The study addresses the safety and operational costs and benefits (e.g., fuel, tires, productivity, congestion, etc.) of speed differentials resulting from posted speed limits and truck speed limiters on rural interstate highways. The report also includes extensive surveys of truck drivers and fleet safety and management personnel with respect to their perceptions of the costs and benefits of speed limiters.
The TRB paper addresses the issue
of the definition of “speeding” that includes both” driving too fast for
conditions” and “exceeding the posted speed limit”. The LTCCS data illustrate
that, although “speeding” is frequently cited as the cause of approximately 30
to 35% of accidents, the observed frequencies of accidents involving “exceeding the posted
speed limits” are much lower, in general, and are very low on rural interstate highways.
The TRB paper illustrates the actual distributions of truck and automobile speeds across the US under different posted speed limit configurations. The data include empirical measurements of speeds for trucks and automobiles under high posted differentials (California, 55 and 70) and “uniform” limits from 65 to 75 mph for both heavy trucks and automobiles. The data illustrate that the actual distribution of speeds is relatively unaffected by the posted speed limits. That is low speed limit states (e.g., Illinois) have virtually a zero percent compliance rate. Similarly, truck actual observed truck speeds increase only slightly when the speed limit is raised to 75 mph.