Visibility splays, explained

If you've ever had a planning officer ask for "a 2.4 by 43 splay", this one's for you.

A visibility splay is the triangle of clear sightline a driver needs when pulling out of a junction or a driveway. Two numbers set it.

The X distance is how far back from the edge of the main road the driver's eye sits. For most junctions it's 2.4 m, which is roughly the bonnet of a car waiting at the give-way line. On quiet, slow streets it can drop to 2.0 m. Go much shorter and you're asking people to creep out blind.

The Y distance is how far along the main road they need to see in each direction. It's the stopping sight distance, which is the room a driver on the main road needs to spot you and stop. That depends on speed, and this is where it gets interesting. In a 30 limit, Manual for Streets gives 43 m. At 20 mph it's 25 m. On faster roads the figures come from the design standards for trunk roads, and at 60 mph you're into 215 m, which is a lot of hedge to keep down.

Note the word speed, not limit. If drivers are actually doing 38 through a 30, the highway authority will quite reasonably ask for the splay to match what people do, not what the sign says. A speed survey often settles the argument before it starts.

Then the bit that catches people out: the triangle has to stay clear. Nothing above about 600 mm between eye and road, so no walls, no bins, no parked cars, and no hedge. That last one is the killer. A splay that works on the drawing in February can be a green wall by July, so the land within it needs to be in the applicant's control or already highway. A lovely splay across the neighbour's front garden is a lovely splay you can't have.

When we look at a site early, the splays are usually the first thing we check, because they decide where the access can go before anything else gets drawn. If you've got a site and an awkward corner, give us a shout.