Speed surveys are undertaken to record the design speed of any road or section of road. The design speed is the highest safe vehicular speed for which the road is designed to accommodate and is not always equal to the speed limit. The design speed is the speed at which 85 percent of traffic travel below and can sometimes be significantly less than the speed limit.
For some planning applications it is necessary to demonstrate the above in order to justify reduced sightlines. The objective of speed surveys is to calculate the design speed of the road.
This situation might arise on a bendy local road, where an 80kph speed limit applies, yet the bendiness of the road reduces vehicular speeds to such an extent that the majority of vehicles travel at a reduced speed of 40kph. In this instance it may not be possible to achieve 160m sightlines in both directions, and a speed survey can establish the actual design speed and hence the precise level of visibility which could reasonably be provided.