SUMMARY:The agency is proposing to amend our air brake standard to improve the stopping distance performance of truck tractors. Based on current safety trend data and brake system technologies for truck tractors@ we are proposing to reduce the required stopping distance for these vehicles by 20 to 30 percent. We have tentatively concluded that truck tractors are capable of achieving a reduction in stopping distance within this range with existing technologies.We also discuss research and request comment concerning improving the braking performance of other types of heavy vehicles@ i.e.@ trailers@ straight trucks@ and buses. The agency may address improved braking performance for these other vehicles in a future rulemaking.