Truck loads in the West slowly killing bridges

News AASHTO Journal September 08, 2004
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At the request of the Western Governors' Association, the Federal Highway Administration has assessed the impacts of lifting th

At the request of the Western Governors' Association, the Federal Highway Administration has assessed the impacts of lifting the current freeze on longer combination vehicles (LCV's) in 13 states that currently allow the larger vehicles.


According to the report issued by the FHWA, titled Western Uniformity Scenario Analysis, the study was requested by the governors as the U.S. Department of Transportation was completing its Comprehensive Truck Size and Weight Study. It specifically addresses the impacts of expanded operations assuming that weights would be limited only by the federal axle load limits and the federal bridge formula, with a maximum gross vehicle weight of 129,000 lb and a length of two 48-ft trailers.


The report finds both positive and negative impacts of the proposed scenario. It estimates that total truck traffic would decrease by 25% in the region, primarily in the long-haul trucking sector. Forecasts for total truck vehicle miles traveled in 2010 would drop from 18.8 billion to a little over 14 billion. The most dramatic change would be seen in a 76% drop in travel by conventional five-axle tractor semitrailers, and a 44% drop in STAA doubles. Less than one-tenth of 1% of rail traffic would be expected to divert to LCVs.


However, the report also finds that the changes in the scenario would affect long-term pavement and bridge costs, and necessitate interchange and other design improvements to accommodate larger trucks. Pavement costs might actually decline by 4%, according to the analysis, because of reduced truck traffic. However, bridge improvement costs are projected to double as states address bridges overstressed by additional weight. Additional costs also are anticipated for improvements to geometric designs to accommodate the longer configurations.


The FHWA report says safety data is inadequate to fully determine potential impacts upon vehicle safety, and adds that impacts on traffic operations also are an issue. The largest benefit of the scenario would be reduced shipper costs. If fully implemented in each state, the report estimates that the scenario could save shippers some $2 billion a year, a savings of almost 4% in the region.


The report maintains that few Western states charge fees that cover the kind of infrastructure costs that would be associated with LCV operations, and recommends that this issue be addressed so that the increased costs are not passed on to other highway users.


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