Roundabouts are probably the ideal junction as far as the Association of British Drivers are concerned. No lights, no pedestrian time to stop cars getting through, cars get right of way over any schoolkids trying to get from one island to another. Sadly, queue theory, one of those pesky little things the ABD don't agree with (along with pollution statistics, finite fuel supplies and global warming), kicks in here. It is impossible for more vehicles to enter a roundabout than leaves it. If the majority of vehicles are heading in the same direction -here down St Michael's Hill and then to the city centre-, then the maximum load of the roundabout is the same vehicles/hour as that single road can sustain. At this point, law abiding cars will stop and wait patiently for the junction to clear before pulling out. Here we some some waiting patiently and only blocking traffic leaving Cotham Road.
Soon they are joined by a car full of school kids, WP05VXC. There is still enough room for cars turning right off St Michael's hill to get through to Cotham Road, though visibility is hampered.
As the traffic crawls forward, a Range Rover pulls out to take up the space. No cars can turn right from Cotham Road to Hampton Road. No bikes can get over to go down Cotham Hill -it's bikes only in that direction. Hampton Road to Cotham Road is the only routing that is still live.
The range rover L12BXF is soon joined by a red Toyota from Cotham Hill. The RR hasn't moved that far forward, so now the line of cars continues across the junction.
A pickup from Cotham Hill now seizes the opportunity. Unfortunately, the Toyota (kids inside again) prevents the pickup behind it from even completing the turn; it has to sit with its indicators on. Blocking the exit for any vehicles heading up Hampton road, such as the two vans behind it.
There we have it. A complete roundabout failure in the space of 30 seconds. The throughput of the roundabout becomes that of the slowest moving road off it: 0 vehicles/hour, so delivering the same throughput to anyone trying to head in any direction off the roundabout.
What could be done to prevent this? Perhaps we could have someone issue spot fines for vehicles blocking a junction? Perhaps a local newspaper could start a campaign against people who seem to be going out of their way to inconvenience cars? Here at Bristol Traffic, we think such persecution is uncalled for, and would be another sign of an anti-car city.