Roundabouts

There’s a roundabout near where I live.

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It sits on a bypass connecting Reading to High Wycombe. There are two other exits, on the north and south of the roundabout (making four in total) that serve low-traffic roads to Marlow and the quiet part of Maidenhead, respectively.

The vast bulk of the traffic that joins the roundabout at rush hour simply passes straight across, to and from Reading and High Wycombe. Traffic joining from the Marlow and Maidenhead roads, either to cross or join the bypass, is, at a guess, maybe 10% or less.

Roundabouts have a slight bias towards the direction of traffic that has a car joining the roundabout, i.e cars need give less space when following than when paths intersect, because the lower speed vectors mean it’s easier for the driver to judge. Because traffic has to give way to the right, a steady stream of traffic joining a roundabout can gain momentum of sorts.

The speed limit on the roundabout is the same as the dual carriageway it sits on: 70mph.

The combined effect is that it becomes very difficult and hazardous for cars joining from the unfavoured Marlow and Maidenhead roads to join the roundabout, due to the speed and density of traffic passing west and east. Roundabouts are designed with the assumption that there will always be plentiful gaps for vehicles joining at each exit, and that traffic will regulate itself, but when cars join fast and frequently in one direction and in smooth single file, those gaps get smaller and scarcer. Accidents are common.

On Thursday last week, Britain voted to add an extra lane to the bypass.