Splays – visibility at junctions

The visibility splay 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).

Generally the X distance is 2.4m for residential streets and 4.5m for major junctions, busy access roads or where the side road has a speed greater than 40mph etc. The greater distance of the splay might also apply for industrial roads. These distances might vary from highways authority to highways authority. Note that the Local Planning Authority (LPA) might not be the body that sets these.

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 85^th percentile vehicle speed (mph) and not the speed limit. The 85^th 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.” 

Any space within the resulting two triangle areas must remain free of any obstruction above 1 metre. The road curvature may require that the lines in each direction are of different lengths.

These distances might vary depending upon your local highway department. Check locally for variations for splays in these figures.