rail to truck would equal about 7 percent of total long-haul (>200 miles) truck traffic. This would be a much smaller share of total truck traffic which is the base for estimating impacts in the 2014 and 2000 CTSW Studies.

The 2014 CTSW Study estimates much lower reductions in truck VMT associated with the scenarios analyzed than did the 2000 CTSW Study. Among the factors accounting for the differences are:

• Differences in the scenario vehicles - In general, the increases in size and weight limits analyzed in the 2014 CTSW Study were smaller than those analyzed in the 2000 CTSW Study. While the 2000 CTSW Study included heavier tractor-semitrailers such as those analyzed in Scenarios 1-3 in the 2014 CTSW Study, those heavy tractor-semitrailers were coupled with doubles combinations with 33-foot trailers and gross vehicle weights of 124,000 and 131,000 pounds. Whereas reductions in truck VMT for Scenarios 1-3 of the 2014 CTSW Study were on the order of 1-2 percent, reductions in the 2000 CTSW Study for scenarios with the heavy tractor-semitrailers and the heavy twin 33 combinations were about 11 percent. Most of the diversion went to the heavy twins since it was assumed they could travel from origin to destination and would be used for truckload as well as less-than-truckload shipments.
• Differences in the scenario networks - There was a significant difference in assumptions concerning networks available for triples in the 2014 and 2000 CTSW Studies. The 2000 CTSW Study assumed that triples would be granted wide access and would be able to travel from origin to destination. This assumption was based primarily on their ability to make turns which is comparable to a standard tractor-semitrailer. While there was recognition that other factors might affect access decisions, wide access to origins and destinations was assumed for purposes of the study. With wide access, a further assumption was made that triples at 132,000 pounds gross vehicle weight would be attractive to truckload traffic, despite issues with their maneuverability at loading docks and other locations. Assumptions about access and the attractiveness of heavy triples to truckload operators are quite different in the 2014 CTSW Study. With respect to access, triples are assumed to be limited to the Interstate System and other highways on which they currently operate. They may travel approximately 1 mile off that system to access terminals and other points of loading and unloading, but no farther off the designated triples network. These more restrictive access assumptions led to more restrictive assumptions about the type of traffic that would shift to triples. With very restrictive access it was judged that most truckload operators would not find triples an attractive alternative, despite the much higher gross vehicle weight allowed in Scenario 6. Therefore for purposes of the study, use of triples in Scenarios 5 and 6 was limited to less-than-truckload freight. In practice some truckload operators might find ways to make the use of triples economical, but there was no way of estimating the extent and characteristics of usage by truckload operators.
• Differences in analytical tools and data sources - The same basic analytical tools used to estimate modal shifts in the 2000 CTSW Study were also used in the 2014 CTSW Study, but significantly better commodity flow data were available for the 2014 CTSW Study.  Some improvements in the Intermodal Transportation Inventory Cost (ITIC) model were made between 2000 and 2014, but the basic logic remained the same. The Freight Analysis Framework (FAF) commodity flow database that was used in the 2014 CTSW Study had not been developed when the 2000 CTSW Study was being conducted, however. Instead, data collected through truck stop interviews was the primary source of truck flow data. These data did not allow short haul moves to be analyzed using the ITIC model and long-haul truck data were much more limited than data available from the FAF. Additionally, improved data on VMT by truck configuration, operating weight and highway functional class were available for the 2014 CTSW Study than the 2000 CTSW Study.

Another important measure of modal shift is the magnitude of changes in railroad traffic. Different studies have used different measures of railroad impacts including reductions in ton-miles moved, reductions in car-miles, reductions in railroad net income, and reductions in railroad contribution. In addition, assumptions about whether railroads will reduce rates to prevent traffic from shifting to the larger, heavier trucks are inconsistent. The last column of Table 2-1 shows various estimates of rail impacts associated with truck size and weight policy changes. Percentage changes vary from 0.1 percent change in rail contribution for Scenarios 5 and 6 in the 2014 CTSW Study to 60 percent change in ton-miles for shifts to turnpike doubles in the Martland study. The range of estimated rail impacts in the other studies is not as great, but results still vary widely. Assumptions in each study account for much of the variation.

Comparing estimated rail impacts in the 2014 and 2000 CTSW Studies, impacts estimated for scenarios in the 2000 CTSW Study are higher than impacts estimated for the 2014 CTSW Study scenarios. Differences in the scenario definitions and the metrics used to present rail impacts explain a major part of the difference. Scenario vehicles in the 2014 CTSW Study generally have lower gross vehicle weights than vehicles in the 2000 CTSW Study and in the case of triples, access assumptions are much more stringent than in the 2000 CTSW Study. As noted above, the two scenarios from the 2000 CTSW Study that include heavier tractor-semitrailers comparable to those in Scenarios 1-3 of the 2014 CTSW Study also include heavy twin trailer combinations that actually are responsible for most diversion in the 2000 CTSW Study scenarios. The LCVs Nationwide scenario in the 2000 CTSW Study that allowed Rocky Mountain doubles, turnpike doubles, and triples was estimated to have by far the greatest impact on the railroads, but there was no comparable scenario in the 2014 CTSW Study. As noted above, another potential explanation for differences in the rail impacts estimated in the two studies is the metric used to present rail impacts. In the 2014 CTSW Study impacts are expressed in terms of rail contribution which reflects both the lost revenue from diverted traffic, but also the reduced cost associated with shipments that are diverted to truck. The measure used in the 2000 CTSW Study is the change in rail car-miles.

The two studies by Martland included heavy tractor-semitrailers similar to those analyzed in Scenarios 1-3 of the 2014 CTSW Study as well as longer combination vehicles similar to those included in the 2000 CTSW Study. The biggest differences between the Martland studies and the two USDOT studies is that percentage changes in Martland's studies reflect only diversion of traffic for rail-competitive commodities, and Martland does not examine the potential of railroads to reduce rates to keep traffic from diverting. The percentage changes reflected in the USDOT studies are based on a much larger portion of all rail traffic than the Martland studies, and railroads are allowed to reduce their rates to prevent diversion of traffic to trucks. Both of these differences would be expected to show higher rail impacts in the Martland studies than the USDOT studies.

The Western Uniformity Scenario Study estimated significantly less rail diversion than either the 2014 or 2000 CTSW Studies. Two factors may account for this difference. First, the Western Uniformity Scenario was regional in scope while the 2014 and 2000 CTSW Studies were nationwide in scope. Thus long-distance freight moves that had origins or destinations outside the West would not have been able to take advantage of the heavier trucks for the entire trip and would be less likely to divert. Second, one or more LCVs already operate in each of the States included in the Western Scenario and thus some freight that otherwise would have been on the railroads has already diverted to the existing LCVs leaving less traffic subject to further diversion under Western Uniformity Scenario Study assumptions.

McCullough's diversion estimates are based loosely on the reduction in transportation costs estimated for the LCVs Nationwide Scenario in the 2000 CTSW Study, and thus should be compared to estimates of diversion associated with LCVs. His diversion estimate is less than half the estimated diversion for the LCVs Nationwide Scenario in the 2000 CTSW Study. The two studies use different metrics and different methodologies which could account for some of the difference.

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