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This report is an archived publication and may contain dated technical, contact, and link information
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Publication Number:  FHWA-HRT-13-091    Date:  November 2014
Publication Number: FHWA-HRT-13-091
Date: November 2014

 

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

Specific Vehicle Configuration Considerations

Many States have State-specific vehicle categories, such as Oregon’s triple trailer vehicles. Michigan tracks a number of very large, very heavy vehicles because the State allows these vehicles to meet the needs of the heavy industry located there. In most cases, LTPP does not need to include these definitions in its Class ification system because these vehicles are not commonly found in many other States.

However, several vehicle types do appear in more than one of the examined Class ification rule sets that are not explicitly defined in the LTPP rules. The LTPP rule set does not always correctly Class ify these vehicles. They are generally present in modest numbers in some States but not present at all (or only rarely) in others.

In all cases, the “missing vehicles” that are found in multiple States but not Class ified in the LTPP rules are large, multi-axle vehicle configurations. Most of them are associated with hauling heavy natural resources. One category of these vehicles falls into FHWA Class 7. The other falls into Class 10.

The LTPP rule set defines two Class 7 configurations, one with four axles and one with five axles (a single and a quad). The Ohio, Virginia, and Washington rule sets all define larger single-unit trucks. In all three cases, six-axle Class 7 trucks are allowed. In Ohio and Virginia, seven-axle Class 7 trucks are defined.

An analysis of eight of the LTPP TPF data sets indicated that six-axle Class 7 trucks exist in all eight test sites. Given the Washington WIM definition of Class 7, these vehicles make up slightly more than 20 percent of the Class 7 vehicle mix at both Arizona sites, while in Pennsylvania and Tennessee (States with very large numbers of five-axle single-unit trucks), they make up less than 1 percent of Class 7 vehicles.

At none of the test sites examined were obvious seven-axle (single-unit) Class 7 vehicles identified. At several sites, the Washington rule set identified a number of seven-axle vehicles as Class 10 vehicles that may have been Class 7 vehicles, but in which at least one-axle spacing in addition to the first spacing was greater than 12 ft. This separation generally indicates the space between two independent vehicle units but can also be the distance between an axle group and an extendable drop axle at the end of a trailer.

Given these findings, it appears that the LTPP rule set should be revised to at least allow six-axle Class 7 vehicles, and probably seven-axle single-unit vehicles as well.

The next significant vehicle category missing from the LTPP rule set is Class 10 vehicles that have more than six axles. Five State rule sets (Washington, Wisconsin, Virginia, Ohio, and Florida) define seven-axle Class 10 vehicles. Washington actually defines four additional styles of eight-axle Class 10 vehicles. These vehicles are generally large dump trucks pulling multi-axle pup trailers, such as the one shown in figure 5. They can also be large fuel tankers pulling full trailers. In other States, these are large, heavy duty tractors pulling multi-axle low-boy trailers. The LTPP rule set currently Class ifies these vehicles as Class 13 vehicles.

Figure 5. Photo. Seven-axle Class 10 truck. This photo shows a large seven-axle, Class 10 truck. It is a Single–Tridem configured dump truck pulling a tridem axle equipped dump trailer

Figure 5. Photo. Seven-axle Class 10 truck.

As with several other large vehicle types, these vehicles are common in some States but rare in others. An examination of the 18 LTPP TPF sites that use the LTPP Class ification rule set and for which data were readily available, found these vehicles in all States. At 14 of these 18 TPF test sites, more than half of the vehicles Class ified as Class 13 trucks shifted into Class 10 when the Washington algorithm was applied to the W-cards. [4] Much smaller percentages of vehicles shifted from Class 13 to Class 10 when the Wisconsin and Florida algorithms were used. The main reason for these differences is that while both the Florida and Wisconsin rules define the seven-axle vehicle as a three-axle truck pulling a four-axle trailer, the Washington rules allow both three- and four-axle trucks pulling both three- and four-axle pup trailers.

A review of the specific axle configurations of vehicles from all States showed that while eight-axle Class 10 trucks are found at only a subset of the TPF sites (and presumably in a subset of states), both three- and four-axle dump trucks are routinely found. Table 3 presents examples of the typical axle spacings found for these vehicles. (These specific examples were taken from the Pennsylvania SPS 6 site.) All four examples represent trucks identified as Class 13 vehicles under the LTPP rules (but as Class 10 under the Washington rules).

