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

Chapter 6. Sensitivity of Pavement Design Models to Differences in Classification of Class 5 and Class 8 Vehicles

Discussion of Findings from AASHTO 93 Analysis

The same analysis was repeated using pavement design models from the AASHTO 93 Interim Guide. Table 26 provides the pavement cross sections used in the analysis. For simplicity, the same base material type and thickness used in the MEPDG designs were applied here. The additional increase in volume of Classes 5 and 8 required achieving the 0.5-inch increment in the design thickness of the AC or PCC top layer using the AASHTO 93 design are provided in table 27 and table 28, respectively. Both tables also provide the original volumes of Classes 5 and 8.

Table 26. Reference designs thickness using the AASHTO 93.

RI = Rural Interstate

ROPA = Rural Other Principal Arterial

HMA = Hot Mix Asphalt

PCC = Portland Cement Concrete

WF = Wet-Freeze

WNF = Wet-No Freeze

DF = Dry-Freeze

DNF = Dry-No Freeze

Table 27. Original and new Class 5 volume needed to require 0.5-inch difference in design thickness using the AASHTO 93.

RI = Rural Interstate

ROPA = Rural Other Principal Arterial

HMA = Hot Mix Asphalt

PCC = Portland Cement Concrete

WF = Wet-Freeze

WNF = Wet-No Freeze

DF = Dry-Freeze

DNF = Dry-No Freeze

Table 28. Original and new Class 8 volume needed to require 0.5-inch difference in design thickness using the AASHTO 93.

RI = Rural Interstate

ROPA = Rural Other Principal Arterial

HMA = Hot Mix Asphalt

PCC = Portland Cement Concrete

WF = Wet-Freeze

WNF = Wet-No Freeze

DF = Dry-Freeze

DNF = Dry-No Freeze

Findings From Class 5 Sensitivity Analysis

For the case of sensitivity of RI designs to Class 5, it was found that an increase of Class 5 volume of more than 5,000 (flexible design) or more than 6,000 vehicles (rigid design) per day from the original 170 vehicles is required to result in any significant difference in the design thickness. These additional Class 5 volumes would result in AADTT per lane values of 7,000 to 8,000, which are unlikely to be observed in the field. It also would mean that the percentage of Class 5 vehicles in the VCD would be about 70 percent, which also is unlikely for RIs. Therefore, the conclusion from the analysis is that misclassification of Class 3 vehicles as Class 5 is not likely have any practical impact for AASHTO 93 designs for RIs.

For the case of sensitivity of ROPA designs to Class 5, it was found that an increase of Class 5 volume of more than 1,400 vehicles (flexible design) or 2,100 (rigid design) per day from the original 155 (flexible) or 99 (rigid) vehicles is required to produce any significant difference in the design thickness. This results in AADTT per lane values that are possible but unusual for ROPAs. Therefore, the conclusion from the analysis is that misclassification of Class 3 vehicles as Class 5 is not likely to have any practical impact for AASHTO 93 designs for ROPAs if AADTT per lane is less than 1,900 vehicles for flexible design or 2,800 vehicles for rigid design. For roads with AADTT per lane above these values, load spectra developed based on the LTPP rule set should be used only if site-specific analysis concludes that the portion of misclassified Class 3 vehicles among all Class 5 vehicles at the site is less than 90 percent for flexible pavement and 93 percent for rigid pavements.

Findings From Class 8 Sensitivity Analysis

For the case of sensitivity of RI designs to Class 8 volumes, it was found that an increase of Class 8 volume of more than 1,400 (flexible design) or 1,500 vehicles (rigid design) per day from the original 150 vehicles is required to result in any significant difference in the design thickness. These additional Class 8 volumes would result in AADTT per lane values of 3,400 to 3,500. These AADTT per lane values frequently are observed in the field. It also would mean that the percentage of Class 8 vehicles in the VCD would be about 41 percent for flexible pavements and 36 percent for rigid pavements. These Class 8 percentages are not typical but are possible for RIs. Therefore, the conclusion from the analysis is that the misclassification of Class 3 vehicles pulling light trailers as Class 8 may have practical impacts for AASHTO 93 designs for RIs with truck volumes equal to or greater than the above traffic conditions and when the percentages of lightweight Class 8 vehicles within the total volume of Class 8 vehicles is more than 80 percent for flexible pavements and 82 percent for rigid pavement.

