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

Chapter 3. Findings From Comparison of the State and LTPP Vehicle Classification Rules

This study compared the rules used to differentiate vehicle classes from 10 different classification procedures to the rules developed by the LTPP Traffic ETG for use at the LTPP SPS TPF WIM sites. The examined classification rule sets included four State rule sets (in addition to the LTPP rule set) designed to work only with WIM devices-California Department of Transportation (Caltrans), Florida Department of Transportation, Michigan Department of Transportation, and Washington State Department of Transportation-and six rule sets designed to work with AVC devices but that can also be used as a classification rule set with WIM equipment. The AVC rule sets were supplied by Caltrans, Florida Department of Transportation, Wisconsin Department of Transportation, Missouri Department of Transportation, Ohio Department of Transportation, and Virginia Department of Transportation. This chapter describes the differences in the rules being applied by these alternative set of procedures.

Differences in Vehicle Classification Rules

The following three basic categories of differences were examined in comparing the State and LTPP vehicle classification rule sets:

• Does the State system use axle or gross vehicle weight information (the LTPP classification approach does)-and if so, what are the boundaries?
• Does the State procedure change any of the axle-spacing boundary conditions that separate two classes of vehicles with similar numbers of axles?
• Does the State system include (or exclude) consideration of specific vehicles that are common to that State (and that are not explicitly considered in the LTPP rule set)?

Finally, some State rule sets are designed to be processed in a specific order. This usually means that the rule set has vehicle classification definitions that overlap for two or more vehicle classes. The processing rules are designed so that a vehicle falling into an overlapping category is always assigned to one specific classification. A simple example is that some rule sets assign vehicles with a specific number of axles to a default ^[1] classification whenever the observed axle spacing and weight information do not place that vehicle in a defined classification. (For example, all five-axle vehicles are assigned to Class 9, but only if they do not fit any other possible five-axle category definitions.) Thus, the order of processing might be to try to fit a five-axle vehicle into one of several Class 9 configurations, then a Class 11 configuration, next a Class 7, and then the Class 2 and 3 configurations (i.e., a large pickup pulling a three-axle trailer). Only after the vehicle fails to match the axle configurations defined for these rule sets is the default assignment to Class 9 applied.

The following subsections describe the basic differences observed in comparing the LTPP rules to the other 10-vehicle classification rule sets.

Inclusion of Weight Information

The LTPP classification rules use both gross vehicle weight (GVW) and front axle weights to differentiate between some vehicle classes. This gives the LTPP rules the ability to distinguish among light, medium, and heavy power units. The system is thus good at differentiating between “real trucks,” and cars and light-duty pickups pulling trailers.

Two of the four WIM-based State rule sets use both axle and GVW parameters to differentiate between cars and trucks. The other two WIM rule sets use only GVW in their analysis.

In the case of Washington, both axle weight and GVW are parameters defined in the classification rule set, but GVWs are set to values that prevent them from being used in the classification process. Axle weights are set to values that help differentiate passenger vehicles from trucks.

Other than Washington’s GVW values, the weight values used by all of the WIM rule sets are similar. They all define commercial trucks (Class 4 and up) as above 8,000 lb, with Class 6 and larger trucks required to be heavier than 12,000 lb. This removes most pickup trucks pulling light trailers from the truck classifications and places them in the passenger vehicle categories. In some cases, an upper boundary of 12,000 lb is placed on GVW for Classes 2 and 3 when these vehicles are pulling trailers. If the weight of the trailer, when combined with the weight of the towing vehicle, is heavy enough to exceed this tolerance, the vehicle is placed in one of the true truck classes.

Axle Space Boundary Conditions

It is difficult to summarize the differences among axle configuration parameters for the different vehicle classification rule sets. In many cases, the differences are a matter of only tenths of a foot in the break point between different classes. In other cases, while the basic rule defining a given class of vehicles may be similar from one set of State rules when compared to another (e.g., a large space between axles, followed by a tandem axle, followed by a large axle space), the break points used to define the upper or lower boundaries of those permissible axle spaces can differ by more than 10 ft from one set of rules to another. Simple summaries of the rule sets are shown in appendix A. This section highlights some of the key differences. The next major section of this report discusses the effects of these various differences.