Note that the truck described in the first row of table 3 is most likely a Class 10 but may in fact be some type of Class 13. The project team believes that the first vehicle is a four-axle truck (single-tridem configuration) pulling a trailer with a tridem and a drop axle. However, it is possible, but unlikely, that this a three-unit vehicle with a four-axle truck pulling two trailers, where the last trailer is a single-axle semi-trailer. The second vehicle shown in table 3 is a four-axle truck (single-tridem) or a heavy-duty tractor pulling a trailer with a quad axle. (This is essentially the vehicle shown in the picture in figure 5, except with a quad-axle pup trailer instead of the tridem-axle pup trailer shown.) The third vehicle in table 3 is a Class ic three-axle dump truck (single-tandem) pulling a quad-axle pup trailer, but could also be a three-axle tractor pulling a quad equipped low-boy trailer. The fourth vehicle is a four-axle truck pulling a three-axle pup trailer.

Table 3. Example axle spacings for misClass ified Class 10 vehicles at the Pennsylvania SPS-6 site.

Number of Axles

Axle
1-2 Spacing

Axle
2-3 Spacing

Axle
3-4 Spacing

Axle
4-5 Spacing

Axle
5-6 Spacing

Axle
6-7 Spacing

Axle
7-8 Spacing

8

15.6

4.2

4

35

4

4

14

8

16.4

3.8

5

33

4

4

4

7

13

4.3

31

4

4

4

-

7

15.5

4.4

4

36

5

5

-

- Indicates not applicable

Without doing an extensive study of State-specific axle configurations, it is logical to suggest that some subset of these particular vehicle configurations exists in each State, but many States may not have all of these vehicle types. Given these findings, the project team concludes that the LTPP Class ification rule set should be revised to more effectively differentiate between Class 10 and Class 13 vehicles with seven and eight axles.

The last missing State-specific vehicle category observed in this analysis is Class 13 vehicles with more than nine axles. These have been present at all 18 of the analyzed TPF sites, although they are not extremely common and are quite rare at several of these sites. However, importantly, these vehicles are often extremely heavy (greater than 150,000 lb) because they are typically special permit loads. If these vehicles are considered unClass ified, their very heavy tridem and quad axles are not included in the Class 13 load spectra. Thus, it is important that these vehicles be Class ified and included in the LTPP rule set, even if they do not represent a large volume of vehicles.

The specific axle spacing rules to be included in the LTPP Class ification rules to accomplish these additions are described in the Conclusions section of this report.

The Effects of Different Classification Rule sets on Traffic Loading Parameters

This section discusses the size and scope of differences in traffic volume estimates, by vehicle Class , that result from the application of different vehicle Class ification rules. The analysis was performed by taking W-card data from 18 LTPP TPF sites-each of which originally collected data under the LTPP Class ification rule set-and reClass ifying the vehicles described in those records by using seven different State rule sets. Computational procedures were written that allowed Class ification rule sets such as those shown in appendix A to be entered along with W-card records from the TPF sites. These procedures were then used to compute new records containing both the original LTPP vehicle Class es and the vehicle Class es defined by using each new rule set.

The State rule sets applied in this test were the following:

Changes in Truck Volumes by Class

The researchers applied simple cross tabulations to summarize the effects of using different Class ification rules on the total number of vehicles that were assigned to any given FHWA vehicle Class as a result of using any given rule set. An example of these cross tabulations is shown in table 4.

Table 4. Example cross tabulation of LTPP Class rule set versus Washington WIM rule set (Tennessee SPS-6 data).