For the case of sensitivity of ROPA designs to Class 8 volumes, it was found that an increase of Class 8 volume of more than 350 (flexible design) or 500 vehicles (rigid design) per day from the original 40 to 47 vehicles is required to result in a significant difference in the design thickness. These additional Class 8 volumes would result in AADTT per lane values of 850 to 1,200. These AADTT per lane values frequently are observed in the field. It also would mean that the percentage of Class 8 vehicles in the VCD would be about 45 percent for both flexible and rigid pavements. These Class 8 percentages are not typical but are possible for ROPAs. Therefore, the conclusion from the analysis is that classification of Class 3 vehicles pulling light trailers as Class 8 may have practical impacts for AASHTO 93 designs for ROPAs with truck volumes equal to or greater than the above traffic conditions and when the percentages of lightweight Class 8 vehicles within the total volume of Class 8 vehicles is more than 79 percent for flexible pavements and 83 percent for rigid pavements.

Conclusions

Analyses conducted in this study identified few traffic conditions when misclassification in Class 5 or 8 volumes, as a result lightweight non-trucks classified as trucks, could cause practical differences in pavement design outcomes using MEPDG Level 2 or 3 traffic loading inputs based on the LTPP vehicle classification rule set. These conditions require high percentages of Class 5 or 8 vehicles, as well as high percentages of improperly classified lightweight vehicles within Class 5 or 8 total counts to have any practical implications for pavement design outcomes. Table 29 and table 30 specify limits (maximum values) of applicability of MEPDG loading defaults based on the LTPP rule set when combined with data collected using a non-LTPP rule set that allows lightweight vehicles to be classified as Class 5 or 8.

Table 29. Applicability limits for the case of lightweight Class 5 vehicles.

RI = Rural Interstate

ROPA = Rural Other Principal Arterial

AADTT = Annual Average Daily Truck Traffic

Table 30. Applicability limits for the case of lightweight Class 8 vehicles.

RI = Rural Interstate

ROPA = Rural Other Principal Arterial

AADTT = Annual Average Daily Truck Traffic

The LTPP traffic database was used to determine whether any of the sites have the traffic conditions identified in this report that would limit applicability of MEPDG Level 2 or 3 traffic loading inputs based on the LTPP vehicle classification rules. Based on assessment of LTPP data, with the exception of the three sites discussed under the Class 8 ROPA MEPDG results, none of the sites have combinations of AADTT volume and percentages of Class 5 or 8 vehicles that satisfy these conditions.

If LTPP default load spectra developed based on the LTPP classification rule set are to be used with data from non-LTPP sites, a check should be done on AADTT and Class 5 and 8 values. If the combination of AADTT and percentage of Class 5 or 8 values satisfy the conditions summarized in table 29 and table 30, a manual short-term traffic classification study should be used or analysis of available axle load data should be conducted to assess the portion of lightweight vehicles in the total Class 5 or Class 8 volumes. Knowing the percentage of lightweight vehicles in total Class 5 or Class 8 volumes, a conclusion then can be reached regarding the applicability of heavier Class 5 and Class 8 load spectra developed based on the LTPP vehicle classification rules. Generally, more than 90 percent of all Class 5 counts or more than 80 percent of all Class 8 counts must be lightweight vehicles to have practical effects on pavement design outcomes when MEPDG Level 2 or 3 traffic loading inputs based on the LTPP vehicle classification rule set are used. More specific conclusions are provided in the following paragraphs.

