Road Safety, Traffic Engineering and Accident Investigation
Traffic engineering essentials
Road safety rests on the three E's - Engineering (road and vehicle design), Enforcement (law) and Education. Key design parameters come from IRC codes: reaction time is taken as 2.5 s for stopping sight distance and 2.0 s for overtaking sight distance.
Stopping sight distance: SSD = v·t + v²/(2gf), with v in m/s, t the reaction time, g = 9.81 m/s² and f the longitudinal friction coefficient.
Other tools: superelevation and camber, traffic volume studies, PCU (passenger car unit) for heterogeneous traffic, journey speed and spot speed studies, and black-spot identification where accident frequency or severity exceeds a set threshold.
Accident investigation
- Record the collision type: head-on, rear-end, side-swipe, right-angle, rollover, pedestrian or fixed-object.
- Preserve skid and tyre marks, debris, glass scatter and vehicle crush damage; measure with a baseline and offset method and plot a collision diagram.
- Apply the skid formula v = √(2gfd) to estimate pre-braking speed from the longest skid mark.
- Establish the sequence: perception, reaction, braking, impact, rest. Check whether ABS left fainter, discontinuous marks.
- Compute accident rate per million vehicle-kilometres and severity as the ratio of fatalities to accidents.
Worked example
A car at 60 km/h on a road with f = 0.4 and t = 2.5 s. v = 60/3.6 = 16.67 m/s. Reaction distance = 16.67 × 2.5 = 41.7 m. Braking distance = 16.67²/(2 × 9.81 × 0.4) = 35.4 m. SSD = 41.7 + 35.4 ≈ 77.1 m.
Traps
- Substituting km/h directly into the formula without dividing by 3.6.
- Quoting braking distance alone as stopping sight distance.
- Assuming a vehicle with ABS leaves no marks and cannot be reconstructed.