The Department of Transportation advised Congress late Friday that no change should be made to current truck size and weight laws because the agency lacks the necessary data to make accurate assessments of the national impact of any adjustments.

Peter M. Rogoff, undersecretary of transportation, said there wasn't enough data available from crash reporting statistics to determine a vehicle's weight at the time of an accident; DOT could not determine by the available data whether trucks, prior to a crash, were fully loaded, running overweight, at legal capacity for their axle configurations, or had unevenly distributed weight, Rogoff said.

In addition, there was little in the way of acceptable models to predict bridge deck deterioration over time, making it difficult to forecast long-term maintenance costs, Rogoff said. DOT also had difficulty separating the costs of a truck weight enforcement program from the costs of providing overall truck safety enforcement, Rogoff said.

Rogoff's comments came in a letter sent late Friday to Rep. Bill Shuster (R-Pa.), chairman of the House Transportation and Infrastructure Committee. The Federal Highway Administration (FHWA), a DOT subagency that monitors the nation's highways, conducted the study, which was mandated by Congress in 2012 when the most recent federal transport-funding bill became law.

The study found that the anticipated reduction in vehicle miles travelled that might have resulted from heavier and longer trucks was relatively small. The finding would be a setback to supporters of bigger trucks who have long claimed that the vehicles could handle more freight per trip and would lessen the need for more trucks to handle the same number of loads.

Rogoff said the findings were anticipated, noting an April 2014 report issued by the Transportation Research Board that cited similar shortfalls in available methods and data. The FHWA report was considered a technical document, and not a vehicle for advancing public policy.

The federal limit for trucks operating on the 46,000-mile Interstate Highway System has been set at 80,000 pounds of gross vehicle weight—the combination of tractor, trailer, and cargo—since 1982. In addition, the length of twin trailers attached to a tractor has been capped at 28 feet each since that time.

There have been various legislative efforts to raise the federal weight limit to 97,000 under the condition that trucks hauling the heavier weight be equipped with six axles instead of five to compensate for the extra weight. Proponents said the sixth axle makes it possible to maintain the current weight per tire as well as the current braking capacity, which means stopping distances would remain the same.

Currently, Maine, New Hampshire, New York, Vermont, Massachusetts, and Rhode Island allow six-axle trucks weighing up to 97,000 pounds on their portions of the interstate system. About 40 states allow vehicles weighing more than 80,000 pounds to operate on their own roads. Eighteen states allow twin-trailers of 33 feet in length each on their portions of the interstate system.

It is unclear what impact the DOT's findings will have on bills that may address the controversial issue of increasing a truck's size and weight. A two-month extension to the 2012 federal funding law expires on July 31. In the next few days, the House is expected to vote on a fiscal year 2016 appropriations bill, H.R. 2577, that funds DOT, the Department of Housing and Urban Development, and related agencies. That bill includes language increasing twin-trailer lengths nationwide to 33 feet. Rep. Hal Rogers (R-Ky.), chair of the House Appropriations Committee, supports the provision.

Opponents such as the Teamsters Union argue that the highway network's merge lanes and on-off ramps were not designed to accommodate the longer trucks. Supporters, notably Frederick W. Smith, chairman and CEO of Memphis-based FedEx Corp., maintain the longer trucks will increase truck productivity by optimizing each trailer's cubic capacity. They also contend that trucks carrying 33-foot trailers with longer wheel bases will handle with more stability than rigs hauling 28-foot trailers.

The American Trucking Associations (ATA) reacted angrily to the DOT conclusions, charging that far from being devoid of policy directives, the document is an "obvious attempt to promote administration policy" which has been to oppose any truck productivity initiatives, Bill Graves, ATA's president and CEO, said in a statement.

Graves called it "appalling" that after years of repeatedly saying the study would not make recommendations, DOT would issue a report that provides policy guidance.

A person closely involved in the process was also highly critical of the report. "It was a crude, transparent, and highly selective attempt to prop up their opposition to the House language," said the person, who asked not to be identified. "It causes me to wonder what data favorable to the industry's position is to be found in the parts of the study they didn't release."

Shuster's office did not issue a comment at press time. Rep. Lou Barletta (R-Pa.) who opposes longer and heavier vehicles in his district, which encompasses a large swath of the state's center, said the DOT report means that states and localities can gear up for anticipated road construction work "without worrying about ever-larger trucks rolling through our neighborhoods."

Barletta, who sits on the House Transportation and Infrastructure Committee, said that any study on larger and heavier trucks should include an examination of their impact on local roads and bridges, as well as interstates and primary state roads.

The Coalition for Transportation Productivity (CTP), a group of about 200 shippers and affiliated associations that support changes in size and weight limits, said in a statement that the DOT study affirms its view that the heavier truck weights with an additional axle will spawn a more productive supply chain with no additional safety risks. The group has previously said that the changes are key to mitigating the growing truck capacity crunch mostly caused by a shortage of drivers.

According to CTP, the study cited lower transportation and logistics costs, fuel savings, reduced carbon emissions, less congestion due to fewer trucks on the road, and no degradation in vehicle stability and control with a sixth axle in place.

John Runyan, CTP's executive director, said he wasn't surprised by the DOT's conclusions "given the highly charged atmosphere surrounding the study." Runyan added that it is "now up to Congress to decide if heavier six-axle vehicles, which clearly have few negatives and many positives, can be utilized to address the capacity crisis."