Visibility splay calculator.

2.4 m represents a reasonable maximum distance between the front of a car and the driver’s eye, and is the value to use in most built-up situations (MfS2 10.5.6).

2.0 m may be considered in some slow-speed situations where flows on the access are low (MfS2 10.5.8). Typical examples are a driveway serving one or two homes, or a lightly used access onto a 20 or 30 mph street.

With 2.0 m, the front of some vehicles will sit slightly over the edge of the main road, so drivers tend to edge out. MfS2 asks designers to check that drivers and cyclists on the main road can see the overhang from a reasonable distance and pass it safely. Avoid 2.0 m where the access is busy, speeds are higher or cyclists ride close to the kerb.

Going the other way, longer x-distances add capacity but can make emerging drivers less attentive to pedestrians and cyclists (MfS2 10.5.7).

Y has traditionally been measured to the nearside kerb. MfS2 10.5.3 says a more accurate assessment measures to the nearside edge of the vehicle track, because drivers normally travel some distance from the kerb. MfS2 does not fix that distance. The research summarised at MfS2 10.4 measured to a point 1 m out from the kerb edge.

A 1.0 m offset is therefore a reasoned refinement rather than a standard, and needs the highway authority’s agreement. It is most defensible where:

• the lane is wide enough for vehicles to run clear of the kerb;
• there is no cycle lane, regular cycling or kerbside parking along the approach;
• the approach is straight, not the inside of a bend;
• observations or a speed survey show where vehicles actually run.

It is weaker where cyclists or motorcyclists ride near the kerb, narrow lanes keep vehicles close to it, or buses and HGVs pull in at stops. Some authorities accept the vehicle track but warn against overestimating its distance from the kerb.

Traffic from the left normally travels in the far lane. MfS2 10.5.5 allows the splay to the left to be measured to the centreline where opposing flows are physically separated, so vehicles from the left are unlikely to cross it. Examples include a central reserve, a run of traffic islands, or a hatched centre area with islands.

Some highway authorities apply this strictly, accepting it only where vehicles from the left are physically unable to cross the centreline. Others accept it more widely on straight two-lane roads with little overtaking. Confirm the local approach before relying on it.

Avoid it where vehicles from the left may overtake, pass parked cars or a bus stop, or cut the corner on a bend. In those cases, measure to the nearside kerb or a justified offset.