Roundabouts break down if there is an uneven flow distribution (e.g. peak hour in one direction), making it hard for other approaches to get a look in. So what may work perfectly fine during most of the day may suddenly become a big mess come rush hour. At some point, that will be the dominant issue with a site.

Those same peak flows also mean that cyclists and pedestrians often don’t get a safe time to get across either (especially less confident riders and impaired pedestrians) – all that “smooth flow” also means no gaps in traffic. Although roundabouts have a fairly good safety record in general, they can be worse for those walking and cycling. This is particularly an issue for multi-lane roundabouts; well-designed single-lane roundabouts that slow traffic down can still be reasonably OK.

Roundabouts certainly have their place as basic intersections get busier (there’s a couple of locations near my place that I think would benefit everyone from having roundabouts; St Martins/Burnbrae and St Martins/Centaurus). But eventually even they have their limitations in terms of safety/efficiency.

What’s wrong with roundabouts that traffic lights do well ?