Why do 20mph campaigns not make more of this? It’s obvious to pretty much everyone, including me, a big fan of 20mph, that reducing the speed limit will increase the journey time. But it’s wrong.

I’ve calculated for a long time that any increase in journey time is negligible (see calculation below) but clearly it’s more complex and more positive than that!

So, given that this must be the assumption of everyone reticent on 20mph despite all the other benefits, shouldn’t this be a headline of every campaign?

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Theoretical maximum increase in journey time:
A typical journey might start in a 20mph zone, within a few hindred metres move onto main roads, before perhaps ending in another 20mph zone. I acknowledge this excludes delivery drivers, who might spend a lot of time within a zone.

This being the case, the maximum theoretical time saving on such a journey would be 2 minutes, but in reality it will be less than one minute. I calculate this as follows:

• “worst-case” scenario (also makes the maths easier!) of having to drive 1 mile out of and back into a 20mph zone
• 2 miles out and into a zone at 30mph takes 4 mins
• 2 miles out and into a zone at 20mph takes 6 mins

The real-world maximum will be far lower because:

• there are few if any places where it is one mile to the nearest 30/40mph road
• it’s impossible to average 30mph on a 30mph street (without breaking the limit, but even then impossible in most instances). My guess is that a typical average speed is well below 20mph.
• my guess is that the average speed on a 20mph street will be very similar to a 30mph street, because it’s governed by infrastructure and traffic far more than by the speed limit. The only exception might be night-time driving.