Practices for Permitting Superheavy Load Movements on Highway Pavements (2015)

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28 The practices of permitting superheavy commercial vehicles (SHCVs) in the United States varies widely between agen- cies in terms of both the criteria used to define them, the analysis details for evaluating their impact on pavements, and the fees levied for permitting them. The gross vehicle weight (GVW) thresholds used to define SHCVs vary from 120 kips to 254.3 kips. Axle load limits by configuration also vary, ranging from 20 to 29 kips for single axles on dual tires, from 34 to 60 kips for tandem axles on 8 tires, and from 50 to 81 kips for tridem axles on 12 tires. In addition, some agen- cies set limits on the tire weight per unit width (i.e., it varies between 500 and 800 lb/in.), whereas others do not. This obvious lack of uniformity in weight regulations reduces the weights of SHCVs traveling through multiple jurisdictions to the least common set of rules in effect through the juris- dictions involved and imposes a considerable administrative burden on shipping companies. There have been regional efforts to establish uniform heavy truck permitting regulations in the United States, whereby a permit issued by one state is accepted for travel in neighbor- ing states. Twelve western states, under the auspices of the Western Association of State Highway and Transportation Officials (WASHTO) agreed on a uniform set of truck weight regulations that allow trucks permitted in one of these states to legally operate throughout the rest. In summary, these limits consist of a GVW of 160 kips, tire weights of 600 lb/in. of width, overall consecutive axle weight limits governed by the Bridge Formula, and axle configuration weight limits of 21.5, 43, and 53 kips for single, tandem, and triple axles, respectively. Similar regional efforts are being undertaken in other geographic regions of the country under the coor- dination of AASHTO’s Subcommittee on Highway Trans- port (SCOHT). Future efforts of this committee will include harmonization of oversize requirements, coordination of the truck permitting processes with local governments, develop- ment of a guide for assessing proposals for changes in truck size and weight standards, and formation of a state–industry advisory group on the movement of “superloads.” It can be noted that other industrialized countries, such as Canada, Europe, Australia, and South Africa, already have such regu- lations in place. The literature review also suggests that SHCV single-trip fees vary considerably among the 62 jurisdictions in North America (i.e., 50 states, the District of Columbia, ten Canadian provinces, and the Yukon Territory): • Twenty-three (37%) levy SHCV permit fees that are a function of weight-distance, typically in the form of $/ton/ mile for GVW exceeding a certain value. Interestingly, some of the states that use weight-distance taxes do not use the same approach for levying SHCV permit fees. This fee ranges from $0.006/ton/mi to $0.2/ton/mi with an average value of about $0.049/ton/mi. • Fifteen (24%) levy SHCV permit fees that are related to GVW per axle weight alone and do not consider the distance traveled by the vehicles. • Eight (13%) levy a flat SHCV permit fee that ranges from $5 to $550, regardless of any pavement usage indicators, that is the weight of the vehicle or the distance traveled. • Seven (11%) levy a processing fee and may add an infra- structure usage fee after studying SHCVs on a case-by- case basis. • Two jurisdictions (3%) levy a flat fee and the cost of repairing the infrastructure from any damage rather than the cost infrastructure utilization from SHCV movement. The web-based survey was conducted between January and July 2014 to collect detailed information on the prac- tices the United States and Canadian provinces use in permit- ting SHCVs. A total of 39 states and five Canadian provinces responded to this survey (response rates of 78% and 50%, respectively). Eight states submitted two responses, one by their permit officer and one by the engineer that analyzed the impact of SHCVs, bringing the total number of survey responses to 52. Thirty-eight agencies responded as to whether or not they conduct pavement analysis as part of their SHCV permit pro- cess. Of those, five (13%) always do (Delaware, Missouri, Louisiana, Tennessee, and Vermont), 15 (40%) do so depend- ing on the circumstances (Arizona, Colorado, Iowa, Illinois, Indiana, North Carolina, North Dakota, Oregon, Washington State, Wisconsin, Wyoming, Texas, Virginia, British Colum- bia, and Ontario), whereas the remaining 18 agencies (47%) never perform such an analysis. The majority of the agencies that perform pavement analysis do so when dealing with a vehicle exceeding their definition of a SHCV. Details of pave- ment analysis performed were provided by 15 states. Their majority uses either their own in-house developed mechanistic- empirical pavement analysis approach or the mechanistic methods developed by industry (i.e., Asphalt Pavement Asso- ciation and Portland Cement Association). Several agencies indicated that they use the 1993 AASHTO Guide for the Design of Pavement Structures and characterize the truck chapter five CONCLUSIONS

29 loads in terms of equivalent single axle loads. None of the responding agencies uses the Mechanistic-Empirical Pave- ment Design Guide for analyzing the impact of SHCV. Addi- tional details on the pavement analysis performed by the 15 responding states suggest that their majority uses repre- sentative thickness and layer/subgrade moduli, and consider the entire length of the SHCV. About half consider only one wheel path and the actual number of tires in the wheel path and the tire inflation pressure, while approximately 25% consider the actual vehicle speed. Furthermore, only four of the 15 responding agencies consider the stability of the pavement subgrade and of those one indicated using a Mohr– Coulomb type of analysis and another using a slope-stability numerical method type of analysis. The number of SHCV permits issued annually varies between agencies and to a large extent depends on their defi- nition of SHCVs. The range is from fewer than 100 to more than 10,000 per year. The GVW of the heaviest SHCV ever permitted by some agencies exceeds 2 million lb. In conclusion, the findings of this study suggest that the practice of permitting SHCVs may be improved by carrying out and implementing future research on: • The methodologies used for evaluating the impact of SHCV on pavements and, • The approaches used for levying permit fees that cover pavement utilization.