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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 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:

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.

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