Characterization and Development of Truck Axle Load Spectra for Implementation of New Pavement Design Practices in Louisiana

This study addresses the traffic data needs and requirements associated with the adoption of the Mechanistic–Empirical Pavement Design Guide in Louisiana. The objective of the study is to develop the truck axle load spectra from existing data and to identify possible improvements in data collection techniques. Data collected from portable weigh in motion (WIM) sites were screened by using data validation tests and were used to develop axle load spectra for different axle configurations (single, tandem, and tridem) of the primary vehicle class. A significant variation in vehicle class distributions was observed for the same roadway functional classification, and therefore an alternative grouping method based on the truck traffic classification (TTC) procedure was recommended. The results showed that Class 9 was the dominant vehicle class for single axle loads at most of the portable WIM sites located on Interstates and principal arterials. The majority of tandem axles in TTC groups appeared to belong to Class 9, whereas the majority of tridem axles in TTC groups appeared to belong to Class 10. For single axle load spectra, the developed spectra were very similar to the default values, except for TTC 5. For both tandem and tridem axle load spectra, the developed spectra appeared to differ significantly from the default values. It was strongly recommended that traffic data from portable WIM stations be collected for a longer monitoring period and that calibration be done more frequently. Another strong recommendation was to develop a strategic plan for installing permanent WIM sites and tap into axle load data from existing weight enforcement sites to supplement the data collected by portable WIM sites.