Evan Davis: Perché così tante persone infrangono i limiti di velocità di 20 miglia all’ora

Actually quite a good article but a few issues.

The Belfast study mentioned showed a reduction of casualties on 20mph limits, but spotted an increase overall when you factored in other roads. The main reason for that was thought to be displacement; traffic was actively avoiding the 20mph roads and using other, less suitable roads and having their accidents there at the same/greater rate. Similar thing happened in Manchester a few years previous – in both cases they removed the default 20 in favour of targeted limits.

This displacement is a big issue; I’ve seen some TomTom data on it in Wales where traffic has gone from using a well designed A category road with great active transport infrastructure to an uncategorized road with no pavement and lots of walkers and cyclists using it- it was something like a 250% increase in traffic which is crazy.

The data in Wales is riddled with statistical issues; prior to the limit reduction they found the police weren’t recording speedlimts correctly and most were the default value. It’s a shame, but it makes any comparison impossible.

20 limits are a powerful tool for highlighting dangers, when targeted.

Are you trying to tell me that people who ignore 30, 50, and 70 mph speed limits also ignore 20 mph speed limits?

/s

My cruise control doesn’t go below 25 speedo. Honestly it should be 25 speedo or about 23 which is what people actually do on it but they probably assume that people will then do +3-5 over any limit they set so set it lower.

I think targeted 20mph speed limits are a good thing. But not as a blanket rule that many councils are now implementing.

There are lots of headline stats in there about the roughly one third reduction in collisions as well as deaths and serious injury. But we need a greater understanding of the causes of those accidents. Was it always the driver at fault or did the pedestrian/cyclist do something that meant it was unavoidable. Changing a variable without understanding why it has an effect is not very helpful.

We used to have adverts on the TV all the time for the Green Cross Code Man and ‘Stop, Look, Listen’ is very familiar to previous generations. Now though there appears to be nothing and by constantly pushing the perception that its drivers that are the only cause of accidents means there are plenty happening that could be avoided by other road users being more aware.

It wouldn’t surprise me in a few years if there isn’t a push to lower them to 15 followed some time later by 10 and eventually someone will say ‘you know the Red Flag Act has a lot of merits so we should bring that back’.

Its because more people don’t consider the law to have merit. Alot more people followed the 30mph speed limit because it made logical sense to them and going faster is clearly a bad idea on many roads. When it comes to 20mph the actual affects of that speed limit seem arbitrary in the moment to people and when they see a clear road, they see no good reason to stay at that speed. They will only follow it if there is some consequence to not following it (speed cameras).

Its the governing by consent issue.

Is this really limited to people breaking 20mph limits? Or is this just drivers regularly breaking any speed limit?

Collectively, as a majority we abide by laws that seem reasonable.

20mph limits in most scenarios are not reasonable.

It’s as simple as that.

Because it’s been rolled out without much thought

There’s plenty of roads near me that could easily be a 40, it was a 30, now it’s a 20

Town centre? Housing estate? I get it

The comment section of that BBC article is rather revealing of the sorts of people we share the road with.

I’m not following laws I don’t agree with unless you make it untenable not to

The statistics in that article are a car crash. The improvement in safety for a reduction from 40mph to 30mph is framed as a ‘relative risk reduction’ of 3.5-5.5 times (let’s call it an average of 4.5 times) and is only discussed in terms of pedestrians (the most vulnerable group). That’s a 78% absolute risk reduction.

Whereas all the statistics for the 30mph to 20mph are given in absolute risk reductions (22-35%, let’s generously call it 30% on average). And relate to all Killed-Seriously Injured (seemingly) which would include other non-pedestrian road users.

These numbers just aren’t comparable at all. Evan Davis is notionally an economist, he should do better than this in terms of presenting data and statistics.

Furthermore there is no consideration of displacement to other roads, nor of negative externalities that might occur due to slower speeds (what impact does it have on fuel economy, emissions and journey times). There has to be a trade off between speed limits and other factors (otherwise we’d limit vehicles to no more than walking speed), but only by looking at the full picture (with comparable data!) can we make that decision.

I sat on a bus in London, just for fun turned on the GPS speedometer.

Of course it was doing 25-30 mph between stops (when it could). Even buses can’t hold those unrealistically low speed limits.

It’s not about safety, it’s about opening up a new revenue stream.