Verification, Refinement, and Applicability of Long-Term Pavement Performance Vehicle Classification Rules
FHWA developed a standardized vehicle classification system in the mid-1980s. This system was the result of compromises designed to meet the needs of many traffic data users. Pavement designers were an important segment of those users but by no means the only intended audience. Another segment of key users comprised the safety community, which was (and still is) highly interested in the amount of travel occurring in multi-unit vehicles (that is, power units of various types pulling trailers of various configurations).
In addition to these needs was the requirement that the electronic equipment and sensors available at the time (mostly simple road tubes) be able to differentiate passing vehicles into the desired classifications. Available sensors were capable of measuring the presence of vehicles, detecting axles, and determining the distance between consecutive axles on the basis of the speed of each vehicle as it passed over the sensors.
The result of that 1980-era work is the FHWA 13-category classification rule set currently used for most Federal reporting requirements and that serves as the basis for most State vehicle classification counting efforts. The FHWA classification system is shown in table 1.
Table 1. FHWA vehicle classification definitions.
Class Group
Class Definition
Class Includes
Number of Axles
1
Motorcycles
Motorcycles
2
2
Passenger Cars
All cars
Cars with one-axle trailers
Cars with two-axle trailers
2, 3, or 4
3
Other Two-Axle Four-Tire Single-Unit Vehicles
Pick-ups and vans
Pick-ups and vans with one- and two- axle trailers
2, 3, or 4
4
Buses
Two- and three-axle buses
2 or 3
5
Two-Axle, Six-Tire, Single-Unit Trucks
Two-axle trucks
2
6
Three-Axle Single-Unit Trucks
Three-axle trucks
Three-axle tractors without trailers
3
7
Four or More Axle Single-Unit Trucks
Four-, five-, six- and seven-axle single-unit trucks
4 or more
8
Four or Fewer Axle Single-Trailer Trucks
Two-axle trucks pulling one- and two-axle trailers
Two-axle tractors pulling one- and two-axle trailers
Three-axle tractors pulling one-axle trailers
3 or 4
9
Five-Axle Single-Trailer Trucks
Two-axle tractors pulling three-axle trailers
Three-axle tractors pulling two-axle trailers
Three-axle trucks pulling two-axle trailers
5
10
Six or More Axle Single-Trailer Trucks
Multiple configurations
6 or more
11
Five or Fewer Axle Multi-Trailer Trucks
Multiple configurations
4 or 5
12
Six-Axle Multi-Trailer Trucks
Multiple configurations
6
13
Seven or More Axle Multi-Trailer Trucks
Multiple configurations
7 or more
14
Unused
----
----
15
Unclassified Vehicle
Multiple configurations
2 or more
---- Indicates not applicable
As part of the development and adoption of this 13-category system, John Wyman of the Maine Department of Transportation produced an initial rule set (commonly called Scheme F) to convert the axle spacing information available from axle sensing data collection equipment into estimates of the number of vehicles in each of the 13 FHWA vehicle categories. This initial rule set has been revised many times by many different individuals, companies, and agencies. These revisions are designed to deal with two major factors:
1)The FHWA definitions are based on vehicle characteristics that can be easily identified visually but that cannot be perfectly computed based on the basis of the number, weight, and spacing of axles.
This problem is exacerbated by the following fact:
2)Truck characteristics may change significantly from State to State because vehicle owners and manufacturers build and optimize vehicles to maximize their profit-generating potential, which depends on the truck size and weight laws in each State.
The first of these problems is illustrated in figure 1 and figure 2. The two pickup trucks shown have the same number of axles and similar axle spacing. However, because the pickup truck in figure 1 has a conventional (two-tire) rear axle, it is defined as a Class 3, whereas because the truck in figure 2 has dual tires on each side of its (four-tire) rear axle, it is defined as a Class 5. When empty, these trucks weigh essentially the same. Therefore, correctly classifying them is problematic no matter which State’s WIM or automatic vehicle classification (AVC) rule set is used. (Please note that the following four photos were taken with a camera associated with the data collection device. The vehicles were moving at about 60 mi/h, which accounts for the blurring.)
Figure1. Photo. Class 3 vehicle.
Figure 2. Photo. Similar Class 5 vehicle.
In another example, vehicles with very different weight characteristics have similar axle spacings. This can be seen when larger pickups, such as that shown in figure 1, pull utility trailers. (The FHWA rule set would still classify it as a Class 3 vehicle.) These vehicles can have axle spacings that are similar to those of a conventional, two-axle truck pulling a heavy, single-axle trailer, a vehicle classified as a Class 8. Examples of these two configurations are shown in figure 3 and figure 4. Straight trucks such as that shown in figure 4 often have axle spacings that are similar to those of the pickup shown in figure 3.
