Visibility Splays are used to help determine if an access is safe. This is done by providing lines on a plan to prove to the Local Highways Authority (LHA), usually the Council or County Council, that the application is safe. Please note that whilst the details below are broadly acceptable every LHA has their own way of doing things so checking first is advisable. This can be done by searching the term "visibility splays for planning" and adding your Local Planning Authority (LPA); it will likely link to a highway standards document or Supplementary Planning Document (SPD).
The visibility splay of an access is the ability to clearly see traffic approaching from both directions at a junction.
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).
"X" is typically 2.4metres to indicate the length of a car bonnet and your standard position when driving a car, but for major junctions this may increase to 4.5metres.
The "Y" distance is typically determinate on the speed of the road but could also be to do with the general traffic using the access or street, for example, HGVs accelerate and decelerate a lot slower than cars so extra distances to compensate for this may be required.
The "Y" distance of the splay will vary depending upon the speed of the vehicles and not how busy the road might be. The speed is a measured 85th percentile vehicle speed (mph) and not the speed limit. The 85th percentile speed is defined as, “the speed at or below which 85 percent of all vehicles are observed to travel under free-flowing conditions past a monitored point". So, in some cases, whilst a road may have a 30mph speed limit, the traffic may be generally travelling faster and if the 85th percentile speed is greater than 30mph then the visibility splays will need to increase commensurably.
In order to work out these 85th percentile speeds it is often required to undertake a speed survey. This is done either by someone sitting in a car with a laser gun measuring car speeds or having some rubber strips applied to the road for a lenth of time to assess passing vehicles. The first option is less favourable as someone pointing a laser gun at their car typically reduces speeds and gives unrealistic results; therefore LHAs do not really like them. The strips on the road is more accurate and less labour intensive giving highly accurate numbers with little effort (but can cost around £800 to do). Highways consultants are typically required for this but mainly for interpretation of the data which is then fed to the LHA to help them determine the case.
Any space within the resulting two triangle areas must remain free of any obstruction above 0.6metres to 1 metre, dependent on the LHA. Also some LHAs will insist on the visibility splay reaching the kerb and some will allow for the visibility splay to be set 1m off the kerb; this could be a hinderance or blessing depending on the road and what obstacles you face. The road curvature may also require that the lines in each direction are of different lengths. Crests in the road can also impact visibility so that ought to be factored in if the road undulates significantly.
These distances might vary depending upon the LHA so it certainly worth asking them about these matter....if you can get hold of them. Alternatively the LPA should have a decent knowledge of these matters and should be willing to provide this information without a pre-application enquiry or crossing their palms with silver. Check locally for variations for splays in these figures.
If you would like more information on these matters or would like to discuss the visibility of a site for a planning application feel free to send us an email or give us a call.