How do we reduce NOx in Bath?

Richard Speigal


by Richard Speigal

I won a hack!

This weekend I went to the BreATHe day by Bath:Hacked, which challenged us to look at traffic patterns from a two week ANPR study by BANES Council.

Bath has been asked urgently by the government to reduce pollution and improve air quality in the city. In response BANES has proposed a clean air zone which will mean charges for high polluting vehicles that come into the city.

NOx (a nasty gas produced mostly by diesel vehicles) is the prime target, and the ANPR study allowed the council to understand the problem more fully.

The data they provided was truly incredible: 4 million vehicle observations taken over a two week period were given to us. This told us a huge amount: How many vehicles were in Bath, what types of vehicles, where they were travelling and also (for my hack) the emissions they produced.

Map of vehicle movements in Bath
What 4 million vehicle movements look like

The dataset allowed us to estimate how far each vehicle had travelled, which I combined with Copert tables to estimate the emissions each had produced on its journey around the city.

It feels like I'm always stuck behind a bus or lorry, but the fact is that traffic in Bath is overwhelmingly made up of cars. Of 380,000 vehicles that went through the city over the two week study, over 80% were cars.

So target cars, right? Wrong.

It turns out that cars aren't entirely to blame for the city's NOx levels. "Amazon van man" needs to shoulder a lot of the blame too.

NOx emissions by vehicle type

A huge thanks to everyone at Bath:Hacked who helped organise the day, and my fellow hackers who worked so hard in finding useful things in this data. Robin and Nicola from BANES air quality team were also immensely knowledgeable and helpful. I couldn't have got this done without them.

For what it's worth, I'm now firmly behind the clean air zone proposals. On current estimates, only around 1 in 5 vehicles will be subject to the charge. This seems like a good price for clean air.

Can you fix NOx levels? I put together a playful web app so you can try.

In the meantime here's a video of what I learned on the day. Thank you!


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