4. IMPACT OF SPEED ON CRASHES
4.1 Speed variance vs crashes
Solomon (1964) describe the effect of speed vs crashes as a U shaped curve [9]. Solomon obtained the estimated speed of the vehicles suffered the crashes by tracing the police reports. The author compiled a “modus speed” which implies a safe speed for that particular stretch of road by driving with the traffic flow [45]. Solomon created a crash proportion per speed category, a comparison ratio using the crash vehicle speeds and the modus speed for each category. The estimated travel speed from the accident records were compared to the speeds measured at the selected road section within each study section [46].The shape of the U shaped curve signifies that as the variation from average speed increases, the number of collisions per 100 million vehicle miles increases. The more the variation from average speed is negligible, the lesser the number of collisions or crash rates. According to Solomon [9], the variance from modus speed had higher crash rates on either sides. The literature by Aarts [45] state that Cirillo [1968] [48] revisited Solomon’s study by exploring the percentage of vehicles involved in crash rather than the proportion per speed category. Both studies weighed low speed category as more potential prone to crashes than high speed category [46]. Further studies in this area displayed similar results but the influence on crash rates by under speeding vehicles was less than those calculated by Solomon [9] and Cirillo [48]. Figure 1: Crash rate with deviation from Average speed explained by Solomon and Cirillo (46) It should be noted that “modus speed” or estimated average speed is very discretionary as it was judged by driving through the road. There was no scientific basis for choosing the modus speed. Also the estimated crash speed traced by Police reports is highly depended on the estimation of Police. The crash reconstruction methods were not developed much those years. Also speed causing crashes is only a small representation of a large section of roadways. The problem of discretionary choice of speed was mitigated by the Research Triangle Institute (RTI) [47] when they employed speed monitoring stations to validate the findings of crash investigators [46]. RTI reconstructed the crashes using physical marks or vehicle …show more content…
The author considers speeding behavior as a trade- off between “risk taking” and “safety conscious”. Slower moving drivers prioritize safety over risk taking behavior whereas, faster moving vehicles, trade risk taking for safety [10]. This is the basis for further research in the field of driver behavior. The traffic instability created by different driver behavior cannot be controlled by speed limits. Hauer states that slow moving vehicles also create chances of crashes as the faster moving vehicles have to adjust to slow traffic or change lanes. Upper limit in speed enforcement and lower limit in speed enforcement reduce the probability of overtaking. But the author emphasizes that lower speed limits have twice the effect of upper speed limits during overtaking during a specific speed distribution. The study of Ezra Hauer backed up the claim by Solomon [9] that variance in speed in either direction results in crashes and not just over speeding.
But the subsequent studies conformed to the general notion, that a crash is developed when speed vary between vehicles travelling in a same
4.1 Speed variance vs crashes
Solomon (1964) describe the effect of speed vs crashes as a U shaped curve [9]. Solomon obtained the estimated speed of the vehicles suffered the crashes by tracing the police reports. The author compiled a “modus speed” which implies a safe speed for that particular stretch of road by driving with the traffic flow [45]. Solomon created a crash proportion per speed category, a comparison ratio using the crash vehicle speeds and the modus speed for each category. The estimated travel speed from the accident records were compared to the speeds measured at the selected road section within each study section [46].The shape of the U shaped curve signifies that as the variation from average speed increases, the number of collisions per 100 million vehicle miles increases. The more the variation from average speed is negligible, the lesser the number of collisions or crash rates. According to Solomon [9], the variance from modus speed had higher crash rates on either sides. The literature by Aarts [45] state that Cirillo [1968] [48] revisited Solomon’s study by exploring the percentage of vehicles involved in crash rather than the proportion per speed category. Both studies weighed low speed category as more potential prone to crashes than high speed category [46]. Further studies in this area displayed similar results but the influence on crash rates by under speeding vehicles was less than those calculated by Solomon [9] and Cirillo [48]. Figure 1: Crash rate with deviation from Average speed explained by Solomon and Cirillo (46) It should be noted that “modus speed” or estimated average speed is very discretionary as it was judged by driving through the road. There was no scientific basis for choosing the modus speed. Also the estimated crash speed traced by Police reports is highly depended on the estimation of Police. The crash reconstruction methods were not developed much those years. Also speed causing crashes is only a small representation of a large section of roadways. The problem of discretionary choice of speed was mitigated by the Research Triangle Institute (RTI) [47] when they employed speed monitoring stations to validate the findings of crash investigators [46]. RTI reconstructed the crashes using physical marks or vehicle …show more content…
The author considers speeding behavior as a trade- off between “risk taking” and “safety conscious”. Slower moving drivers prioritize safety over risk taking behavior whereas, faster moving vehicles, trade risk taking for safety [10]. This is the basis for further research in the field of driver behavior. The traffic instability created by different driver behavior cannot be controlled by speed limits. Hauer states that slow moving vehicles also create chances of crashes as the faster moving vehicles have to adjust to slow traffic or change lanes. Upper limit in speed enforcement and lower limit in speed enforcement reduce the probability of overtaking. But the author emphasizes that lower speed limits have twice the effect of upper speed limits during overtaking during a specific speed distribution. The study of Ezra Hauer backed up the claim by Solomon [9] that variance in speed in either direction results in crashes and not just over speeding.
But the subsequent studies conformed to the general notion, that a crash is developed when speed vary between vehicles travelling in a same