Do bike lanes increase pollution?

THere has been some argument in London along the lines that the rollout of cycleways there has contributed to increased air pollution.

The argument seems to go that by converting traffic (i.e. motor vehicle) lanes with bicycle lanes congestion will increase and hence so too will air pollution. There’s been a few rebuttals in the media (for example, here and particularly here) and even reference made to real pollution data which suggests there’s been no effect.1

As one of the most prominent backers of the pollution theory is a scientist he’d presumably agree that the best way to test such a hypothesis is to use the scientific method. And as we’ve got access to pollution data, that sounds like a bit of fun. So let’s have a bit of a quick look to see whether there’s any merit in digging deeper.

The argument seems to be along these general lines:

  • The cycleways have reallocated roadspace from motor vehicle occupants to bicycle riders

  • This has resulted in increasing congestion

  • The increase in congestion has meant higher fuel consumption

  • The higher fuel consumption has resulted in elevated levels of pollution compared to the (hypothetical) situation that would have existed without the cycleways.

There’s a heap of data on motorist travel times in London that one could use to test the “increasing congestion” part of the argument. But what I’m interested in here is whether the cycleways have lead to a “measurable increase” in pollution.

There’s an air quality monitoring site on the Strand one block north of Embankment (where there’s a bi-directional cycleway that appears to have resulted in the eastbound traffic lanes reducing from two to one).2 There’s another monitoring site on Upper Thames St, where the easbound direction appears to have gone from two to one traffic lane to accomodate a cycleway.

Apparently the cycleways were both installed in early 2016. My limited understanding is that Embankment was fully open to riders in April 2016, and it appears from Google Maps Streetview that Upper Thames Street opened around the same time. During construction there is presumably disruption equivalent or greater than that experienced after the cycleways opened. I’m going to assume here that construction of both started at the beginning of 2016 and was complete by April 2016. So the before case is prior to 2016 and the after case is from April 2016 onwards. What we’re interested in is whether air quality at the two sites has been affected by the loss of traffic capacity in each case.

The air quality monitoring site near the Embankment measures Nitrous Oxide (NO) and Nitrogen Dioxide (NO2) concentrations. Data is available from May 2015 to present with a gap around May 2016. I’ve used the 24-hour running average concentrations in this analysis; the 15-minute data shows similar trends but with (obviously) more fluctuation. Moreover, I’ve only looked at the period from April to December (inclusive) in each of the two years before and after the cycleway was installed. Air quality will be strongly influenced by, among other things, climatic conditions - so this will at least partially control for these effects.

Turning now to the results, visually there doesn’t appear to be much effect below, and indeed the average concentration after the cycleway was installed is marginally lower for both NO and NO2.

Air quality at the Strand

Figure 1: Air quality at the Strand

The Upper Thames Street site measures only particulates, and then only those 10 mm or greater in diameter (PM10). Again, if anything there appears to have been a decrease in pollutant concentrations after the cycleway was installed.

Air quality at Upper Thames Street

Figure 2: Air quality at Upper Thames Street

Now before we go jumping to conclusions and claim that the cycleways have in fact decreased pollution we really need to control for all the other things that may be going on. This would be things like changes in traffic volumes, speeds and vehicle composition. We could build all sorts of models to control for these confounding factors, but let’s keep it simple. Maybe, just maybe, all sites in London exhibited an improved in NO, NO2 and PM10 over the period but these two sites experienced a much smaller improvement because of the purported adverse impacts of the cycleways? As a simple test, and for kicks, I’ve pulled out the data for a couple of other inner city streets: Horseferry Road and Marylebone Road. Neither site has had a cycleway installed on them from what I can make out, although I really have no idea whether there may have been other things going on over the period that could have affected traffic volumes and speeds (and hence pollution). Both seem to have also experienced a decline in pollution concentrations.

Horseferry Road

Figure 3: Horseferry Road

Horseferry Road

Figure 4: Horseferry Road

So now let’s bring the data together a bit. Below I’ve illustrated the change in average concentration compared with the year before the cycleways were built - i.e. 2015. Based on this I’d conclude two things:

  • There has, in general been a trend towards decreasing concentrations of NO, NO2, and PM10 over the period from 2014 to 2017 at all four sites, and

  • If anything we’d conclude the two sites nearest cycleways (the Strand and Upper Thames Street) have experienced declines greater than the two control sites.

Figure 5: Distribution of concentrations by pollutant and time period

Now we could go on and on at this point and start estimating models to try and ascertain an effect and run all sorts of statistical tests. And if this wasn’t just a blog post maybe I’d do that. But that feels a bit over the top at this point; we can see no prima facie evidence of an effect so it just doesn’t seem warranted to pursue this any further.3

So, based on the evidence we have before us, we can only conclude that we have no evidence that cycleways have increased pollution. Fair enough. But really, that’s not the point of this post. Rather, the point is this: it’s all very well to go around theorising and offering conjecture about something (and shouting about it on social media and in Parliament), but to be taken seriously we need evidence to back our theory. And if there’s no evidence then we must revisit our theory. To quote Daniel Moynihan:

Everyone is entitled to their own opinion, but not their own facts.

Feels to me like we could with a bit more of this sort of thinking right now, rather than just tweeting off whatever is on our mind. Not that I can think of anyone famous that does things like that…

  1. Credit is due to the comment by StevieDee in this article for linking to air quality monitoring data.

  2. I’m no expert on air pollution, but I’d take a stab that a monitoring site a block from the treatment we’re interested in is unlikely to detect any change unless it’s really dramatic, or has led to congestion on the Strand as well. Given the claim in the tweet is pretty dramatic (traffic is at a standstill, apparently) then presumably nearby routes like the Strand could also be affected?

  3. Even if was an obvious increase in concentrations after the cycleways installed this wouldn’t necessarily mean it’s the fault of the cycleways of course. Correlation isn’t causation. But it would at least warrant further examination.

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