Is 20 plenty?

20 MPH Streets, Bellevue, Edinburgh, 05/05/2017: Photography for Living Streets Scotland from: Colin Hattersley Photography - - - 07974 957 388.

20mph is becoming so normal for urban areas, it’s beginning to lose its controversy. That it’s far safer for pedestrians and cyclists is firmly backed up by evidence. But it’s often stated that vehicles travelling at 20mph pollute more than they do at 30mph.

20 – more dirty than 30?

Well, yes, if you drive a big old petrol car (like – shhhhh! – I do. Not very often, I promise.). At 20mph, my tailpipe emissions will increase – by about 2% CO2, and 8% NOx. So far, so bad for all of you 20’s-plenty fans. BUT… my particulate matter (PM10 – the coarse stuff) will go down by about 8%. If you drive a diesel car, it’s a different story: both NOx and PM10 emissions will decrease by about 8%, and CO2 by nearly 1%. It’s likely that at 20mph you’ll be wearing down your brake pads less and probably your tyres, too – so all those other particulates flying off your car will be reduced as well. (Information taken from an Imperial College study.)

Research has also shown that in a 30mph zone, drivers will accelerate and brake a lot more – speeds are more variable in 30s than in 20s. This sort of driving uses more fuel. 20mph may lessen your fuel consumption – and that in turn lessens pollution.

20mph also cuts noise pollution (see our earlier post on noise here).

So, although there are some increases in pollution as a result of 20mph, there is overall a decrease in pollution in 20mph zones.

Trotting along at trente kilomètres heure

Paris has just embraced 20’s-plenty. The reporter couldn’t resist the sentence “Car owners and commuters are fuming” (groan). Delivery drivers have, apparently, warned of greater waiting times and taxi-drivers complain that journeys will be longer and more expensive. One driver said “if I drive at 30 kph, the client starts complaining. If I drive at 50 kph (c.30 mph), I get arrested by the police. So I don’t know what to do! People take a cab because they’re in a hurry. At 30 kph, they might as well walk.”

And that’s perhaps the main pollution point: a 20mph limit means that it’s as quick to cycle (or scoot) as it is to drive – so you might as well cycle. There is evidence to suggest that 20mph zones see a reduction in traffic – it’s not as effective as restricting motor vehicles (as you’d expect), but it is what social scientists would call a nudge. 20mph is a disincentive to drive and therefore an incentive to travel more actively. More pedestrians and cyclists and fewer motorists = less pollution, healthier people and (we think) a happier environment.