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10 driver acceptance of ISA through improved designs, such as of the challenge in deploying ISA relates to creating and consistency of speed limits across the region, maintaining the maintaining a map database with accurate information of speed limit database, and addressing the complexities of posted speed limits, an issue that does not relate to commer- dynamic speed limits. For large-scale deployment to be effec- cial vehicle speed limiters. tive, Bishop (2005) indicated that factors such as context- appropriate speed limits, up-to-date speed limit databases, and The ISA studies reviewed by Bishop (2005) echo some interoperability within Europe would need to be addressed. of the concerns found with CMV speed limiters in the writ- ten survey (such as driver's concern with unequal speeds compared with neighboring vehicles); however, there are ISAUK significant differences between passenger car and CMV driver concerns regarding speed limiters that the written sur- From 1997 to 2000, the British government funded a study to vey addressed. Therefore, the results of the ISA work are not assess acceptance of ISA, implementation technologies, sim- deemed to be of sufficient magnitude to have a strong bearing ulation modeling to assess side effects, and user trials both in on the commercial vehicle speed limiter questions addressed a driving simulator and on actual roads. The major conclu- in this study. sion from this project was that ISA, in its most compulsory and versatile form (i.e., a mandatory system that is capable of dynamic speed limits based on weather and other con- ADVANTAGES AND DISADVANTAGES ditions), could achieve a 36% reduction in injury crashes OF SPEED LIMITERS across the United Kingdom and a 58% reduction in fatal crashes. This section describes the literature's perspective on the advantages and disadvantages of speed limiters, both objec- Follow-up work ran from 2001 to 2006 and examined tive and subjective. The review leads the Study Team to con- driver behavior with and without speed limiters activated. The clude that there is insufficient data to conclusively establish project involved 20 ISA-equipped vehicles and 80 drivers. Tri- many of the claims, leading to extensive reliance on empiri- als began in early 2003 in four cities that represented both cal data and professional judgment of individual fleet safety urban and rural driving. The systems relied on GPS/map-based managers and independent drivers. The resulting lack of speed information and speed control could be overridden by "solid ground" fuels the policy debate discussed in the next the driver. As of the writing of the report, results from these section. follow-up studies have not yet been published. Advantages Other ISA Projects Clearly, speed limiters have several potential safety benefits. Smaller-scale ISA projects have been conducted in Belgium, They reduce the top speed of vehicles to a pre-set limit. Denmark, Finland, Hungary, the Netherlands, and Spain. Although this may reduce overall crash risk it is more likely Results were similar to those outlined by Bishop (2005) in to lessen the severity of a crash (Wilmot and Khanal 1999). terms of driver acceptance and effectiveness in reducing Speed limiters also reduce speed variability, thereby reduc- speeding and speeding-related crashes. ing lane change and deceleration maneuvers (Varhelyi and Makinen 2001; Toledo et al. 2007). Speed limiters have also been shown to reduce approach speeds at intersections, Relevance to Speed Limiters curves, and roundabouts (Varhelyi and Makinen 2001). on Commercial Vehicles However, there are also potential benefits beyond safety. Although the work in passenger car ISA has been quite thor- Higher speeds are less fuel-efficient. Speed limiters have ough, its application in CMVs is very different. First, pas- been shown to be fuel-efficient and could lead to substantial senger car drivers have different motivations and concerns fuel savings (Guerrero 2006). Less fuel consumption means when driving as compared with CMV drivers; that is, pas- a reduction in greenhouse gas emissions ("Slow Speed senger car drivers subjectively assess their perceived costs Ahead?" 2006) and longer tire life ("Institute Supports Speed and benefits relative to speed, whereas CMV operations Limiters . . ." 2007). Industry expert Robert Inderbitzen of focus more on quantifiable costs and benefits wherever pos- REI Safety Services estimates that, overall, speed limiters can sible. Second, the ISA work cited by Bishop (2005) was produce a 10% to 15% cost reduction when limiting speeds to almost entirely focused on reducing speeding on arterials and about 60 mph, with most of the savings coming from fuel, residential streets, whereas the emphasis for CMV speed lim- tires, and maintenance (primarily brakes) (R. Inderbitzen, per- iters is on major highways. Even though local and short-haul sonal communication, Oct. 2007). According to Vermeulen CMVs operate on arterials and residential streets as well, the and Klimbie (2002), a field test in the Netherlands involving speed limits on those types of roadways are likely to be well 177 vans and 30 trucks between 3.5 and 12 tons estimated the below the CMV speed limiters set speed. Furthermore, much fuel savings from speed limiters at an average of 5%.