Urban Roundabouts 3

Dutch roundabouts with peripheral priority cycle tracks can only handle 25,000 motor-vehicles per day (and 1500/hour conflicts at any crossing). In situations where there is room for a Dutch roundabout, there may very well be too much traffic. The roundabout on the north side of the river at Lambeth Bridge carries about 50,000 motor-vehicles per day. At that volume, it is arguably better to revert to using traffic lights.

This is a sketch design of a cycle-friendly controlled junction at Lambeth. I’ve used the space to provide left-turn filter lanes. These filter lanes have humps and raised crossings at both ends to keep speed down, but are otherwise uncontrolled. Cycle lanes are marked across the entry and exit to explicitly remind left-turners to check for cyclists. This eliminates some, but not all of the left-turn risk.

The space in the middle of the junction is used to provide right-turn lanes on all four approaches. Small right-turn pockets are provided to allow bikes to make right turns in two stages. Pedestrian islands are provided between each lane, to allow pedestrians to cross in stages on red, if they wish to. The best arrangement is probably to run each of the four entries in turn, rotating clockwise. This allows timings to adjust to cope with peak flows, but also simplifies the signals (allowing far-side secondary signals) and allows the right-turn pockets to get green straight away.

The original aerial imagery is from Bing.