Figure 3. Photo. Class 3 light truck pulling trailer.
Figure 4. Photo. Class 8 truck pulling trailer.
In the figure 3 and figure 4 example, the WIM-based classification rule sets that use axle weights as part of their classification algorithm can correctly classify both the pickup and trailer and the truck with trailer because the heavier engine on the conventional truck increases the weight of that configuration to the point at which it can be routinely differentiated from larger pickup trucks. However, no conventional vehicle classifier (which does not have access to axle weight information) can differentiate between those two vehicles. The rules place both in the same vehicle category. As a result, one of these trucks will be correctly classified; the other will be misclassified. Which one is correctly classified will be determined by the “break points” that are selected in the axle spacing parameters adopted for use in the rules used to define the vehicle classifications. (For example, adopting a break point of 13 ft between Class 3 and Class 5 will place both trucks shown in figure 3 in Class 5 if they have similar axle spacing of 13.3 ft. However, if the break point is selected as 13.4 ft, both would be classified as Class 3. In each case, one truck would be misclassified.)
Because many trucks share similar, but not exactly the same, axle spacing characteristics, carefully selecting the axle spacing break points between classifications with similar axle spacings can greatly reduce the number of misclassified vehicles.
Because truck characteristics often change from State to State in response to differing size and weight laws, many State transportation departments optimize their classification rules by shifting their break points to more effectively reflect the realities of the truck configurations commonly found in their State. If those configurations are not routinely found in another State, the application of the first State’s classification system may not perform as desired in the second State.
For similar reasons, many State transportation departments add rules to help detect and monitor specific vehicles that are important (either politically or for technical reasons) to that State. For example, Oregon law allows triple-trailer trucks (a tractor pulling one semi-trailer and two full trailers). The number and use of these vehicles is a politically sensitive topic, so Oregon’s classification rules track them. When the Oregon Department of Transportation submits data to FHWA, it aggregates these trucks into Class 13 along with other seven-axle or larger multi-unit configurations. In Washington, these trucks are illegal and do not operate in the State. Consequently, they are not a category identified by the Washington classification rules.
Finally, differences in the capabilities of traffic data collection equipment can result in differences in the parameters used to determine vehicle classification. The most significant difference between various classification systems is the use (or lack of availability) of weight data. For conventional vehicle classifiers (i.e., those pieces of equipment that only obtain data on the number and spacing of axles), classification can only be determined based on the number and spacing of axles. However, if the data traffic collection system is a WIM scale, it is possible to use both axle spacing and axle weight (or gross vehicle weight) data to classify a passing vehicle.
The result of these differences is that the same vehicle can be classified very differently by two different pieces of equipment. When a State takes advantage of the availability of axle weight information to apply a more accurate classification system in its WIM scales than is possible in its less capable vehicle classifiers, that State will create a situation where a given vehicle will be classified differently depending on which piece of data collection equipment observes that vehicle.
To illustrate the variety of algorithms that can be used by State transportation departments, appendix A contains examples of different classification rules used by a variety of States.
In 2003, the Traffic ETG of the LTPP project developed a new set of rules for classifying vehicles based on sensor outputs available from WIM systems. In 2006, the LTPP project adopted the Traffic ETG recommendation that this rule set be used at SPS TPF WIM scale sites in those States that were willing to adopt those rules.
The LTPP rule set is designed for WIM scales. It uses a combination of four variables to classify each vehicle:
Not all variables are used to define each class of vehicle.
The LTPP classification rule set was originally designed so that there was no overlap between defined vehicles. (In some State classification rule sets, two vehicle classes can have the same characteristics. In these cases, a specific order is used when processing the classification rules so that vehicles that fit within the overlapping classification definition are consistently placed in one of the two classes.) Out of necessity, this was changed when some additional classification rules were defined for very large trucks. In addition, the LTPP rules allow Class 5 vehicles to pull a trailer while the FHWA visually based rule set classifies these vehicles as Class 8.
The initial LTPP classification rules deployed in the field as part of the SPS TPF WIM study are shown in table 2 on the following page. Differences between the LTPP classification rules and the State rules examined for this project are described in the following chapter of this report.
Table 2. LTPP classification rules for SPS WIM sites (adopted March 2006 by Traffic ETG).