LTPP Class /New Class

New Class 1

New Class 2

New Class 3

New Class 4

New Class 5

New Class 6

New Class 7

New Class 8

New Class 9

New Class 10

New Class 11

New Class 12

New Class 13

New Class 15

Total

LTPP Class 1

12,523

121

-

-

-

-

-

-

-

-

-

-

-

-

12,644

LTPP Class 2

812

1,002,540

393,488

-

209

-

-

-

-

-

-

-

-

-

1,397,049

LTPP Class 3

-

-

400,865

-

36,535

-

-

9,385

175

-

1

-

-

9

446,970

LTPP Class 4

-

-

-

6,804

1,742

5,302

-

2

-

-

-

-

-

-

13,850

LTPP Class 5

-

333

19,282

7,239

62,708

-

-

23,837

1,402

-

6

-

-

40

114,847

LTPP Class 6

-

-

99

3,712

-

18,527

-

52

-

-

-

-

-

-

22,390

LTPP Class 7

-

-

51

-

-

-

19,511

-

-

-

-

-

-

3

19,565

LTPP Class 8

-

-

951

-

-

-

22

38,968

-

-

-

-

-

2

39943

LTPP Class 9

-

-

 

-

-

-

-

-

13,56,356

-

1,071

-

-

26

1,357,453

LTPP Class 10

-

-

-

-

-

-

-

-

-

10,587

-

-

-

-

10,587

LTPP Class 11

-

-

-

-

-

-

-

-

-

-

101,448

-

-

-

101,448

LTPP Class 12

-

-

-

-

-

-

-

-

-

25

-

37,952

-

-

37,977

LTPP Class 13

-

-

-

-

-

-

-

-

-

2,018

-

-

340

-

2,358

LTPP Class 15

629

1,401

1,716

9

260

733

101

1,404

6,413

838

994

546

463

1,920

17,427

Total without Class 15

13,335

1,002,994

814,736

17,755

101,194

23,829

19,533

72,244

1,357,933

12,630

102,526

37,952

340

80

3,577,081

Total with Class 15

13,964

1,004,395

816,452

17,764

101,454

24,562

19,634

73,648

1,364,346

13,468

103,520

38,498

803

2,000

3,594,508

- Indicates not applicable

LTPP = Long-Term Pavement Performance

In table 4, results show application of the Washington WIM rule set. The data came from the Tennessee SPS-6 TPF site. In the table, the total number of trucks counted by the WIM system with the LTPP Class ification rule set is shown in the far right column. The total number of vehicles counted in each FHWA Class ification when the tested State Class ification rule set was used for those same W-card records is shown in the bottom row. Each cell in the table describes how the two Class ifications compare. So the intersection of the row “LTPP Class 8” and the column “New Class 3” describes how many vehicles that were reported as FHWA Class 8 by the WIM system were Class ified as FHWA Class 3 under the rule set being tested. In this case, 951 vehicles were Class ified as Class 8 by the LTPP algorithm but as Class 3 by the Washington algorithm.

If the two systems matched perfectly, all data would fall in a diagonal line of cells from the upper left to the lower right of the table.

Instead, table 4 shows that the Washington rules assigned a wide variety of vehicles to Class 3 that the LTPP rule set assigned to other Class es. Vehicles were reassigned into Class 3 from LTPP Class es 2, 5, 6, 7, 8, and 15 when the Washington rules were applied. At the same time, vehicles the LTPP rule set defined as Class 8 trucks were moved into Class es 3, 7, 8, and 15 by the Washington rules.

In addition to the cross tabulations, the analysis also allowed any record to be extracted for which the LTPP and State rule sets produced different outcomes. The axle characteristics of these records could then be studied in more detail. This allowed the factors causing any shift from one vehicle Class to another to be examined. It also enabled an additional quality control check to ensure that each rule set was being properly applied.

As can be seen in table 4, a variety of differences occurred when Class ification counts produced with the LTPP Class ification rules were compared with counts produced with an alternative vehicle Class ification rule set. When an alternative rule set was used, vehicles shifted both into and out of individual FHWA Class es.

It was somewhat surprising that, even when just one alternative Class ification rule set was compared with the LTPP rules, the sizes of these shifts were not consistent from site to site across the 18 TPF data sets tested. This variation occurred because although vehicles with the same axle configuration shift between the same two Class es when any given rule set is tested, the number and percentage of vehicles with those specific axle characteristics differ markedly from site to site.

For example, the previous section of this report noted that the LTPP system does not correctly Class ify large dual-unit resource haulers that should be correctly Class ified as Class 10 trucks. The Washington rule set does identify these vehicles accurately. This limitation in the LTPP rule set is apparent in table 4. Of the Class 13 vehicles observed at this Tennessee site, 85 percent (2,018 vehicles) shifted into Class 10 when the Washington rules were applied. These vehicles were seven- and eight-axle Class 10 vehicles. The majority of them have seven axles, with the lead unit most often having a single-tridem axle configuration. This unit most often pulls a tridem-equipped trailer. However, at this site, there were also a substantial number of eight-axle vehicles, the most common of which had axle configurations in which a single-tridem vehicle pulled a tridem-drop axle or conventional quad-axle configured trailer.