Class 2

Class 2 (without trailer) vehicle definitions are all reasonably similar. Most rule sets require the one axle spacing to fall between 6 and 10 ft. The minimum spacing varies by 0.1 ft from this value in several cases, while the maximum allowed spacing may be up to 10.2 ft.

These base values do not change when a trailer is considered. However, the spacing value allowed between the last car axle and a following trailer’s axle varies considerably-usually from a minimum of 6 ft between the second and third axle (with as much as 8 ft), but with a maximum spacing ranging from 18 to 25 ft. If a two-axle trailer is considered, the spacing on that trailer (the third to fourth axle spacing) ranges from as little as 0.1 ft to as much as 30 ft. Some rule sets define this dual-trailer axle spacing explicitly as a tandem (no more than a 6-ft spacing), while others allowed this trailer to consist of two single axles.

Class 3

The examined classification rule sets have a much higher degree of variation in allowable axle spacing for Class 3 vehicles (without trailer) than for Class 2 vehicles. This is in large part because of the use of GVWs in the LTPP and California WIM rule sets. In both of these systems, the use of a weight restriction (Class 3 vehicle GVW must be less than 8,000 lb, or 12,000 lb if pulling a trailer) means that the classification rules allow vehicles to have a much larger maximum axle 1 to axle 2 spacing and still be considered “light duty, passenger vehicles.” For rule sets without this constraint, Class 3 vehicles generally require the axle 1 to axle 2 spacing to be longer than 10 ft but less than 13 to 14 ft. (Note that even without the outlier cases in the LTPP and California rules, the range of allowable maximum axle spacings is much larger than the range found for the Class 2 definitions.) The LTPP and California WIM rule sets allow up to 23-ft spacings on Class 3 vehicles.

The range of allowable trailer spacings on Class 3 vehicles is generally similar to that for Class 2 vehicles.

Class 4

The most common axle spacings defined for Class 4 vehicles are a minimum of just over 23 ft and a maximum of under 40 ft. However, Washington uses much smaller spacing definitions (20 to 25.5 ft), and Wisconsin also uses 20 ft as the lower boundary while using the more common 40-ft upper boundary. Virginia uses 22 to 32 ft. All examined rule sets allow a third axle and require it to be in a tandem configuration. ^[2] The allowable axle spacing on that tandem is most commonly 0.1 to 6 ft. LTPP requires a minimum 3-ft spacing with up to a 7-ft spacing, values that are slightly larger than those required by the other classifications. (However, few actual buses are likely to have tandem spacings that fall outside of the LTPP range.)

Class 5

The use of GVW to differentiate light from not-light vehicles for Class 5 again allows LTPP to use a very different axle spacing than most of the other rule sets examined. LTPP defines Class 5 vehicles (with no trailer) as having an axle spacing of longer than 6 ft and less than 23.09 ft but also having a GVW of heavier than 12,000 lb and less than 20,000 lb. This is similar to the California WIM rule set’s definition. Rule sets that do not use GVW instead use a much narrower definition of axle spacing for Class 5 vehicles. This generally ranges from a minimum of 13 ft to a maximum of 20 or 23 ft. The effect of using weight in the rules is well illustrated in California, where its WIM algorithm allows a spacing of between 6 and 23 ft, whereas its AVC rule set allows only 14.5 to 23.1 ft. Washington’s definition is 12.5 to 40 ft.

