The majority of the accidents in these places are normally classified into two types : smashing at an object after running out of road and rolling over resulting from the losing control of the vehicle. In most of the cases the reasons identified are the excessive speed of the drivers entering the curves , which would lead to misjudging the sharpness of the bend , the sudden sharp turn of the curve after series of gentle curves or the change in surface characteristics that would lead to unsafe road curves. It is widely observed that the road surface would undergo significant change like polishing of the road surface aggregate on the bend than that of the rest of road due to the larger forces on road due to higher side thrust that results in unsafe road conditions on curves. Also , the problems due to inappropriate assumptions in the geometric design of the road like possibility of large number of drivers exceeding the design speed at curves is another important reason for higher accident rates at curves. Loss of super elevation due to poor maintenance could also lead to horizontal curves becoming more problematic than designed.
One of the earlier studies in the effect of curves on the safety of the roads in along London to Birmingham was reported by Hauer (2001). The data obtained in the field studies analysed are presented in the Table 1
Particulars of road geometric
Grade of road
(Million Vehicles Miles)
Road curves to right
Road curves to left
Effect of curves on road safety
The results also showed that the effect of vertical curves on the road safety was not very significant. The most striking feature was the combination of grades with the horizontal alignment. Higher the grades as well as higher degree of curves resulted in the increased rate of accidents. Also, the combination of grades with alignment too resulted in the cumulative effect on the accident occurrence. In an another initiative , the detailed road safety audit exercise conducted in UK brought out the different aspects of the road that often lead to accidents (Motoring Policy, n.d.). According to vehicle user, the major reasons for the accidents at the curves are due to higher actual speed of vehicles than the design speed, obstruction to sign boards due to overgrown vegetation and absence of safety fences.
Prevailing Guidelines for design of curves
In UK, the guidelines are as per the UK departmental Standard TD 9/93 "High Way Link Design". But often less attention is paid in strictly adhering to the norms and specifications. And the road curves are built to alternate framework. This ultimately lead to non-uniform road networks and driver confusion due to poor visibility etc (Department of Transport, 1987). The safe guidelines for the design of curves are given as follows. Design speed is the one of the most important parameter that links different factors that ensures safe driving conditions. The design speed is given as Vdesign = 127.R.(e + f ), where Vdesign is in Km /hr, R is the radius in meters, e is the super elevation in metres per metre and f is the side friction factor (Transport Research Laboratory, 1988). The