Cycling doesn’t have a “very high modal share” on Thames crossings 1

A quick note on something that keeps popping up and bugging me — a factoid that has been passed through Chinese whispers to become something entirely different, and entirely incorrect. Triggered on this particular occasion by reading The Ranty Highwayman’s account of a talk by Phillip Darnton:

He went on to suggest that we perhaps need to get away from looking at average modal share across the UK as a measure of success and concentrate on areas which are doing well as our base. For example, he cited Cambridge has having a cycling share of 23% for all urban trips and picked central London’s bridges as being places with very high numbers of modal share for cycling.

The original fact comes from 2011:

The original fact was that a quarter of vehicles crossing the Thames on road bridges in Zone 1 during the morning rush hour are now bicycles, and bicycles outnumber cars in that place and at that time. This is determined by manual traffic counts made at specific locations around the city — including each of the crossings — conducted periodically by TfL.

The original fact was promoted by campaigners who suggested that it highlights the need for authorities to be providing space for cycling — and highlights that some of the space could be reallocated from the private car, use of which is in decline.

The fact has been mangled into something about modal share, which is not even slightly the same thing. To suggest that 25% of vehicles on Zone 1 bridges amounts to a 25% mode share for cycling is obviously absurd. TfL rush hour screenline vehicle counts differ from mode share in failing to take into account any of:

  • Pedestrians crossing the bridges (and entirely ignore those traffic-free (and always packed) bridges, like Millennium and Jubilee)
  • Loading on the buses – every bus is One Bus in a vehicle count, regardless of load
  • Passengers on the railway bridges
  • Passengers in the tube tunnels
  • Passengers on the waterbuses
  • Motorists who are making a journey into Zone 1 but doing so by detouring through higher-capacity Zone 2 bridges and tunnels like the Blackwall Tunnel
  • Non-rush hour traffic, which always has a lower share for cycling

Most people get into Zone 1 by public transport, and get around inside it by walking. So screenline vehicle counts entirely ignore what are by far the main Zone 1 modes, and so bear little resemblance to modal share.

That’s not to say the screenline data isn’t useful. But it’s not what people are saying it is, and given the very specific limited methodology, it has very specific limited uses.

I’m reminded again of Dave Horton’s comments:

there are two clear and present problems which bedevil UK cycling advocacy: one is the requirement to trumpet any and all gains, however minor or potentially imaginary, in order for us to legitimate and reproduce ourselves as advocates; the other is a rush to interpret any sign of growth in cycling as both ‘good’ and a clear sign that investments in cycling are paying dividends, when a wider and more critical analysis might concur with neither.

Incidentally, official traffic counts in other places can just as easily give a deflated as an inflated picture of cycling. The DfT counts, for example, focus on busy main roads rather than the sort of "quiet routes" that cycling has been pushed away to over the years. And they completely ignore off-road paths. So in the rare examples where a passable Sustrans rail trail provides an alternative to a main road — between Bristol and Bath, for example — traffic counts will present a massive underestimate of bicycle share.

But this is all getting too far into tedious details for what was meant to just be a quick cautionary note.

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