Another significant difference between rule sets is whether Class 5 trucks are allowed to pull trailers. The LTPP, California WIM, Florida, and Missouri rule sets allow Class 5 trucks to pull trailers. The FHWA, Washington, Wisconsin, California AVC, and Ohio rules do not. In these rule sets, Class 5 vehicles pulling trailers are defined as Class 8 vehicles. Interestingly, the Virginia rule set allows Class 5 trucks to pull two-axle trailers but not single-axle trailers. Where trailers are allowed, the axle spacings are quite different, with LTPP allowing the greatest leeway in axle spacings (e.g., from 1 to 20 ft on the last spacing of a two-axle trailer), while other rule sets-such as Florida’s-allow only spacings that reflect tandem axle configurations for the last axle spacing.

As a result of these differences, there is more variability among vehicles that are classified within the Class 5 category than within Classes 3, 4, and 8. These large differences are illustrated later in this report. It also means that the use of different classification rules can produce large changes in Class 5 truck volume estimates.

Class 6

The Class 6 truck definition is among the most homogeneous of the definitions. All rule sets assume a single-tandem configuration. Most require the first spacing to be longer than 6 ft and less than 23.1 ft. However, Washington uses longer than 11 ft and less than 40 ft, and Virginia has two Class 6 rules, one of which allows a first spacing of longer than 22 ft and less than 32 ft.

Class 7

The FHWA Class 7 definition is for four-or-more-axle vehicles. The four-axle definitions of the various rule sets are reasonably similar, with most requiring a first spacing of longer than 6 ft and less than 23.1 ft, followed by a tandem spacing, and then a third/fourth axle spacing of up to 13 ft. However, there are a number of variations on this last axle spacing. For example, the Florida WIM rule set allows the final axle spacing to range from 0.1 to 13 ft, while the Florida AVC rule set requires the spacing to be no longer than 6 ft. Essentially, the WIM rules allow a spacing consistent with the use of a drop axle, while the AVC rules require a more conventional tandem axle spacing.

However, the major differences among classification rule sets for this FHWA vehicle category are in the number of axles allowed. The LTPP rule set allows both four- and five-axle vehicles. It does not allow six- or seven-axle vehicles. The California, Florida, and Missouri rule sets do not even allow five-axle Class 7 trucks. Virginia, Ohio, and Washington allow not only four- and five-axle trucks but also six- and seven-axle Class 7 trucks. Wisconsin allows five-axle trucks but no larger. As described in the next section, Specific Vehicle Configurations Considered, data from every site examined included six-axle Class 7 trucks. Most did not include seven-axle Class 7 trucks.

Class 8

Class 8 trucks can exist in both three- and four-axle configurations. The most common configurations of these vehicles are referred to in old nomenclature as 2S1 or 3S1 vehicles (two- and three-axle tractors pulling a single-axle semi-trailer). For most rule sets, attempts have been made to define the specific axle spacing distances found in these configurations. The most common of these (for the 2S1) assumes a first axle spacing of between 6 and 23 ft, with a second axle spacing of 11 to 40 ft. The four-axle truck is then 6 to 23 ft, followed by a tandem axle (3 to 6 ft), followed by the final axle (again 11 to 40 ft).

However, a couple of States, for example Washington, have given Class 8 very broad latitude in axle spacing so that all three-axle vehicles that do not fall into one of the previous three-axle categories falls into Class 8 by default. By not allowing a Class 5 truck pulling a trailer to be classified as a Class 5, and by limiting the distance of the first axle spacing for Class 2 and 3 vehicles pulling trailers, this State rule set assumes that everything with three axles that is not obviously a car pulling a trailer is Class 8. This philosophy may or may not work (it was not tested against ground truth in this project), but it certainly results in very different axle spacing rules than those of the LTPP rules.

This same situation occurs with four-axle Class 8 vehicles. In both cases, the Washington rule set relies on the order in which rule processing occurs to ensure the approach functions as desired. That is, a single-tridem configuration fits both the Washington Class 7 and Class 8 rule definitions. It is correctly classified as a Class 7 and will continue to be classified as such as long as the Class 7 rules are applied before the Class 8 rules.

