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Real impact of 20 mph speed limits

Was it 85% of drivers who 100% ignore any urban 20 mph limit? This study shows that in Edinburgh, mean speed fell by just 1.34 mph and median speed by 0.47 mph, measured 12 months after implementation. But somehow the casualty rate fell by 39%.

In Belfast, speeds did not drop and the casualty rate fell by just 2%. The difference is ascribed to differences in the design of the schemes - Edinburgh's is more comprehensive through the city.

What can we conclude? I can't see how the tiny speed reduction can possibly be the factor that cut Edinburgh's casualty numbers, though I suppose many of the streets the limit applies to were already slower than 30 due to traffic or restricted width due to buildings or parked cars. So previously faster roads may be over-represented in the casualty reduction.

I can't really escape the conclusion though that most drivers really don't give a fig (or a stronger word) for speed limits.

Here's the paper:

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David9694 | 1 year ago
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Cornwall could get 20mph speed limits on every residential street

The council is taking a 'sign-only' approach which will cost £3.8 million and will be rolled out completely by 2026

crack on, Cornwall. 

chrisonabike | 1 year ago
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I find this interesting in general (haven't read this paper yet). On the one hand the ideas from NL etc (even understood in the UK to some extent) say the way to do this effectively is to use human psychology and physically change the street design and environment. Covered by many but Notjustbikes has a good video:

And Robert Weetman has another excellent article on street signs and human behaviour:

On the other hand I want to believe some studies which indicate that while people don't obey signs without considerably more enforcement than we can muster merely changing signs from eg 30 to 20 DOES often reduce the average speed. (Can't find link ATM but think the Scottish 20s plenty campaign had this evidence).

So maybe we should do both? Take the cheap option - make the sign change in some places now? Optimistically, knowing this might have little effect. But we shouldn't pretend we've "fixed it" and we shouldn't give up on the "real" solution of making the limit "self- enforcing" through street design.

TheBillder replied to chrisonabike | 1 year ago

The Robert Weetman article is very interesting - more than partly because it's a junction I know very well. He makes good points, especially that people will not obey signs unless they see a reason (which may include fear of detection).

I think he misses a bit of detail around stop signs though. They are usually placed where sightlines are poor, meaning that you have to stop and then move out a bit to see if further progress is safe. So is the stop sign really just trying to ensure that the moving out speed is really low? A full stop does seem pointless at that one, as you just have to restart.

Secondly, drivers will follow others through pinch points on the assumption that they can get through. As no traffic can arrive from the right at that junction, and anything approaching from straight ahead will be easy to see, it's not illogical.

My conclusion is that unless we can enforce compliance, we're in a "policing by consent" mode, and therefore we have to try to make road rules that people can see the need for, with enforcement where that's not possible.

The Leith Walk to London Road turn is a case in point. It's No Left Turn because of the pedestrian crossing phasing as far as I can see. Drivers are unlikely to realise that reason, especially if unfamiliar with the road. The diversion is bats, for want of a better word. I think a redesign is called for.

mattw replied to chrisonabike | 1 year ago

Interesting piece. It illustrates quite well why I often disagree with Mr Weetman :-), who I think sometimes reads his assumptions into his analysis to an extent when the evidence says otherwise, and rather underplays cultural factors.

I'll try to find time to write it up, but I have a big dentist visit today.

IMO tt's an abortion of a junction layout, though. eg Just where the sightlines are needed to the left as you stop at the STOP line as instructed, the approximately 1.3m wide narrow pavement you are supposed to look down along the high wall is made 0.3m or so visually narrow because someone has put a lamp post at the back of the pavement a few m away which blocks the most critical bit of the sightline - so you have to roll forward to see enough to even begin to make a go / no go decision.

They've done it on the other corner too, with the addition of some sort of small even more sticky-out box on a post at what look slike driver eye level - but that is on the entrance to a one way route so can be ignored.

Plus the Council has put parking down the other side of the road to within 10m of the junction, which squeezes all the traffic towards the people who can't see from behind the STOP line because of the lamp post.

Plus all the corners have loose radius GO FASTER curvatures, which subliminally undermines the STOP message.

I would be interested in @TheBillder's comments on these aspects.

TheBillder replied to mattw | 1 year ago

I agree, in short. I simply don't understand why stop signs are used. They only ever seem to mean that the sightlines are bad and extra care is needed.

Moist von Lipwig replied to TheBillder | 1 year ago

Out of interest I've just had a look at Traffic Signs Manual Chapter 3 regarding directions for their use.   Seems thats the point, stop signs are to be used where the visibility is poor.   As most qualifying junctions already have them, not expected to be used as new junctions should * not be constructed without adequate visibiity.   Also interestingly, the police should be consulted on their use to ensure enforcement of them.

* There's that 'should' get out clause again.

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