Chapters 553 (HB 1276) and 554 (SB 504) of the 2018 Acts of Assembly directed the Virginia Department of Transportation (VDOT) to convene a work group to identify the implications of the Commonwealth’s participation in a federal data collection pilot program or project involving sixaxle tractor truck semitrailer combinations weighing up to 91,000 pounds and utilizing interstate highways. The legislation required VDOT to consult relevant stakeholders and review the fee structure and axle spacing for qualifying tractor trucks, issues related to reasonable access from loading facilities onto a primary or secondary highway and interstate highways, the sufficiency of existing data in determining if certain routes and bridges should be excluded from the federal pilot program or project, and any other issues that VDOT deemed relevant or appropriate.

In response to this legislation, stakeholders were identified and three stakeholder meetings were held. Extensive literature was reviewed on the topic of increased weight limits on safety, mobility, infrastructure, mode shift, and enforcement/compliance. Although there are insufficient data available to fully quantify the impacts of 91,000-pound, six-axle vehicles, potential impacts are identified.

Potential Safety Impacts

The introduction of the heavier pilot vehicles poses a potential increase in the number of crashes and/or their severity in comparison to the 80,000-pound vehicles currently operating on interstate highways. Other studies have found increased crash rates but recognize that it is not possible to draw national conclusions due to a lack of relevant crash data. A related safety issue is the potential increase in damage to roadside safety hardware. Additionally, a possible increase in the number of safety violations, particularly regarding brakes on heavier vehicles, is a concern highlighted in earlier research.

Potential Operational Impacts

There is concern that the heavier pilot vehicles would further degrade the speed of the overall traffic stream on interstates, particularly on steep grades; pilot vehicles may operate at slower speeds than the current 80,000-pound vehicles, further reducing the speed of traffic. Additionally, the heavier pilot vehicles may be less capable than the 80,000-pound vehicles to accelerate at entrance ramps, degrading the flow of traffic within the vicinity of ramps. Operations off of the interstate are also a concern, particularly on more narrow roadways with less room within which to operate.

Potential Pavement Impacts

The impact on pavement condition of a 91,000-pound, 6-axle truck will depend on the axle spacing and configuration of the vehicle. Some configurations could result in decreased per-axle loads that would result in little to no impact while other configurations could have more significant negative impacts. Shortening the service life of interstate and non-interstate routes included in a pilot could potentially increase maintenance costs substantially.

Potential Bridge Impacts

If a vehicle configuration is known, it is possible to evaluate each structure and determine its capacity to safely carry a specified load through a process referred to as “load rating." This analysis would identify any structures that are not capable of accommodating the pilot vehicle weight. In the case of the proposed pilot, the vehicle configuration is not known and, therefore, the load rating cannot be calculated.

In addition to the immediate issue of structural capacity, long-term deterioration is also a concern. Most size and weight studies in the literature reviewed anticipate an expected increase in deterioration rates for structures when subjected to increased frequency of heavier loads, leading to increased maintenance and replacement costs.