LTPP also has a number of overlaps in its axle spacing rules; however, it relies on the different weight allowances (particularly the front axle weight for the Class 8 definition) to differentiate between light vehicles pulling trailers and larger trucks pulling trailers. In general, the LTPP mechanism is superior to the other mechanisms, but it cannot be applied when AVC equipment is used, i.e., AVC counts must be conducted with a classification rule set that is different than that used with the WIM equipment.

Class 9

Class 9 trucks come in three primary configurations: 3S2 (single-tandem-tandem), 2S3 (single-single-tridem), and 3-2 (three-axle truck pulling a two-axle trailer, usually single-tandem-single-single). This last axle configuration also applies to a three-axle tractor pulling a semi-trailer with a split tandem. The single-tandem-single-single configuration is relatively uncommon, except in certain regions where this configuration of trucks carries a specific commodity (e.g., coiled steel in Ohio). For example, they were present in large numbers at one of the two Arizona SPS TPF sites.

The differences in definitions of allowable Class 9 spacings are similar to those of Class 8. Most rule sets use fairly standard axle spacing definitions for Class 9 vehicles, while one or two (most notably Washington and Virginia) use very broad spacing definitions combined with the application of more constrained spacing definitions for smaller vehicles. In the case of Washington and Virginia, Class 9 is essentially treated as the default five-axle vehicle classification. That is, if the axle spacings observed do not fit the tightly defined criteria for other vehicle categories (e.g., the single-quad definition for five-axle Class 7 trucks), and if they include two tandem axles(where one tandem can be a split tandem), then the five-axle vehicle is assigned to Class 9.

However, the majority of Class 9 rule sets attempt to define the spacings allowable for the basic truck configurations identified above. The most common State rules for identifying a 3S2 vehicle allow a first axle spacing of between 6 and 26 ft (LTPP allows a maximum of 30 ft), with the upper boundary of the various State rules ranging from 24 to 32 ft (except the Washington rules).

State rule sets define the second axle spacing as a tandem axle, with an allowable maximum value of about 6 ft (LTPP uses 6.3 ft). This axle group is assumed to be followed by a long spacing representing the length of the trailer. This third spacing is generally required to be longer than 6 ft and less than 40 ft. Finally, another tandem axle spacing is defined, although this tandem axle spacing is allowed to be larger than the spacing between axles two and three because this axle can be a split tandem. Most State rule sets allow this spacing to be shorter than 11 ft, with LTPP allowing 12 ft.

A second set of spacing rules is normally used to define the 3-2 vehicle configuration. The major difference between most of the 3-2 rule sets and the 3S2 rule set described above is that for the 3-2 vehicle, the last axle spacing is generally defined as longer than 11 feet and shorter than 27 ft (12 to 27 ft for LTPP), rather than being the shorter tandem spacing. Washington and Virginia do not differentiate between 3S2 and 3-2 configurations in their rules, but the broader axle ranges these States use capture both styles of vehicles within FHWA Class 9.

LTPP, Ohio, Washington, and Virginia directly define the 2S3 truck. While this is one of the most common European heavy haul trucks, it is uncommon in the United States. Therefore, it is not surprising that most State rules do not directly identify it.

Class 10

For Class 10, as for Class 7, the more limited definition that LTPP uses results in what appears to be misclassification of a substantial number of vehicles. LTPP provides one-axle spacing rule set for Class 10 vehicles. This definition assumes a lead vehicle with a single-tandem axle configuration pulling a trailer with either a single-tandem or tridem axle configuration. This appears to miss many of the common dual-unit, heavy resource-hauling vehicle configurations. The LTPP rule set is similar to that used in California, but many State rule sets do define additional Class 10 vehicle configurations.

