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SAFETY IMPACTS OF SPEED LIMITER DEVICE INSTALLATIONS ON COMMERCIAL TRUCKS AND BUSES SUMMARY Speed limiters, also described as speed governors, are devices that interact with a truck engine to only permit the attainment of a pre-programmed maximum speed. For more than a decade they have been used in Europe and Australia to limit the speed of large trucks, and are widely available in the United States on late model and new Class 8 trucks. Many truck fleets use speed limiters both for their safety contribution and to reduce fuel use and tire wear, with the speed set at a level optimum for these factors. From a safety perspective, the premise is that the slowing down of large trucks may result in lower travel risks for all motorists, reducing collisions and mitigating the severity of collisions that do occur. The counter-argument is that safety can be compromised, in that speed-limited vehicles cannot accelerate to avoid traffic conflicts (for instance, in merging situations) and the slower speed of these vehicles relative to the surrounding traffic creates speed differentials that have been correlated with increased crash risk. It is desirable, therefore, to synthesize and examine existing literature and industry informa- tion relating to speed limiters. Are there measurable safety impacts? If so, what are the metrics? What is the degree of benefit, if any? With these questions in mind, the CTBSSP initiated this project to synthesize data, research, and analyses performed to date in terms of both the method- ologies employed to assess speed limiters and the actual results, which may be used to guide policy development in North America. The scope of the project encompasses data and analyses that assess the safety efficacy of speed limiters for commercial motor vehicles in Australia, Europe, and North America. The objectives of this synthesis are to document current knowledge and state of practice for speed limiters in commercial vehicle operations and to survey truck and intercity and charter bus car- riers who have experience in using speed limiters regarding perceived benefits and/or drawbacks. Note that the response rate of the current survey was approximately 7% (103 out of 1,500 fleet safety managers completed the survey). The survey can best be described as a small popula- tion convenience survey of the commercial motor vehicle industry; as such, the results may not be representative and should be interpreted with this in mind. Almost 56% of respondents indicated speed limiters were either "successful" or "very suc- cessful" in reducing crashes. In operational terms, speed limiter users believed that limiters were either "successful" or "very successful" in reducing tire wear (44%) and increasing fuel economy (76%). Almost 96% of respondents indicated speed limiters did not negatively affect safety or productivity. The survey queried whether speed limiters result in drivers driving faster in speed zones below the speed limiter set speed to "make up time." Survey results supported this view, with 88% of survey respondents reporting that this was most likely occurring. Generally, respon- dents saw this as relating more to driver habits than to speed limiters directly. Driver attitudes towards speed limiters were largely neutral (64%), with 23% positive. Recognizing the seri- ousness of the ongoing driver shortage, it is also noteworthy that 77% see the impact of speed limiters on driver hiring and retention as neutral.