It is well known that drivers exceeding speed limits are likely to deliberately underestimate their speeds. Such underestimation can account for much of the apparent underinvolvement of moderately high-speed drivers in crashes. In addition, later research found that simply removing the crashes involving intersections and turning maneuvers eliminated the exaggerated overinvolvement of slower drivers in crashes. More important, the Solomon research addressed only speed variation, not speed limits. Raising speed limits raises average speeds as well.
Large trucks require much longer distances than cars to stop.
Lower speed limits for trucks make heavy vehicle stopping distances closer to those of lighter vehicles. Slower truck speeds also allow automobile drivers to pass trucks more easily. Institute studies have shown that lower speed limits for trucks on 65 mph highways lower the proportion traveling faster than 70 mph without increasing variation among vehicle speeds. In one study, only 4 percent of large trucks exceeded 70 mph on Ohio's rural interstates with 55 mph speed limits for large trucks and 65 for cars, but up to six times the proportion of trucks exceeded 70 mph on rural interstates in three other states with uniform 65 mph speed limits.