Visibility splays and their extent 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).
Unless full details of visibility splays were included on the plans submitted in support of the planning application, a condition will be attached to the planning approval requiring the submission of a detailed drawing/s showing how the visibility splays will be provided (as well as other details relating to how the access is to be constructed).
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).
Visibility splays and their extent depend on several factors and are general unique for each specific site.
There are two national standards that apply to road design; Design Manual for Roads and Bridges (DMRB) and Manual for Streets (MfS). DMRB is produced by Highways Agency (HA) and is the design standard for the Strategic Road Network (SRN) (Trunk Roads). MfS is advice on road design which was first published in 2007 by the Department for Communities and Local Government and the Department for Transport and initially focussed on the design of lightly trafficked residential estate roads. There is an area of overlap between the two standards.
T he two documents apply different standards for visibility splays at accesses. The choice of which standard to apply can make the difference between approval and refusal of development proposals. Whilst this is a matter for engineering judgement the Local Highway Authority (LHA), needs to have robust guidance in place to support officer’s judgement in the choice of standard used in assessing developer’s proposals through the planning process.
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 hand held 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 for traffic signal installations.
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).
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 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.
Need help with determining the visibility splays associated with your development please contact us.