A week ago,  the Archbishop of  Canterbury was convicted of speeding in a 20 mph limit and fined a total of £510 (as reported e.g. here). Apparently administrative errors prevented him from paying the fine in a timely manner and thereby avoiding a (virtual) trip to the Magistrate’s Court.
The minimum penalty for speeding – unless you are lucky enough to dodge it by being sent on a useless speed awareness course – is £100 and 3 points on your license. Presumably Welby would have received this but for the “administrative errors.”

So much so topical. But lurking at the back of my mind when I read about that was another news story, this one from a month ago, as for example here.

Use of 20mph speed zones in built up areas ‘usually advisory’ and many CANNOT be enforced, police chief admits

[I don’t know why I picked the Daily Mail’s version of the story to share with you. Their site is near unusable unless you block scripts/ads.]

A police chief has admitted 20mph speed zones in built up areas are ‘usually advisory’ and many cannot be enforced.

Giles Orpen-Smellie, Conservative Police and Crime Commissioner for Norfolk, made the admission despite a third of local authorities having rolled out the measures in recent years.

Speaking at a public meeting, he said: ‘It’s extremely difficult for police to prosecute… if it goes in front of magistrates they will throw it out.’

Well, Welby’s case was certainly not thrown out.

Personally I am ambivalent about 20 mph zones*. I like them when I’m a pedestrian; on the other hand, if I’m in a car trying to get somewhere… not so much. Nonetheless, I dutifully stick to the limit – and my anecdotal observations tell me that I’m in the minority. Only yesterday a brand new MG EV zoomed up behind me in a 20 limit (you can’t say “roared up” any more), wanting me to speed up. I declined with a sneer. When it finally overtook, I was surprised to note that it was a taxi.

There is a 20 mph road near me where there have been a succession of wildlife casualties over the years. Mostly these have been woodpigeons; not the rarest of birds, but apparently now amber listed. Then there are the hedgehogs – whose response to an oncoming car is not exactly adaptive. The woodies do like to flirt with the passing cars – but at 20 mph, they can afford to. At the speed some drivers go, not so much.

According to our soi-disant government, 20 mph zones are popular. They are:

“…supported by the majority of residents and drivers.”

But the majority of drivers – at any rate, the majority of vehicles – do not adhere to the 20 mph limit. In fact the data shows that more than half of all cars travelling [in free flowing conditions] through 20 mph zones went as fast as, or faster than, the Archbishop. Note that only the first two columns are compliant; 87% of cars exceeded the 20 mph limit.

In general, the lower the speed limit… the fewer cars adhere to it. (I don’t know about you, but one of my least favourite types of driver is the Zen 40 driver. This type proceeds at what seems a stately 40 where there are no limits… and then at a seemingly reckless 40 when they pass through a 30 zone.) Other data shows that introducing a 20 mph limit has essentially no effect on road speeds, but here it appears that this is “real-world” data, i.e. all traffic, not skimming off only the vehicles that were were not slowed somewhat by conditions. The figure comes from a 2018 report by Atkins and others for the DfT, available here (also apparently the source for the claim that 20 mph limits are popular). The median speed in a residential area dropped by less than 1 mph when a 20 limit replaced a 30 – but as you can see, the speeds in the sample are in general a lot lower than the “free-flow” speeds in the frequency distribution figure. (In free-flow conditions, the data [same table as the above figure] shows that average speeds are 5 mph slower in a 20 limit than they are in a 30 limit; 31 vs 26 mph.)

What about the justifications of 20 mph lengths of road, excepting pigeons? Well, there is data that shows that accidents involving pedestrians at 20 are less likely to cause fatalities than accidents at 30, although your probability of dying if you are hit at 30 is still “only” 7%. The figure is from a 2010 report by D.C. Richards, again for the DfT.

EVs

One thing that is not often remarked upon is that, since EVs are heavier than ICE cars, they are likely to be more dangerous for pedestrians when shifting at the same velocity. The braking distance – despite rumours on the internet – does get longer the heavier the vehicle. (The internet rumours rely on elementary mechanics to prove that mass is not relevant to stopping distance; it appears on both sides of the equation when you equate the energy expended in stopping a moving object with its kinetic energy and therefore disappears. And this is true for sliding block puzzles – but not where tyres are involved. It’s possible to prove a lot of wrong things with elementary maths. My physics teacher challenged the class to choose between being shot or hit by a car. Probably everyone picked the car. But the “correct” answer was the bullet – as the teacher soon demonstrated with mathematics. Nevertheless, I would still have taken the car then, and would today.)

The performance of EV tyres depends on the tension between maximising grip and minimising wear and rolling resistance. This is the case for all tyres of course, but rolling resistance is a particular issue in EVs, where range is a key selling point. Add to that the fact that EVs are heavier, and we are left with a possible safety issue. I am not the first to notice this, although the headline I’ve picked out in the snip perhaps exaggerates matters just a tad:

Conclusion

In general, although people are in favour of 20 mph limits, they ignore them. There seems little point in creating them – they have little to no effect on driving speeds. But if people did stick to the new limit, it seems likely that pedestrians and pigeons alike would be safer. Was the Welby fine fair? No. 53% of cars go as fast or faster than Welby went when they travel through a 20 limit; the people say they are in favour of a 20 limit when asked, but their actions demonstrate the opposite. Should a minority be fined for something the majority do? That does not seem right to me.

However, I don’t think there is any reason for side roads in residential areas not to be limited to 20 mph. For through roads, the debate is more nuanced; you have to balance the needs of people trying to get somewhere with the residents who want calm streets. But if you actually want people to slow down, only speed bumps or other physical paraphernalia will help. A signpost – or a beacon as in the featured image – is generally ignored.

Notes and Data

* There is a distinction in some reports between a 20 mph limit (signposts only) and a 20 mph zone (with added bumps etc). The latter seems far more effective than the former.

The data for the frequency distribution comes from table SPE0111 (for 2021), at the link given above the figure.