Verification, Refinement, and Applicability of Long-Term Pavement Performance Vehicle Classification Rules

Chapter 8. LTPP Vehicle Classification Rule set Evaluation and Recommended Modifications

In general, the LTPP rule set does an excellent job of classifying vehicles at all of the 18 TPF sites included in this analysis. It is particularly effective in differentiating between passenger vehicles pulling trailers and real trucks. The few errors it does make when trying to differentiate between these vehicles are mostly caused by the inability of current WIM systems to detect differences in the number of tires on rear axles of single-unit trucks. This factor is often the only distinguishing characteristic between some passenger vehicles pulling trailers and light trucks pulling trailers because these vehicles share similar axle spacing characteristics.

Because these vehicles have similar axle spacings, changes to the LTPP rule set are unlikely to improve on the current performance of the rule set because shifts in axle spacing parameters or axle weight parameters will likely create as many errors as they resolve.

For similar reasons, the LTPP rule set, like all current classification rule sets based on axle spacing, has some difficulty correctly classifying buses, and especially "bus-like" recreational vehicles pulling trailers or towing passenger cars. As with passenger vehicles pulling trailers, buses pulling trailers have axle spacings that are similar to other vehicle types, and because buses are as heavy as many trucks, even the use of weight parameters does not allow differentiation of these vehicles. Thus no combination of axle spacings and axle/vehicle weights will consistently separate these vehicles. Consequently, no changes are recommended in the LTPP rule set to more effectively identify these vehicles.

However, the project team believes the following improvements could be made to address three minor limitations in the rule set:

• The Class 7 definition needs to be extended to allow inclusion of Class 7 vehicles with six or seven axles.

• The Class 10 definition needs to be extended to allow inclusion of large double-unit resource hauling vehicles and tractors pulling very heavy low-boy trailers, with seven and eight axles included.

• The Class 13 definition needs to be extended to allow inclusion of Class 13 vehicles with 10, 11, or 12 axles.

Although not all of these vehicle types are present at all TPF test sites, at least some of these vehicles were present at each test site. In addition, all of these vehicles have the potential to be extremely heavy and thus contribute significantly to estimated pavement damage, even if they are not present in large volume. Therefore, it makes sense to extend the LTPP classification rules to account for these vehicles.

In addition, when these rules are placed in the LTPP rule set, it is necessary to make a few modifications to the existing rules to prevent rule overlap.

The following rule definitions are recommended for implementation.

Recommended Class 7 Rules:

Table 31. New Class 7 rules for the LTPP classification rule set.

- Indicates not applicable

GVW = Gross Vehicle Weight

Note that in both of these definitions, the Class 7 vehicle requires closely spaced axles for all but the first and last spacing. The first larger spacing allows for distance between the steering axle and the main load-supporting axles. The extra distance allowed in the final spacing allows for the presence of a drop axle, which is typically offset slightly from the primary load bearing axles.

In addition to these rules, it is recommended that the current definition of the five-axle Class 7 be changed. Two minor changes are recommended in response to comments from the Traffic ETG. The first is an increase in the final axle spacing to allow the definition to capture the longer axle spacing needed for some drop axles. (The last spacing was 2.5 to 6.3 ft. This should be changed to 2.5 to 15.0 ft.) Because the new spacing creates some conflict between this definition and the definition of a Class 5 vehicle pulling a trailer, it is also recommended that the GVW weight limit rule for this Class 7 truck be changed to "more than 20,000 lb." This better differentiates these heavy resource hauler vehicles from other five-axle vehicles. The revised five-axle, Class 7 truck definition is shown in table 32.

Table 32. Revised five-axle Class 7 rule for the LTPP classification rule set.

Bold boxes indicate changes recommended by the Traffic ETG

GVW = Gross Vehicle Weight

Recommended Additional Class 10 Rules

The recommended Class 10 definitions are based on the design of the Washington and Ohio rule sets, but use the current LTPP axle spacing criteria. These rules simply allow for seven-axle Class 10 vehicles with both tandem- and tridem-equipped lead vehicles pulling either tridem or quad axle pup trailers. The two eight-axle definitions allow for tridem- and quad-equipped lead vehicles pulling either tridem or quad axle pup trailers. These rules should be applied before the current Class 13 rules are applied in the LTPP software. (Note that if testing of these rules shows that Class 13 vehicles are being incorrectly classified as Class 10, it is possible to further tighten the allowable axle distances on the second unit of the vehicle to 2.5 to 6.3 ft. This change would help ensure that the trailer axles were all acting as a unit and that the recommended 10.99-ft maximum spacing is not misclassifying an additional unit as part of the Class 10’s trailer. The presence of a third unit would make the measured vehicle a Class 13.)

Table 33 shows the four rules that need to be added to the LTPP WIM classification rule set to correctly identify seven- and eight-axle Class 10 trucks.

Table 33. New Class 10 rules for the LTPP classification rule set.

- Indicates not applicable

GVW = Gross Vehicle Weight

In addition to these rules, the Traffic ETG has requested one modification to an existing Class 10 rule. The current six-axle, Class 10 definition allows for up to 50 ft between the last axle on the first unit and the first axle of the second unit of the vehicle. (That is, axle spacing 3-4 can be from 6.1 to 50 ft.) It was requested that these spacings be changed to 6.3 ft to 45.0 ft. This requested change is adopted as part of this project.

Recommended Additional Class 13 Rules

The Class 13 rules should be applied after the Class 10 rules, so that any vehicle that fits both definitions is defined as a Class 10. The recommendation for the required additional rules is simply that vehicles with 10, 11, and 12 axles be specifically defined as Class 13. The recommended axle spacing values for these additional axles are just extensions of the current allowable limits for axle spacings 4 through 8 in the LTPP rule set. That is, a spacing of between 3 and 45 ft. The recommended new Class 13 definitions are given in table 34. (Note that the format of the Class 13 rules is given vertically so that it fits on the page-unlike the Class 7 and 10 definitions, which are shown horizontally.)

Table 34. New Class 13 rules for the LTPP classification rule set.

GVW = Gross Vehicle Weight