So I wanted to consider the Old Kent Road in Southwark, and wonder: could we fit a cycle lane down here?
Update May 2017: Tfl have proposed exactly this, and the majority of respondents to the initial consultation are in favour. https://southwarkcyclists.org.uk/okr-aap-responses/
Space for cycling?Firstly, some measurements. Starting at the south-west end, where the New Cross Road becomes the Old Kent Road, I took measurements using Google Maps' measurement tool and aerial footage. So these are not exact numbers, but they're a good place to start.
30.5m (Ilderton Road), 7 lanes
23.5 (Leo Street), 4 lines and a wide painted meridian
26.5 (Gervase St), 5 lanes
23 (Hillbeck Close), 4 lanes
18.7 (Sylan Grove), 4 lanes
19.5 (Devonshire Grove), 4 lanes
28 (Devon Street), 5 lanes
22.5 (Murdock Street), 4 lanes
30m (St James' Road), 6 lanes and a wide planted meridian - outside B&Q
22m (Glengall Road), 4 lanes and a wide painted meridian
So the space between buildings widens and narrows, but is generally over 22.5m wide. Keep that figure in mind.
2.5m pavementsThe pavements are 2.5m - 3m wide on each side, in places up to 6m. Presumably, this is so that pedestrians can walk several metres away from fast-moving motor vehicles on the A road. But if there is a cycle track, then pavements wider than 3m should not be necessary; pedestrians will already be well away from the traffic. If pedestrian flows are low, then 2m pavements may be sufficient, but the planned regeneration of the area (including new tube station) presumably aims to change that. Let's say 2.5m for now.
2.5m cycle tracksIf we want to add a one-way cycle track on each side of the road, that would be 2.5m on each side. If we expect only 150 bicycles an hour, then 2m might be ok; but if there was a continuous segregated track that went from New Cross to Elephant and Castle, I think we can assume slightly more in rush hour.
3m traffic lanesA traffic lane wide enough for a London bus needs to be 3m wide.
Sources: Keeping Buses Moving, LCN Design Manual. (Note, 3m is the minimum - tendency for other drivers to stray over the line means more is better if possible, to avoid blockages.)
Adding it up...So with cycle tracks either side, and pavements, and 2 traffic lanes in each direction, we've used 22m of space. If we add kerbs to separate the cycle tracks from the motor traffic, which would be preferable, let's call it 22.5m.
This is the width of most of the 4-lane sections of the Old Kent Road. We can easily fit in the 5 lane sections without losing a traffic lane, as the road gets much wider where they are. There are a few pinch points where the current number of traffic lanes could not be retained with 10.5m of walking and cycling infrastructure, but I think that a slight narrowing on both sides for a short section could be tolerated, provided that segregation was maintained throughout.
Safety at unsignalised junctions needs considering: left-turning drivers may collide with cyclists by approaching from behind, then cutting into their path. These junctions will either need lights, or banned turns, or the pavement and cycle track bending away from the main road, so that the driver is at right angles to pedestrians and cyclists when they cross paths, and travelling slowly.
TreesLots of London plane trees are planted along the edges of the pavement, and may need relocating to be out of the new cycle tracks. However, Kew Gardens was moving oak trees with a horse and cart in the 1840s, so I think the technology is not out of our reach.
These trees do reduce the effective width of the pavement. However, the A2 is either a main road or an arboretum, not both. If the trees are both too large to fit alongside cycle tracks, and too necessary to remove, then we should lose a lane of motor traffic. If the movement of people and goods is so necessary that we cannot lose transport space, then we definitely need a cycle track - it is a much more efficient use of space than a car lane.