Effects of Class 5 Misclassification

MEPDG Results

For the case of the sensitivity of RI designs to the misclassification of vehicles into Class 5, the conclusion from the analysis is that misclassification of Class 3 vehicles as Class 5 is not likely to have any practical impact for MEPDG designs.

For the sensitivity of ROPA designs, the misclassification of Class 3 vehicles as Class 5 is not likely to have practical impacts on MEPDG designs under most traffic conditions unless 1) the AADTT per lane is more than 2,600 vehicles for flexible design or 1,700 vehicles for rigid design and 2) the portion of misclassified Class 3 vehicles among all Class 5 vehicles at the site is more than 96 percent for flexible pavements and 87 percent for rigid pavements.

AASHTO 93 Results

For the case of the sensitivity of RI designs to the misclassification of vehicles into Class 5, the conclusion from the analysis is that misclassification of Class 3 vehicles as Class 5 is not likely to have any practical impact for AASHTO 93 designs for RIs.

For ROPA designs, the conclusion from the analysis is that misclassification of Class 3 vehicles as Class 5 is not likely to have practical impacts for AASHTO 93 designs unless 1) the AADTT per lane is more than 1,900 vehicles for flexible design or 2,800 vehicles for rigid design, and 2) the rate of misclassification for Class 5 vehicles is more than 90 percent for flexible pavements and 93 percent for rigid pavements.

Effects of Class 8 Misclassification

MEPDG Results

For RI designs, the misclassification of Class 3 vehicles pulling trailers as Class 8 may have practical impacts for MEPDG designs if 1) AADTT volumes are more than 4,400 for flexible pavements and 2,700 for rigid pavements and 2) the percentages of Class 8 vehicles are above 39 percent for flexible pavements and 51 percent for rigid pavements. In addition, the portion of lightweight vehicles among all Class 8 vehicles at the site should be more than 94 percent for flexible pavement and 83 percent for rigid pavements.

For ROPA designs, the misclassification of Class 3 vehicles pulling light trailers as Class 8 may have practical impacts for MEPDG designs for ROPAs with AADTT volume above 700 for flexible pavements and 1,000 for rigid pavements and when the percentages of Class 8 vehicles are above 40 percent for flexible pavements and 35 percent for rigid pavements. In addition, the portion of lightweight vehicles among all Class 8 vehicles at the site should be more than 84 percent for flexible pavements and 89 percent for rigid pavements. A small fraction of ROPAs may have these traffic characteristics.

AASHTO 93 Results

For RI designs, the conclusion from the analysis is that classification of Class 3 vehicles pulling trailers as Class 8 may have practical impacts for AASHTO 93 designs if 1) AADTT per lane is higher than 3,400 for flexible and 3,500 for rigid pavements and 2) the frequency of occurrence of lightweight Class 8 vehicles in the total volume of Class 8 vehicles is high.

For the case of Class 8 sensitivity for ROPA designs, the conclusion from the analysis is that misclassification of Class 3 vehicles pulling light trailer as Class 8 may have practical impacts for AASHTO 93 designs if 1) AADTT per lane is higher than 850 for flexible and 1,200 for rigid pavements and 2) the rates of lightweight Class 8 vehicles in total volume of Class 8 vehicles are higher than 79 percent for flexible pavements and 83 percent for rigid pavements.

Disclaimer

The results presented in this report and the conclusions drawn are limited to the study of RIs and ROPAs with AADTT volumes developed based on average values observed among all LTPP sections. For road types and truck volumes not covered in this study, the analysis methodology described in this chapter can be applied to gain a better understanding of implications resulting from combining load spectra developed based on low percentages of lightweight Class 5 and 8 vehicles with truck volumes and class distributions obtained using algorithms that misclassify lightweight Class 3 vehicles pulling trailers as Class 8 or misclassify Class 3 vehicles as Class 5.

All the conclusions developed in this study focus on the analysis of the expected differences in pavement design or performance predictions resulting from application of different classification rule sets. Additional errors in pavement design or performance predictions could result if selected load spectra do not accurately describe the expected loading conditions at the site for which only classification data are available.