For example, Florida and Missouri have rules that are similar to LTPP’s, but both State rule sets allow smaller axle spacings (less than 6 ft) between the third and fourth axles. This shorter axle spacing allows identification of lead vehicles with four axles (single-tridem configurations) that are pulling multi-axle trailers as Class 10 vehicles. These States also allow classification of seven-axle vehicles as Class 10. In much of the western United States, the most common of these is the four-axle truck pulling a three-axle trailer. In the eastern United States, these trucks are often a three-axle tractor pulling a quad-equipped low-boy trailer. The Ohio and Washington rule sets are designed to identify not only these vehicles but also eight-axle configurations (a four-axle lead vehicle pulling a quad-axle pup trailer or a full tandem-tandem trailer).

To correctly differentiate these Class 10 vehicles from Class 13 vehicles with the same number of axles, both Ohio and Washington include several very specific rule sets that essentially look for consecutive tridem and quad-axle configurations separated by a large space. Vehicles with seven or more axles that have more than one tandem axle group are assigned to Class 13. However, the definitions of tridem and quad axle spacing used in Ohio and Washington are generous-consecutive axles are allowed to be up to 9 or 10 ft apart to account for distances frequently found with drop axles on these vehicles. Because these rules are processed before the Class 13 rules are applied, these Class 10 vehicles are identified before the default rules classify seven-or-more-axle vehicles into Class 13.

Class 11

All of the State rule sets for Class 11 vehicles have very similar definitions, except-once again-for Washington.^[3] All of the rule sets define the first axle spacing as longer than 6 ft and generally less than 26 ft. The allowable upper boundary ranges between 17 and 30 ft; LTPP has selected a 30-ft upper boundary. The second spacing must exceed 11 ft and be less than 26 ft. (The upper boundary again varies slightly from 25 to 30 ft, with LTPP using 26 ft.) The third spacing must exceed 6 ft and be less than 20 ft (with the upper boundary ranging from 18 to 20 ft, the LTPP selected value). The final axle spacing must exceed 11 ft and be less than 26 ft (with the upper boundary ranging from 25 to 30 ft; LTPP uses 26 ft).

Class 12

Class 12 is defined very similarly to Class 11 in all of the State rule sets. The major difference is that the second and third axles are expected to be a tandem, with all of the remaining spacings definitions moved to the next pair. (That is, what was defined as the allowable spacing between the second and third axles is now the allowable spacing between the third and fourth axles.) The only other difference is that some rule sets slightly adjust the allowable spacing between the fourth and fifth axles (what was between the third and fourth axles). This spacing measurement represents the space between the semi-trailer and the full trailer. For the LTPP, California, and Florida rule sets, the maximum spacing here increases from 20 to 24 ft. For Ohio, this value shrinks from 20 to 18 ft, while it remains at 18 ft for the other State rule sets (except, again, Washington).

Class 13

Class 13 is treated in most of the State rule sets as the default classification category for all seven- and eight-axle trucks. Most State rule sets define the first spacing as longer than 6 ft and less than some large value (ranging from 23 to 45 ft). This same basic rule is then repeated for each successive axle spacing, with a minimum distance of between 1 and 3 ft and a maximum distance of between 26 and 45 ft. The Virginia and Ohio rule sets are the primary exceptions to this approach. Both of these States define a series of Class 13 rule sets that attempt to identify specific tandem axle spacings, usually between the second and third axles and then either the fourth and fifth axles or the sixth and seventh axles. Only Virginia has specifically identified a vehicle where the fourth, fifth, and sixth axles form a tridem-the configuration of the Canadian “B-train,” multi-unit configuration. However, these vehicles will be correctly classified by the broad definitions most State rule sets use for Class 13.

¹ Some classification rule sets refer to these defaults as "forcing" a vehicle of a specific number of axles into a specific vehicle class when it cannot otherwise be classified.

² This means that none of these rule sets will correctly classify an articulated urban transit bus. However, most vehicle classification and WIM devices are not intended to be used in locations where such buseswould normally operate.

³ The Washington measures are sufficiently different to not be included in this discussion. The Washington Class 11 rules basically define five-axle trucks that do not include tandem axles between the second and third axles and between the fourth and fifth axles, or that contain triple or quad axles, as Class 11, regardless of their axle spacing.