Table 5 shows the results of the comparison of the LTPP and Washington WIM rule sets for a New Mexico SPS-5 site. In this case, instead of the 85-percent shift found in Tennessee, only 33 percent of the LTPP-defined Class 13 vehicles shifted to Class 10. This illustrates that while all TPF sites showed a shift in vehicles from Class 13 to Class 10 when the Washington rule set was used in place of the LTPP rule set, the significance of this shift changed dramatically from TPF site to TPF site.

It is also important to note that Class 13 is generally a low-volume truck Class . Errors in Class ification, while significant in that Class , may not have a major impact on the total load estimate, simply because there are generally few Class 10 or Class 13 trucks relative to other truck Class es. (In table 4, Class 13 makes up less than 0.15 percent of the trucks when using the LTPP rule set, and less than 0.05 percent when using the Washington WIM rule set.)

The differences between the New Mexico and Tennessee results were strictly owing to the nature of the vehicle fleets using each roadway. These differences were in part a function of each State’s truck size and weight laws, which govern the vehicle configurations that any given State allows, and in part owing to the types of vehicles that actually travel a specific roadway. (That is, for any given State, the fleet of vehicles using an urban freeway in that State is very different from the fleet that uses a farm-to-market road in a rural county of that State.) Consequently, even within a single State, the changes observed in truck volume and percentages when a given set of rules was tested varied considerably. For example, at the Arizona SPS-1 site, only 13 percent (3 of 23) Class 13 vehicles shifted to Class 10, whereas at the Arizona SPS-2 site, 28 percent (1,055 of 3,782) changed from Class 13 to Class 10.

When different Class ification rules were used, the specific vehicle configurations that shifted from one FHWA vehicle Class ification to another changed. For example, several of the tested Class ification rule sets have more broadly defined Class 10 vehicle types than the LTPP rule set, but these definitions are not as encompassing as Washington’s rule set. A good example is Florida’s WIM rule set, which allows seven- (but not eight-) axle Class 10 vehicles. However, the Florida rules require that the lead unit have only three axles (a single-tandem axle configuration). When it was tested, therefore, a large number of Class 10 vehicles found at the Tennessee site did not shift from Class 13 to Class 10 (see table 6) because the majority of those vehicles had lead units with four axles.

Table 5. Cross tabulation of LTPP Class rule set versus Washington rule set (New Mexico SPS-5 site).

