Facts and Realities in speed drive. Driving speed is an important factor in road safety. Speed not only affects the severity of a crash, but is also related to the risk of being involved in a crash. Once a crash occurs, the relationship between speed and the outcomes of a crash is directly related to the kinetic energy that is released during a collision (Ek=(1/2)mv2) and hence quite straightforwa
rd. The relationship between speed and the risk of a crash is much more complex. It is easy to understand that at high speeds the time to react to changes in the environment is shorter, the stopping distance is larger, and manoeuvrability is reduced. However, it is difficult to quantify this relationship unequivocally, since many factors determine to what extent these consequences of a higher speed would affect the crash rate. The majority of recent studies looked at absolute speed, either at individual vehicle level or at road section level. Respectively, they found evidence for an exponential function and a power function between speed and crash rate. Both types of studies found evidence that crash rate increases faster with an increase in speed on minor roads than on major roads. At a more detailed level, lane width, junction density, and traffic flow were found to interact with the speed–crash rate relation. Other studies looked at speed dispersion and found evidence that this is also an important factor in determining crash rate. Larger differences in speed between vehicles are related to a higher crash rate. Without exception, a vehicle that moved (much) faster than other traffic around it, had a higher crash rate. With regard to the rate of a (much) slower moving vehicle, the evidence is inconclusive. One way to examine the relationship between speed and crash rate, is to determine the crash liability of individual vehicles that drive at different speeds. Self-report studies and case–control studies are the most common approaches here. n self-report studies the measured speed of an individual vehicle is linked to the number of self-reported crashes of the driver. An Australian study by Fildes et al. (1991) applied a self-report method. This study was performed on two urban 60 km/h roads and two rural 100 km/h roads. collected speed data and selected those drivers who drove at a speed below V15 (slow drivers) or above V85 (fast drivers) of the traffic speed distribution. Selected drivers were stopped out of sight of the speed measurement location and were asked about their history of road crashes during the last 5 years. Fast drivers had had more crashes in the last 5 years than the slow drivers. For both the urban and the rural roads, the relationship had the shape of an exponential function, which was much steeper for urban roads than for rural roads (Fig. 1). General drawbacks of this self-report method are that the measured speed may not be representative for the speed that was maintained before the crash. Furthermore, per definition, only crashes that the driver survived can be included and there is no control for confounding variables. Specific weak points of the study of Fildes et al. are (a) the small number of locations (two per road type) and (b) the small number of days of measurement (4–6 days per location). Speed-limit enforcement in India is almost non-existent, although recently highway police have begun to use automated instruments which capture the speed and mail the fine to the car's owner. Failure to pay may result in doubling of the fine, cancellation of the driving license and even arrest. Police are now also using wireless PDAs to identify a driver's or vehicle's history. 85th percentile speed is adopted for safe speed limit at zone. 98th is for highway geometric design. 15th percentile speed is used as minimum speed on major highways. The recently constructed Outer Ring Road in
Hyderabad is designed for speeds up to 120 km/h. However few vehicles can reach speeds of well over 160 km/h, making it one of the fastest expressways in Asia.