Class
Vehicle Type
No. of Axles
Spacing
Between
Axles 1 and 2 (ft)
Spacing
Between
Axles 2 and 3 (ft)
Spacing
Between
Axles 3 and 4 (ft)
Spacing
Between
Axles 4 and 5 (ft)
Spacing
Between
Axles 5 and 6 (ft)
Spacing
Between
Axles 6 and 7 (ft)
Spacing
Between
Axles 7 and 8 (ft)
Spacing
Between
Axles 8 and 9 (ft)
Gross Weight Min-Max
(Kips)
Axle 1 Weight Min
(Kips)1
1
Motorcycle
2
1.00-5.99
----
----
----
----
----
----
----
0.10-3.00
----
2
Passenger Car
6.00-10.10
----
----
----
----
----
----
----
1.00-7.99
----
3
Other (Pickup/Van)
10.11-23.09
----
----
----
----
----
----
----
1.00-7.99
----
4
Bus
23.10-40.00
----
----
----
----
----
----
----
12.00 >
----
5
2D Single Unit
6.00-23.09
----
----
----
----
----
----
----
8.00 >
2.5
2
Car with 1 Axle Trailer
3
6.00-10.10
6.00-25.00
----
----
----
----
----
----
1.00-11.99
----
3
Other with 1-Axle Trailer
10.11-23.09
6.00-25.00
----
----
----
----
----
----
1.00-11.99
----
4
Bus
23.10-40.00
3.00-7.00
----
----
----
----
----
----
20.00 >
----
5
2D with 1-Axle Trailer
6.00-23.09
6.30-30.00
----
----
----
----
----
----
12.00-19.99
2.5
6
3-Axle Single Unit
6.00-23.09
2.50-6.29
----
----
----
----
----
----
12.00 >
3.5
8
Semi, 2S1
6.00-23.09
11.00-45.00
----
----
----
----
----
----
20.00 >
3.5
2
Car with 2-Axle Trailer
4
6.00-10.10
6.00-30.00
1.00-11.99
----
----
----
----
----
1.00-11.99
----
3
Other with 2-Axle Trailer
10.11-23.09
6.00-30.00
1.00-11.99
----
----
----
----
----
1.00-11.99
----
5
2D with 2-Axle Trailer
6.00-26.00
6.30-40.00
1.00-20.00
----
----
----
----
----
12.00-19.99
2.5
7
4-Axle Single Unit
6.00-23.09
2.50-6.29
2.50-12.99
----
----
----
----
----
12.00 >
3.5
8
Semi, 3S1
6.00-26.00
2.50-6.29
13.00-50.00
----
----
----
----
----
20.00 >
5.0
8
Semi, 2S2
6.00-26.00
8.00-45.00
2.50-20.00
----
----
----
----
----
20.00 >
3.5
3
Other with 3-Axle Trailer
5
10.11-23.09
6.00-25.00
1.00-11.99
1.00-11.99
----
----
----
----
1.00-11.99
----
5
2D with 3 Axle Trailer
6.00-23.09
6.30-35.00
1.00-25.00
1.00-11.99
----
----
----
----
12.00-19.99
2.5
7
5-Axle Single Unit
6.00-23.09
2.50-6.29
2.50-6.29
2.50-6.30
----
----
----
----
12.00 >
3.5
9
Semi, 3S2
6.00-30.00
2.50-6.29
6.30-65.00
2.50-11.99
----
----
----
----
20.00 >
5.0
9
Truck+Full Trailer
(3-2)
6.00-30.00
2.50-6.29
6.30-50.00
12.00-27.00
----
----
----
----
20.00>
3.5
9
Semi, 2S3
6.00-30.00
16.00-45.00
2.50-6.30
2.50-6.30
----
----
----
----
20.00 >
3.5
11
Semi+Full Trailer, 2S12
6.00-30.00
11.00-26.00
6.00-20.00
11.00-26.00
----
----
----
----
20.00 >
3.5
10
Semi, 3S3
6
6.00-26.00
2.50-6.30
6.10-50.00
2.50-11.99
2.50-10.99
----
----
----
20.00 >
5.0
12
Semi+Full Trailer, 3S12
6.00-26.00
2.50-6.30
11.00-26.00
6.00-24.00
11.00-26.00
----
----
----
20.00 >
5.0
13
7-Axle Multi-trailers
7
6.00-45.00
3.00-45.00
3.00-45.00
3.00-45.00
3.00-45.00
3.00-45.00
----
----
20.00 >
5.0
13
8-Axle Multi-trailers
8
6.00-45.00
3.00-45.00
3.00-45.00
3.00-45.00
3.00-45.00
3.00-45.00
3.00-45.00
----
20.00 >
5.0
13
9-Axle Multi-trailer
9
6.00-45.00
3.00-45.00
3.00-45.00
3.00-45.00
3.00-45.00
3.00-45.00
3.00-45.00
3.00-45.00
20.00 >
5.0
1Suggested Axle 1 minimum weight threshold if allowed by WIM system’s class algorithm programming
---- Indicates not applicable
Min =Minimum
Max = Maximum