Different speed limits for cars and trucks : do they affect vehicle speeds.

During 1987 and 1988, 40 states opted to take advantage of the 1987 federal law allowing them to raise the speed limit on rural interstates from 55 mph to 65 mph. The majority of these states raised the limit to 65 mph for all vehicles; however, 10 states chose a lower speed limit for trucks than for cars. Analysis of the mean speeds revealed trucks are travelling 1, 4 mph slower in states with differential speed limits than in those without. This difference increases to 3.0 mph for the fastest 5 percent of trucks.