Speed Surveys

Vehicle speed measurement commonly used for determining speed limits, the improvement of alignments and major/minor junctions and accesses, the layout of new major/minor junctions and accesses on existing roads, and for traffic signal design.

An alternative name is a visibility splay survey. This may be required if a planning application includes proposals to construct a new vehicular access, or there is a need to improve an existing access.

The radar speed survey normally comprises of the collection of at least 200 readings in each direction of vehicle movement. On less busy roads where achieving a suitable sample size may be unachievable, it may be acceptable to reduce this amount or alternatively use Automatic Traffic Counters (ATC’s).

The Automatic Traffic Counters (ATC’s) method to collect speed survey information is becoming more common and radar survey are becoming less frequent - By deploying a MetroCount® MC5600 or RoadPod VT 5900 the most advanced tube counter on the market, we can report individual speed by vehicle basis and also apply different gap and headway parameters, sample size is increase and naturally this provides a much more inconspicuous and accurate survey.

We offer both Manual Radar speed surveys & automatic speed surveys both undertaken in accordance to TA 22_81 DMRB V5 SECT 1

Example of our summary for our Radar speed survey is below

Example of our summary for our Automatic Traffic Counters (ATC’s) speed survey is below

Speed survey results are used to determine the 85th percentile wet weather vehicle speeds in each direction of vehicle movement towards the proposed junction. This is the speed up to which 85% of traffic is travelling and in terms of visibility splay this is a parameter used to consider geometric design parameters such as the appropriate visibility splays either side of a junction or planned access road. The 85th percentile value is used rather than the average value as it is a more robust indication and takes in a higher sample number. If speed surveys are undertaken in dry weather a wet weather correction factor is applied to determine the 85th percentile wet weather speed.

Vehicles are normally recorded on their approach path at a point where they first come into view from the proposed access position in either direction. The speed survey is carried out in free flowing conditions; that is outside of peak times where congestion may occur and slow the progress of drivers or outside any other particular circumstances which may prevent drivers from travelling in a normal unhindered manner.

Road Speed Limits, For any given road situation there will be a variation and a spread of vehicle speeds. There's an average speed of vehicles and some travel faster and some slower. The 50th percentile speed is the speed which 50% of vehicles are not exceeding, equally it's the speed which 50% of vehicles are exceeding.

The 85th percentile speed is the speed which 85% of the vehicles are not exceeding, this basically exclude 15% of the higher vehicle speeds. Plenty of research has shown that the safest group of vehicles are travelling at or below the 85th to 90th percentiles. At the 85th to 90th percentiles research tends to find drivers that have above average skill and competence, and this is why their crash risk is the lowest. Above the 90th percentile we tend to find drivers exceeding safe limits and their accident risk increases as a consequence.

This leads to engineering recommendations that speed limits should normally be set at around the 85th or 90th percentile speed of traffic under good conditions.