Speed is likely to be the single most important determinant in the frequency of traffic fatalities. One technology used by commercial truck fleets to lower the overall top speed of their trucks is a speed limiter (SL). SLs (also referred to as speed governors) are devices that interact with a truck's engine to only permit the attainment of a preprogrammed maximum speed (e.g., the truck cannot exceed a preset speed of 65 mph unless the truck is traveling down a grade). Many commercial truck fleets use SLs to increase safety, increase fuel efficiency, increase engine and brake life, and reduce tire wear. This study assessed the safety benefits of SLs on commercial trucks as they operated during normal revenue-producing deliveries (under real-world driving pressures and situations).
The primary safety analysis was focused on the reduction in truck crashes that could have been avoided or mitigated with an active speed limiter installed on the truck. This was the first study to use actual truck crash data collected directly from commercial truck fleets, representing a wide array of crashes. The study included data from 20 commercial truck fleets in calendar years 2007, 2008, and 2009, including approximately 138,000 truck-years and more than 15,000 truck crashes, as they operated under real-world, revenue-producing deliveries.
The data used in the study were divided into two groups: trucks with an SL and trucks without an SL. The crash data were grouped into two groups as well: crashes that were SL-relevant and crashes that were not SL-relevant. Analyses included analysis of variance (ANOVA) and data modeling (random-effect negative binomial distribution).
The findings showed strong positive benefits for speed limiters.
- The calculated SL-relevant crash rate ratio (1.94) was approximately twice that for non SL-equipped trucks compared to trucks with SL.
- Results indicated that trucks without speed limiters had a significantly higher speed limiter-relevant crash rate (approximately 200 percent) compared to trucks equipped with speed limiters.
The cost of the technology is negligible and would not be expected to be cost-prohibitive for commercial truck fleets/owners.