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 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. Vehicle speeds were measured on rural interstates in california and illinois, which have a differential speed limit, and in arizona and iowa, which have a uniform speed limit. A posted differential speed limit on rural interstates was found to reduce high truck speeds on the faster roads. Trucks are a smaller percentage of the high-speed traffic in states with differential speed limits than in states with uniform speed limits when average car speeds exceed 63.4 mph. Specifically, for each 1-mph increase in mean car speed over 63.4 mph on rural interstates, the odds relative to cars of a truck travelling about 70 mph decreases by 20% in the states with differential speed limits compared with states having uniform speed limits. Analysis of the mean speeds revealed that trucks travel 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% of trucks.
Samenvatting