Sunday 9 October 2011

Forecasting cycling levels on Blackfriars Bridge

Transport for London have published, following a Freedom of Information request, the analysis used to make its forecasts for the modal share of people using the new Blackfriars junction when it opens in 2012. Futilitarian has already critiqued much of the methodology, but I just want to focus on the forecast of cycling levels in 2012, which (as Futilitarian says) seems very much on the low side.

TfL forecast that cyclists will make up 6% of people using the junction in 2012:

Their explanatory note says the cycling figures were derived as follows:
  • In 2007/08 there were 1173 cyclists. This is based on JMP’S January 2007 cycle counts which have been uplifted by 17% to reflect seasonal differences in cycle numbers.
  • In 2012 there are assumed to be 1666 cyclists. Cycling numbers uplifted by 42%, based on modal share changes reported 2007-2009 in Travel in London Report 3.

Let's accept the 2007/08 figure. What then is the best basis for forecasting growth to 2012? TfL seem to have looked at the number of cycling trips across the whole of London in 2007 and 2009, in table 2.1 of the Travel in London report here. The table says there were 0.4m cycling trips in 2007 and 0.5m in 2009, which is consistent with a 16% increase which would gross up to 42% over five years.

But other data presented in the same report suggest that the cycling trend in London as a whole is not the best basis for forecasting what is going to happen on Blackfriars Bridge. Figure 2.14, copied below, shows that the number of cyclists entering Central London during the weekday morning peak rose sharply between 2007 and 2009.

In fact, the data behind the chart (.xls file here) says there was a 42% increase in cyclists entering central London between 2007 and 2009 (19,000 to 27,000). If sustained over five years this would amount to an increase of 105%, or a doubling.

We can also look at traffic counts carried out on Blackfriars Bridge itself by the Department for Transport (search for Count Point 56614 here, select 'All years', then 'Save results'). DfT counted 4,676 bikes crossing the bridge in a 12-hour period in 2007, rising to 7,395 in 2010, which is a 58% increase in three years, equivalent to a 97% increase if sustained over five years - very similar to the estimate we got from the Central London cordon counts.

So it does look as if TfL have underestimated the likely growth in cycling on Blackfriars Bridge between 2007 and 2012, by using an all-London comparator rather than the more spatially specific data available.

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