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

Chapter 7. Summary and Conclusions of Pavement Sensitivity Tests

The analyses presented in this report were used to quantify the sensitivity of the pavement design models to the differences in traffic inputs associated with the application of different vehicle classification rule sets.

The first set of analyses focused on quantifying the potential impact of different classification rule sets on pavement design—specifically, what happens when truck volume data collected using a non-LTPP rule set are combined with load spectra based on the LTPP rule set. The results of analyses of several worst-case scenarios showed that, with the exception of the rigid pavement design scenario for a roadway with low truck volume and low total traffic loading, no practical changes to design thickness were observed. Moreover, the rigid design that produced a significant difference is not considered typical, because the majority of rigid pavements are designed for high traffic loading conditions (high total truck volume and high percentage of heavy trucks). For that rigid design scenario, the increase in traffic load owing to use of different classification rule sets was also very high: the total volume of trucks (in Classes 4 through 13) increased by 35 percent with increase of heavy trucks (in Classes 6 through 13) by 59.4 percent. Detailed conclusions from this analysis were provided in chapter 5.

The second set of analyses focused on the sensitivity of pavement design models to increases in predicted volumes of Class 5 and Class 8 vehicles resulting from use of classification rule sets that do not use weight parameters. A set of traffic conditions (AADTT per lane and percentage of Class 5 or Class 8 trucks) was identified through MEPDG and AASHTO 93 analyses when differences in Class 5 and 8 vehicle counts would likely cause a significant difference in pavement design outcomes. Most of these conditions do not apply to RIs and require high percentages of Class 5 or 8 vehicles, as well as high percentages of lightweight vehicles within Class 5 or 8 total counts, to cause any practical implications for pavement designs. It was concluded that more than 90 percent of all Class 5 counts or more than 80 percent of all Class 8 counts must be lightweight vehicles to cause practical implications for pavement design outcomes when MEPDG Level 2 or 3 traffic loading inputs based on the LTPP vehicle classification rule set are used in combination with classification data obtained using a non-LTPP rule set. Assessment of the LTPP traffic database revealed that only three LTPP sites had combinations of AADTT volume and percentages of Class 5 or 8 vehicles that are likely to result in significant differences in the pavement design outcomes. However, the analysis of load spectra available for one of these sites did not indicate high percentages of lightweight Class 8 vehicles. The other two sites did not have axle load spectra.

The following answers could be given to the questions stated in the research objective:

• Can LTPP GPS sites use WIM data from the SPS sites that use the LTPP classification rule set?

  • Based on the analysis of worst-case scenarios of classification rule set differences, it can be concluded that LTPP GPS sites can use WIM data from the SPS TPF sites, with the exception of PCC pavements constructed on low truck volume roads that have unusually heavy Class 9 axle load spectra. It is not likely that many LTPP PCC sites will fall in this category.

  • Prior to applying load spectra based on SPS TPF sites to a GPS site, it is recommended that the loading condition at the GPS site be well understood (at least in a descriptive form, such as typical, lighter than typical, heavier than typical, etc.). Once the loading condition is identified, the appropriate default or surrogate load spectra from SPS TPF sites can be used.

• Does it make any practical difference if the various class rules at the GPS sites differ from the LTPP classification rule set?

  • The impact of potential differences in classification rule sets with regard to Class 5 and 8 vehicles was investigated. The analysis results indicate that, for traffic conditions observed at LTPP GPS sites, no practical difference in pavement design outcomes is likely to result from inclusion of lightweight Class 5 and 8 vehicles using non-LTPP classification rule sets. Pavement design sensitivity results indicate that Class 5 vehicles are too light to produce any significant damage attributed to additional Class 5 volume. Class 8 volumes observed at LTPP sites are not significant enough to produce any considerable damage. A couple of possible exceptions are GPS sites 16-3023 and 6-3010. Vehicle classification data and expected loading conditions for these sites should be further investigated to determine whether high Class 8 volumes are attributed to lightweight vehicles.

Part III: Recommended Changes to the LTPP Classification Rule set

Part III of this report presents the results of a detailed review of the performance of the LTPP WIM vehicle classification system. Based on the comparison of the LTPP system against State-specific classification systems in Part I of this report, a number of minor limitations in the LTPP system were identified. The following chapters describe these limitations and the modifications to the LTPP rule set’s parameters that were implemented to fix those limitations. The results of the field tests of the revised classification system are then presented. Finally, a summary of all the work performed in this project and the conclusions and recommendations drawn from the project are presented.