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Synthesis of Safety Research Related to Speed and Speed Management - Turner-Fairbank.. Page 3 of' 25 More recently, Australian researchers, Fildes, Rumbold, and Leening (1991), used self-reported crash data collected at roadside from motorists whose driving speed had been unobtrusively measured. The researchers found a trend of increasing crash involvement for speeds above the mean speed in both rural and urban conditions - similar to the correlations reported in the early studies. However, no relationship between slower speeds and increased crash involvement was found. In 0 -15.5 -15.5 - 6.5 5.5 >15.5 to to to 5.5 q"5.5 15.5 Deviation from mean speed, milh Figure 2. Relationship between speed and crash involvement (from West and P.~[1B, 1971). fact, Fildes and Lee (1993) report that the researchers, "...failed to observe any vehicles traveling at the very slow speeds reported by Solomon on rural highways." Figure 3 illustrates the speed-crash relationships identified by Fildes et al, for the two rural and two urban sites used in their study. The relationships are presented along with the U-shaped curves derived from the early research on this topic. Some of the difference between the results can be attributed to changes in driver behavior (e.g., far less "drinking and driving" now than in the 1950s and 1960s) and safety improvements in road and vehicle design during the nearly half- century since the early data were collected. I I I J I I I 50,000- \t, ............. Solomon Dayt/me (n~) ~ · ..... CLrillo F~eeweys 10,000 - \\ / '. ', /// 1,000 ....... 100- I I I I I I I -40 -30 -20 -10 0 10 20 30 40 Deviation from Average Speed, mph Fi§ute 3. Crash involvement rate by variation from average traffic speed (from Solomon, 1964; ,qkJ.[Lq, 1968; and, ~J~e.,s gl al., 1991). Harkey, Robertson, and Davis (1990) recent y replicated the U-shape relationship between speed and crashes on urban roads. The researcher compared the police- estimated travel speed of 532 vehicles involved in crashes over a 3-year period to 24-hr speed data collected on the same section of non-55-mi/h roads in mostly built- up areas of Colorado and North Carolina. To partial address the concerns of earlier studies and make the crash and speed data more comparable, their analysis was limited to non-intersection, non-alcohol, and weekday crashes. However, the estimated travel speeds of the vehicles before the crash are questionable. In defense of the early studies, it is important to note that the researchers emphasized speed variance, rather than absolute speed, as the primary culprit in the incidence of crashes; speed variation is defined as a vehicle's deviation from the mean speed of free-flowing traffic. Hauer's (1971) theoretical analysis of overtakings demonstrated that the number vehicle interactions in terms of passing or being passed is a U-shaped curve with a minimum at the median speed. The number of vehicles that a driver catches up with and overtakes increases with speed and the number of times a driver is passed by others decreases with speed. Thus, the increased risk of crash involvement is a result of potential conflicts from faster traffic catching up with and passing slower vehicles. The slower motorists go relative to the median speed, the more overtakings and potential inter-vehicle conflicts encountered. This is illustrated in figure 4, which compares the relative overtaking rates for a 100-km/h road with a standard deviation of 10 percent with the crash risk form various studies. Hauer claimed "the indiscriminate public crusade against speeding should be replaced by a balanced approach emphasizing the dangers of both fast and slow driving." http://www.ntl.bts.gov/ntl/DOCS/speed/speed.htm 07/19/2000