Air pollution data has been used by opponents of the Bowes LTN to try to prove that air quality has got worse as a result of the trial. But has it? We take a more accurate look at the data, by stripping away the biases like lockdown and seasonal changes, and come up with quite a different result…
Back in February we produced this piece about the air pollution around the Bowes low traffic neighbourhood. It was early days so the data was limited and, while there was no reason to think that pollution had increased, that didn’t stop LTN opponents from cherry-picking data to prove otherwise. So now, in November 2021 with the LTN in operation for 14 months, we have crunched the data to find out what has really been going on. All the data is freely available and easily accessible via the London Air website.
Let’s start with a clear example of cherry-picking with an example used by the prospective councillor Kiran Mistry who will be standing as a Conservative at the next local elections in May.
The example shows a 20% rise in pollution. But in order to do this, they have had to include a few biases.
The first bias relates to the seasons. Engines, particularly diesels, are more polluting in the winter months. They take longer to warm up and car designers are allowed to turn off emission control while doing so. You also have to add more ‘background’ NO2 from boilers.
The increase can be clearly seen in the graph Fig 1 below where the pollution levels oscillate over the seasons.
Dividing the year into autumn/winter months and spring/summers months we have the 6 months pre-LTN figure, used by our cherry-pickers, entirely in the spring/summer. The 8 months post-LTN have 6 autumn/winter months (we are not sure why they didn’t choose to go back 8 months except to skew the data a little more).
The second bias is by including the lockdown. Officially lockdown was March 26th until easing of controls on 14th August. But during the first few months the roads were very clear of traffic and this will have had an impact on the figures. So now we have more data, and extend the two pre and post periods so they match, we have this (Fig 2.).
So not a 20% increase but a 10% decrease. But in fact, the two biases still remain and, if we want to find out what really happened to air pollution, we should do our best to get rid of them.
The best way of removing them would be to compare pollution levels for comparable months. The trial has been running from September to November so we have 14 months of full data. To remove the lockdown bias one of the data sets must be pre-covid – the most recent would be September 2018 until October 2019. If we draw a graph and crunch the numbers for those months we come up with a post-LTN decline in air pollution of 31% (Fig 3.).
There is one more bias that we can easily remove. As vehicles get cleaner they emit less NO2 and, as vehicle stock is replaced by newer models, you would expect the NO2 pollution to decline. Using Bowes pollution figures we can demonstrate this as a reduction of just under 2% a year (Fig 4.).
Applying the 2% decline a year to our figures we end up with a figure of 28% reduction in NO2. That is a fairly hefty reduction (Fig 5.).
Of course, there will be many arguments about how many people are working from home and that there were some lockdown measures over the period the LTN was in force. But the same people are making the argument that the traffic has been terrible over the same period. We are not sure how you reconcile more traffic with a decrease in air pollution. In fact, experts who model pollution use traffic as a proxy for air pollution as it is too expensive to have accurate pollution monitors everywhere.
So what has caused the reduction in air pollution? If people were happy to blame the LTN for the increase we don’t think it is unreasonable for it to take credit for the reduction too. While it is unlikely that the A406 traffic will have declined, it is likely that there are fewer vehicles movements around the LTN and this can only be because some of the traffic has evaporated. We will need to wait for the traffic monitoring data for the area to find that out. However, it looks like a positive move towards improved air quality and healthier streets so let’s get on with creating more low traffic neighbourhoods.
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