Based on testing results and its concern that the current stopping-distance requirements might not be practicable, the National Highway Traffic Safety Administration (NHTSA) is slightly relaxing the requirement for typical loaded tractors for the final rule that goes into effect Aug. 1.
For typical loaded tractors tested from an initial speed of 20 mph, NHTSA is increasing the distance from 30 feet to 32 feet and for unloaded tractors tested from an initial speed of 20 mph, it is increasing the distance from 28 feet to 30 feet.
On July 27, 2009, NHTSA published a final rule in the Federal Register amending Federal Motor Vehicle Safety Standard (FMVSS) No. 121, Air Brake Systems, to require improved stopping distance performance for heavy truck tractors. This rule reduced the maximum allowable stopping distance, from 60 mph, from 355 feet to 250 feet for the vast majority of loaded heavy truck tractors. For a small minority of loaded very heavy tractors, the maximum allowable stopping distance was reduced from 355 feet to 310 feet.
Having come to the conclusion that modifications needed for ``typical three-axle tractors'' to meet the improved requirements were relatively straightforward, NHTSA provided two years lead time for those vehicles to comply with the new requirements. These typical three-axle tractors make up approximately 82 percent of the total fleet of heavy tractors. The agency concluded that other tractors, which are produced in far fewer numbers and may need additional work to ensure stability and control while braking, would need more lead time to meet the requirements. Due to extra time needed to design, test, and validate these vehicles, which included two-axle tractors and severe service tractors, the agency allowed four years lead time for these tractors to meet the improved stopping distance requirements.