Where appropriate Sanderson Associates undertake surveys using a calibrated hand held directional speed gun in accordance with Department of Transport advice contained in document CA185, “Vehicle speed measurement”. This document contains the requirements for the measurement of vehicle speeds and for determining 85th percentile speeds on existing all-purpose trunk roads.
The radar speed survey normally comprises of the collection of at least 100 readings in each direction of vehicle movement. On less busy roads where achieving a suitable sample size is problematic, it may be acceptable to reduce this amount by agreement with the Local Highway Authority or alternatively use Automatic Traffic Counters (ATC’s).
Manual traffic counts have the benefit of providing more precise details of the circumstances of the local highway, such as weather conditions, constraints to free flow conditions, high speed emergency vehicles, and by noting and removing 'platoons' of traffic from the survey. ATC’s are in place for a full 24 hour, seven day period and thus a greater range of results and traffic flows is obtained. However because of their nature they will not take into account the factors which an operator would record as part of the manual survey.
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 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.
The precise point at which the measurements are taken and the timing is important. 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 slow the progress of drivers or where there may be road works or any other particular circumstances which may prevent drivers from travelling in a normal unhindered manner.
If the road is lightly trafficked it may, with the agreement of the Local Highway Authority, be acceptable to undertake the survey in the peak hour but only where it is clear that congestion which delays drivers is not present.
The 85th percentile wet weather speed readings are then used to determine the required sight lines through the guidance within either Manual for Streets 1 or 2 for non trunk road locations and the Design Manual for Road and Bridges for the trunk road network.
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Sanderson Associates have enjoyed over 34 years in business, we have extensive experience in assisting our Clients with Speed Surveys in relation to a host of highway planning matters for a variety of developments throughout the UK, Isle of Man and Ireland.
We would be pleased to provide you with our competitive fee proposal to assist you with a Speed Survey, please call us on 01924 844080 or click here to complete our secure online form.