LTPP Class / New Class

New Class 1

New Class 2

New Class 3

New Class 4

New Class 5

New Class 6

New Class 7

New Class 8

New Class 9

New Class 10

New Class 11

New Class 12

New Class 13

New Class 15

Total

LTTP Volume/ New Class Volume

LTPP Class 1

3,862

18

-

-

-

-

-

-

-

-

-

-

-

-

3,880

1.00

LTPP Class 2

-

241,680

83,179

-

-

-

-

-

-

-

-

-

-

-

324,859

1.34

LTPP Class 3

-

-

110,490

-

12,598

-

1

3,538

26

-

-

-

-

1

126,654

0.62

LTPP Class 4

-

-

-

2,896

343

4,002

-

2

-

-

-

-

-

-

7,243

1.22

LTPP Class 5

-

527

11,565

1,568

23135

-

2

13,187

395

-

2

-

-

28

50,409

1.40

LTPP Class 6

-

-

11

1,453

-

3,030

-

15

-

-

-

-

-

-

4,509

0.64

LTPP Class 7

-

-

-

-

-

-

54

-

-

-

-

-

-

-

54

0.76

LTPP Class 8

-

1

386

-

-

-

14

15,645

-

-

-

-

-

-

16,046

0.50

LTPP Class 9

-

-

-

-

-

-

-

-

533,343

-

564

-

-

46

533,953

1.00

LTPP Class 10

-

-

-

-

-

-

-

-

-

2,737

-

-

-

-

2,737

0.90

LTPP Class 11

-

-

-

-

-

-

-

-

-

-

27,047

-

-

-

27,047

0.98

LTPP Class 12

-

-

-

-

-

-

-

-

-

5

-

17,310

-

-

17,315

1.00

LTPP Class 13

-

-

-

-

-

-

-

-

-

293

-

-

589

-

882

1.50

LTPP Class 15

181

50

540

2

34

97

6

702

681

293

173

190

300

252

3,501

46.68

Total with Class 15

3,862

242,226

205,631

5,917

36076

7,032

71

32,387

533,764

3,035

27,613

17,310

589

75

1,115,588

-

Total without Class 15

4,043

242,276

206,171

5,919

36110

7,129

77

33,089

534,445

3,328

27,786

17,500

889

327

1,119,089

-

New Volume/ LTPP Volume

1.00

0.75

1.62

0.82

0.72

1.56

1.31

2.02

1.00

1.11

1.02

1.00

0.67

0.09

-

-

- Indicates not applicable

LTPP = Long-Term Pavement Performance

Table 6. Cross tabulation of LTPP Class rule set versus Florida WIM rule set (Tennessee SPS-6 site).

LTPP Class / New Class

New Class 1

New Class 2

New Class 3

New Class 4

New Class 5

New Class 6

New Class 7

New Class 8

New Class 9

New Class 10

New Class 11

New Class 12

New Class 13

New Class 15

Total

LTTP Volume/ New Class Volume

LTPP Class 1

10,208

-

-

-

-

-

-

 

-

-

-

-

-

2,436

12,644

1.10

LTPP Class 2

1,245

1,352,179

37,850

-

167

-

-

1

-

-

-

-

-

5,607

1,397,049

1.03

LTPP Class 3

-

-

415,546

-

13,549

-

-

17

-

-

-

-

-

17,858

446,970

0.91

LTPP Class 4

-

-

-

13,848

-

-

-

-

-

-

-

-

-

2

1,3850

0.99

LTPP Class 5

-

670

31,152

95

65,189

-

-

5,713

-

-

-

-

-

12,028

114,847

1.34

LTPP Class 6

-

-

-

84

8

22,298

-

-

-

-

-

-

-

-

22,390

1.00

LTPP Class 7

-

-

-

-

-

-

19,555

-

-

-

-

-

-

10

19,565

1.00

LTPP Class 8

-

19

4,281

-

6,425

-

3

27,834

-

-

-

-

-

1,381

39,943

1.19

LTPP Class 9

-

-

296

-

625

-

-

-

1,351,569

-

-

-

-

4,963

1,357,453

1.00

LTPP Class 10

-

-

-

-

-

-

-

-

-

10,538

-

-

-

49

10,587

1.00

LTPP Class 11

-

-

-

-

-

-

-

-

-

-

101,444

-

-

4

101,448

1.00

LTPP Class 12

-

-

-

-

-

-

-

-

-

2

-

37,962

-

13

37,977

1.00

LTPP Class 13

-

-

-

-

-

-

-

-

-

86

-

-

2,272

-

2,358

1.04

LTPP Class 15

483

157

328

42

112

22

34

177

5,532

82

98

226

520

9,614

17,427

0.39

Total with Class 15

11,453

1,352,868

489,125

14,027

85,963

22,298

19,558

33,565

1,351,569

10,626

101,444

37,962

2,272

44,351

3,577,081

-

Total without Class 15

11,936

1,353,025

489,453

14,069

86,075

22,320

19,592

33,742

1,357,101

10,708

101,542

38,188

2,792

53,965

3,594,508

-

New Volume/ LTPP Volume

0.91

0.97

1.09

1.01

0.75

1.00

1.00

0.84

1.00

1.00

1.00

1.00

0.96

3.10

-

-

- Indicates not applicable

LTPP = Long-Term Pavement Performance


4 W-card is the fixed width record format FHWA uses to transmit individual vehicle records. The W-card record includes the number, spacing, and weight of all axles measured by a WIM scale during the time interval for which data are collected. These data are rounded to the nearest tenth of a ft or 100 lb. The format can be found on pages 6-19 of the 2001 FHWA Traffic Monitoring Guide. https://www.fhwa.dot.gov/ohim/tmguide/index.htm

 

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