After two years of study, the U.S. Department of Transportation (USDOT) recommended against any increases in truck size and weight.

The U.S. Department of Transportation (USDOT) released its Final Report to Congress as part of the Department’s MAP-21 Comprehensive Truck Size and Weight Limits Study in April of 2016. The Department’s conclusion after more than two years of study by many of the nation’s foremost truck size and weight experts: there should be no changes in current truck size and weight limits. There is simply not enough reliable data on which to base any changes in truck size or weight.

USDOT Findings on Heavier Trucks

Crash Rates

Heavier trucks with six axles—both 91,000-pound and 97,000-pound configurations—were found to have higher crash rates in limited state testing:

  • Idaho  – 99 percent higher crash rates for six-axle trucks up to 97,000 pounds
  • Michigan – 400 percent higher crash rates for six-axle trucks up to 97,000 pounds
  • Washington – 47 percent higher crash rates for six-axle trucks up to 91,000 pounds

The Technical Reports summarized the findings as follows: “In the three States where data could be analyzed, the crash involvement rate for the six-axle alternative configurations is consistently higher than the rate for the five-axle control vehicle. This consistency across States lends validity to this finding.” However, USDOT states the following in its Final Report to Congress: “Due to the limited number of States with suitable data, the analysis of crash rates cannot be extended to other States or used to draw meaningful conclusions on a national basis.”

Truck Inspection Violations

USDOT found that heavier trucks had higher out-of-service and braking violations:

  • Trucks weighing over 80,000 pounds had higher overall out-of-service (OOS) rates compared to those at or below 80,000 pounds.
  • Trucks weighing over 80,000 pounds had brake violation rates that were 18 percent higher compared to those at or below 80,000 pounds.

Bridge Stress
USDOT found that thousands of Interstate and other National Highway System bridges could not accommodate heavier trucks. These bridges would require posting, reinforcement or replacement, costing billions of dollars. USDOT estimates that the 91,000-pound, six-axle configuration would negatively affect more than 4,800 bridges, costing up to $1.1 billion.


USDOT Findings on Longer Double-Trailer Trucks, Called “Double 33s”

Crash Rates
In its 2000 study, USDOT concluded that multi-trailer trucks have a fatal crash rate that is 11 percent higher than single-trailer trucks. This conclusion was not refuted in the Department’s most recently published study.

Stopping Distances

USDOT tested straight-line stopping distances for all truck configuration scenarios under several braking conditions: normal operating brakes, anti-lock braking system (ABS) malfunction and brake failure. They found consistently longer stopping distances for Double 33s than the truck configuration they are intended to replace, Twin 28s. Double 33s took 22 feet longer to stop than Twin 28s with normal operating brakes.

Out-of-Service Violations
Twin-trailer configurations had the highest out-of-service (OOS) violation rates compared to tractor semitrailer and triple-trailer trucks. Further, twin-trailer configurations had OOS violation rates that were 58 percent higher than single-trailer trucks.

Pavement Damage

Double 33s would result in the largest lifecycle overall increase in pavement costs, increasing pavement damage by 1.8 to 2.7 percent compared to the base case truck configuration.

NOTE: USDOT’s finding translates to $1.2 billion to $1.8 billion in estimated pavement damage per year. This calculation is based on state and local governments spending $68 billion on pavements in 2012—$41 billion by the states, and $27 billion by counties and cities (2012 Highway Statistics and selected Cost Allocation studies; R.D. Mingo and Associates).

Bridge Stress

Double 33s would require nearly 2,500 Interstate and other National Highway System (NHS) bridges to be posted or face further damage, costing up to $1.1 billion in immediate bridge strengthening or reinforcement.

NOTE: As stated above, USDOT only studied 20 percent of the nation’s bridges for this analysis. The remaining 80 percent are likely to be the most vulnerable to Double 33s. In fact, only 1,360 of the bridges considered by USDOT are currently “structurally deficient” (i.e., likeliest to need repair and/or replacement with heavier truck weights), while 70,427 of total bridges are classified as “structurally deficient.”