Where does Bristol's pollution come from?

20 mph is looking like one of the big manufactured controversy issues of the election. Not, say, library closures, or the sale of the Avonmouth ports and apparent abandonment of Avonmouth. No, 20 mph limits, along with parking restrictions. Who says the electorate isn't worrying about city wide issues?

One of the pro-speeding campaign claims is "20 mph causes pollution".

Bristol has a pollution problem. It has an Air Quality Management Area which covers the centre, and also follows the A38 and M32 out of town. And apparently even the council admits it is killing people.

Where is that pollution coming from? Well, for NOx, the nitrogen oxides which VW tried to cover up, it's coming from Diesel cars. Bristol's NOx problem would be significantly reduced if everyone driving diesel today switched to petrol. Not hybrid or electric: petrol would be a start, especially modern cars that turn off the engine when stopped. And with gearing such that they can cruise at 20 mph nicely.

What is that ratio of petrol:diesel in the city?

We have no data, though the council's ANPR camera arrays will have that information if they could sit down and analyse it. Otherwise, it's that same self-selecting short term surveys which we are always so disparaging of. Here's ours



This was taken by a tax dodger cycling up the shared use pavement on Bridge Valley Road. While most shared use pavements are technically, "bollocks", this one is worth using. The speed of the cyclist (4-6 mph) is at a significant different from that of the motor vehicles, so you will only get passed by everything. The speed limit here is 30 mph, its off the portway and cumberland basin flyover, drivers may not have seen a bicycle that morning and forgotten they exist. And there's almost never anyone walking. If there's a fault: pavement surface quality and sweeping; the usual.

Going up the hill on a bike, you can see the motor vehicles come in waves; a queue heading west from the cumberland basin builds up until there's a right turn light, then they set off. You get a minute or two of quiet, 30s of traffic, then repeated until you reach the top. The video shows one of the groups of cars and vans.

Let's look at engine type, confirming on the MOT checker site whenever the registration number was readable and it wasn't immediately obvious

• white (VW?) van. Diesel
• white van. Diesel
• BMW 330d. Diesel. Passed MoT on Feb 22, 97K miles.
• VW Passat. Only available in EURO-test-rigged diesel models. 100K miles on 16/11/15
• Audi A6. Diesel
• LGV. Diesel
• Ice cream van. Diesel with dodgy fan belt.
• white van. Diesel. 84K miles
• Peugeot 308 Diesel. (Passed MoT on second attempt feb 17; 78K miles)
• Mercedes SLK230K, petrol. 50K miles after 15 years of use.
• Ice cream van. Diesel

11 vehicles; only one petrol engined. That's a ratio of 10:1.

As stated, this is a self-selected dataset, but even so, it didn't involve a cyclist going up and down the hill repeatedly until a group came by that were nearly all diesel. It was just the outlier. If one more petrol car had got through the ratio would have dropped to 5:1, awful, but not as bad.

We don't have any real datasets of the petrol/diesel ratio, and this being an out of rush hour measurement may show bias towards working vehicles over commuters (unless a lot of people go to work on an ice cream van, that is).  But it does point a lot of the blame at our NOx problem at Herr Doctor Diesel and his inventions.

This also means the mercedes owner has something to be smug about, as well as having a vehicle still worth more than the Peugeot or the Passat: every mile they've driven has chucked out a fraction of the pollution of the other vehicles, as well as only doing a quarter as many miles/year as the others. Too bad their wingmirrors cost so much to replace.