5.2. SR 07-24-2000 - Laserfiche WebLink

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Synthesis of Safety Research Related to Speed and Speed Management - Turner-Fairbank.. Page 2 of' 25 illustrated in figure 1. The analysis was limited to crashes involving two or more vehicles traveling in the same direction. .,.I I I I I I I ._~ . Nighttime (rural) \1 o Daytime (rural) I I I I I I I -40 -30 -20 -10 0 10 20 30 40 Deviation from Average Speed, mi/h Figure 1. Crash involvement rate by deviation from average travel speed (from Solomon, 1964, and ,qi,.r. jj.[.q, 1968). In these studies, the speeds of crashed vehicles were obtained from police reports, driver's reports, or third party estimates - sources that are subject to error and unknown reliability. Another serious challenge to the internal validity of results is that many of the crashes involving slow speed likely involved vehicles that were stopping or slowing to turn or just entering the road. Whereas, the speed data were collected at locations within the study sections that were representative of the average speed for the entire section but away from intersections, driveways, and other locations having a major effect on speed. These problems would tend to overstate the risk of vehicles traveling at slower speeds. 'Fo address these concerns, the Research Triangle Institute (1970) used a combination of trained on-scene crash investigators and a system of automated continuous speed monitoring stations using sensors embedded in the roadway pavement to obtain the speed of crash-involved vehicles and accurate measurements of traffic speeds at the time of the crash. Detailed data were collected on 114 crashes involving 216 vehicles on a state highway in Indiana with speed limits of 40 to 65 mi/h (65 - 105km/h). In about nine cases, speeds could be linked to specific vehicles involved in crashes and matched well with the estimates of the professional investigators. More importantly, the investigators recognized that vehicles slowing to negotiate a turn should be treated differently in the analysis than vehicles moving slowly in the flow of traffic. The former involves a required slow speed to safely complete an intended maneuver, while the later is more likely to reflect driver choice or limited ability. West and Dunn (1971) reported the results of the Research Triangle Institute studies. Crashes involving turning vehicles accounted for 44 percent of all crashes observed in the study. Excluding these crashes from the analysis greatly attenuated the factors that created the U- shaped curve characteristic of the earlier studies. Without vehicles slowing to turn, or turning across traffic, the investigators found the risk of traveling much slower than average was much less pronounced. Crash risk was greatest for vehicles traveling more than two standard deviation above the mean speed. As illustrated in figure 2, the likelihood of being involved in a crash was extremely flat, with little difference in crash risk for vehicles traveling within 15 mi/h (25 km/h) of the mean speed of traffic. Even excluding turning crashes, the crash risk for vehicles traveling much faster or slower was six times the average rate. Munden (1967), following a different approach, reported similar results for drivers in the United Kingdom who habitually drive at deviant speeds. The speed of selected drivers were observed and compared to the four preceding and four following vehicles. For drivers observed more than once, those traveling more than 1.8 standard deviations above or below the mean traffic speed had significantly higher crash rates. However, drivers observed only once did not exhibit a U-shape relationship. 16 14 ~ 10 4 ...... A¢ciderf~s ] ...... I .~ ...... I· Exolu de Tut ning J_ _ http ://www. ntl.bts, gov/ntl/DOC S/speed/speed. htm 07/19/2000