What is a splay survey?
A visibility splay survey may be required if your planning application includes proposals to construct a new vehicular access, or there is a need to improve an existing access.
These are formed by lowering or setting back any existing boundary treatment (wall, fence or hedge) at each side of the access and ensuring that any new boundary treatment is set back (or is of a height) that ensures that drivers using the access will be able to see approaching vehicles when exiting the site (and vice versa).
Meeting Conditions
To comply with the condition, a drawing should be produced which shows the access and visibility splays. This drawing should be submitted to the Council, as part of the ‘discharge of conditions’ application.
Once the plans have been approved, the access must be constructed in complete accordance with the approved plans and then retained in this approved way ( e.g. with the vehicular visibility splays kept clear of any vegetation exceeding 1.0 m in height).
How is it Calculated?
Visibility splays at any entrance or junction are measured along the edge of the main road (the “Y” distance) from a point a set distance back from the edge of the main road (the “X” distance). The “Y” distance is set by the speed of traffic and is not dependent on volumes of traffic using either the main road or the access. It is in the assessment of the “Y” distance that the two standards differ.
It may also be necessary to undertake vehicular speed surveys to determine the “X” and “Y” distances shown on the above plan. We are able to undertake surveys using a calibrated handheld directional speed gun in accordance with Department of Transport advice contained in Technical Advice Note TA 22/81, “Vehicle speed measurement on all-purpose roads”. This Advice Note provides technical advice on the correct process for vehicle speed measurement for determining speed limits, the improvement of road alignments, the layout of major/minor junctions and accesses, and traffic signal installations.
What about Speed surveys?
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 on 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 that may prevent drivers from travelling in a normal unhindered manner.
If the road is lightly trafficked it may, with the agreement of the LHA, be acceptable to undertake the survey in the peak hour but only where it is clear that congestion which delays drivers is not present
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