Visibility Splays and Sight Lines
Visibility Splays and sight lines are an essential feature of an access or junction as it allows traffic on the minor road to see cyclists, vehicles and pedestrians on the main road. An unobstructed visibility splay or sight line allows a motorist to see and be seen and with appropriate visibility splays or sight lines road users have time to see and react to any potential incident.
Although they have the same function Visibility Splays tend to be areas of land which are dedicated to the Highway Authority as part of the public highway forming part of a new road junction. Sight lines tend to be protected areas of land within the junction design but which will not normally form part of the public highway.
What is a Visibility Splay or Sight Line?
The diagram below indicates the correct interpretation of sight lines at a simple priority road junction, but equally applies to a private access point or individual driveway.
Measuring a Visibility Splay
There are two key dimensions known as the X and Y distances.
Firstly the X distance is measured from the nearside edge of the carriageway back to the position of the driver’s eye line, which for an average car, is 2.4m back and at a height of 1.05 metres above the carriageway of the minor road, access or drive.
Secondly, the Y distance - this is the distance over which vision should be provided to enable the emerging driver to see approaching main road traffic and for that traffic to see the emerging vehicle.
The Y distance is measured from the centre line of the access to the location on the main road of the approaching vehicle based on the calculated findings of the radar speed survey. The faster the approaching vehicles, the longer the distance required to see and be seen.
Visibility splays/sight lines are measured in both a horizontal and vertical sense and at the start of the Y distance, that is the point away from the minor road the driver of the merging vehicle should normally be able to see an object height of 600mm above the main road carriageway. This vertical requirement is to ensure that drivers’ vision is not obstructed by crests in the main road where a “blind summit” may occur.
Within the visibility splay or sight line envelope there should be no obstructions to vision such as walls or vegetation etc. within the vertical profile.
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