Does re-allocating road space from general use to being a dedicated cycle way increase or decrease pollution?
“Obvious!” say the cabbies and others opposed to cycle ways – the motor traffic gets squeezed into the road space left for them – cars stuck for longer are obviously causing more pollution!
“Obvious!” say the cycle campaigners and others in favour – bicycles don’t cause any pollution and so as modal shift away from motor traffic (including “traffic evaporation”) occurs, there is obviously less pollution!
To my mind, both of these arguments have the broad element of plausibility – without further evidence surely it isn’t possible to say which factor dominates. But some people do seem to be sure… check out this notorious tweet from Professor Lord Robert Winston just a month after the East-West Cycle Superhighway opened:
Despite many requests from enmaddened cycling campaigners, I don’t think Professor Winston ever provided any evidence for this “measurable pollution increase”. He claimed he had spent a morning in City Hall discussing the data behind the tweet … but a Freedom of Information request submitted by David Kahn revealed that no-one at City Hall “can recall such a meeting nor the data which is referenced.”
By January 2018 he had backed down somewhat, merely calling upon the Government to publish figures before and after the introduction of cycle lanes, rather than claiming to know the data himself already. Though we guess what his view might still be as he continued: “The reduction of lanes which traffic can travel down means that there are more cars taking longer journeys than ever before at slower speed. [..] The evidence is that the internal combustion engine is less efficient and pollutes more at slow speed particularly when it’s waiting.” (Full Evening Standard article )
So yes we could wait for official Government figures, but hang on, hadn’t I see Andrew Gilligan’s pinned tweet?
This data is tantalising as. unlike Prof. Winston’s assertions, it looks to have basis in numerical fact. But it is unsatisfying, because the pollution figures are so noisy day-to-day it is hard to tell what is really going on. Also the data ends in October 2016, when the superhighway had only been open for a summer – what if there are seasonal effects.
Fortunately, the source for Gilligan’s data is the London Air Quality Network (LAQN) who make all their raw data available for download.
I downloaded the raw data for Upper Thames Street – the only pollution measuring site on the superhighway and batched up the data so I got the average pollution level in six-monthly chunks from June 2008 to December 2017. I chose six monthly chunks as the superhighway has now been open for 18 months and this would give me three pieces of data for the open period. Here are the results:
At first look the results are pretty startling, two of the three “best” (i.e. lowest polluting) periods in the last 10 years are since the cycle way has opened. The average PM10 pollution measure prior to the cycle lane opening 35.8 micro-grams per cubic metre. The average after the cycle lane opening is 28.9 micro-grams/m^3 – a fall of nearly 20%. Surely this is great evidence that the “greenies” are right and Winston is wrong – cycle lanes cause a DROP in pollution?
Well yes I did initially think that. But then stepping back, I realised I was being rather unscientific myself. OK, pollution seems to be lower recently, but it could well be a coincidence that that has happened at the same the cycle way has been open. I needed a control. Fortunately the LAQN also publishes PM10 data for the same period for Marylebone Road, which is largely a cycle-free zone. And here is the data for that location:
The pattern is broadly similar – the last 18 months have seen some record lows for this location too. In fact the average prior to June 2016 was 34.8ug/m^3 and after then was 28.1ug/m^3 – a very similar 20% decrease to the Upper Thames Street location! So the cycleway is not responsible for the pollution drop after all?
Qualitatively, it looks as though the PM10 pollution has been getting steadily better since the peak of 2011 on Marylebone Road, whereas possibly Upper Thames Street shows a bit more a step-change around the time of the superhighway opening. This would be consistent with Marylebone Road benefiting from the ongoing cleanup of buses (see e.g. this Guardian article) but Upper Thames Street less so, because it is not a bus route.
But that’s rather speculative – I think a safe conclusion form the public data is: The East-West Cycle Superhighway appears NOT to have worsened pollution, and may have made it a little better.
- PM10 pollution (the measure of number of “large” pollutant particles is the only type of pollution discussed here because it is the only one the LAQN produces figures for for Upper Thames Street. Many other pollutants are available.
- LAQN says that the data for 2017 is still provisional and subject to further ratification.
- I am not a domain expert. Just an interested bystander.
- The bucketed sample sizes are small! Do not overdo statistical significance.
- I wrote a very small Python script to process the data that is available from LAQN. Available on request.