20 mph Zones

Discussion on Advanced and Defensive Driving.

Postby gjohns » Sun Aug 09, 2015 6:01 pm


I am confused (not uncommon). Careful reading eg

http://www.rospa.com/rospaweb/docs/advi ... tsheet.pdf
"20mph limits are areas where the speed limit has been reduced to 20 mph.....
20 mph zones use traffic calming measures to reduce the adverse impact of motor
vehicles on built up areas. The principle is that the traffic calming slows vehicles
down to speeds below the limit, and in this way the zone is becomes ‘self-enforcing’. "

suggests that 20 Zones do not have a legally enforceable speed limit. Is this true? If not, what is the point of the distinction between a 20 speed limit and a 20 Zone?
I accept that the police make no effort to enforce 20 mph speed limits, but I understand that this is due to practicability and lack of resources, (and possibly fear of the public reaction if they tried).
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Postby martine » Sun Aug 09, 2015 6:14 pm


The idea of a 'zone' is they don't require 20 repeator signs as the road layout (speed bumps, chicanes) mean you can't do more than 20. (well you could but you might risk serious damage to your car).

In Bristol we now have loads of 20 limits (not zones) that do have repeators and are now being enforced. There's even a special '20 speed awareness' course - which seems to be quite popular :shock:
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Postby akirk » Sun Aug 09, 2015 6:17 pm


As far as I am aware - both are equally enforceable - as martine says, one is an area wihtin which the structure is meant to keep you down to 20mph, so they don't need to keep telling you, but they tell you on entry and exit - e.g. a houseing estate might have this...
a 20mph limit is simply a bit of road on which a decision has been made to make the speed limit 20mph and signs / repeater signs / etc. are now needed...

of course if you are on a bike you can ignore both - unless cycling furiously :)

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Postby gjohns » Sun Aug 09, 2015 7:32 pm


I understand the practical differences between the two, but not the reason for them. In Leighton Buzzard we have a 20 Zone that covers the centre of town, with traffic calming bumps and tables every few hundred yards, allowing traffic to progress perfectly safely between them in excess of 20 mph, and 20 repeaters on the tarmac cheerfully ignored by most of the traffic.
Interesting to hear that 20 limits are being enforced in Bristol, but in order to answer my original question we need to know if 20 Zones are being enforced anywhere, thereby proving the existence of an enforceable speed limit in a Zone.
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Postby akirk » Sun Aug 09, 2015 7:47 pm


you asked whether they had a legally enforceable limit (yes)
not whether it was being enforced (no idea)

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Postby martine » Sun Aug 09, 2015 8:01 pm


I don't know if 20 zones are enforceable but assuming they are, there are still practical reasons for the difference. Oxford council have said they would install 20 zones to avoid the street clutter of repeator signs (which would be needed in a 'limit') in their more sensitive, historic roads.
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Postby gjohns » Sun Aug 09, 2015 10:23 pm


Alasdair I'm not trying to be difficult but can you point me to the evidence that a Zone has a legal speed limit?
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Postby waremark » Mon Aug 10, 2015 12:31 am


gjohns wrote:Alasdair I'm not trying to be difficult but can you point me to the evidence that a Zone has a legal speed limit?

ACPO Lead for Roads Policing should gbp giving the right answer.
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Postby RobC » Mon Aug 10, 2015 7:16 am


StressedDave wrote:Any red ringed limit is enforceable, provided its number ends in a zero (there was a thing round here where a handy developer put up non-standard signs limiting speed to 19mph). However, logistical and practicality issues preclude many from being enforced.

Logistics in that there aren't enough coppers around to satisfy those who think there should be more enforcement of these limits. Practicality in that there's typically insufficient space to operate a laser gun.


Agree with SD red circle limits are enforceable. 20mph signs in black or sometimes green or in rectangular information signs are advisory and not enforceable.

My local police send me information of numbers of offences and locations speeding motorists are caught (see below) and they are now enforcing 20 mph limits. Many 20mph limits are former 30mph limits and some are on wide roads with plenty of opportunity to catch motorists. One local 20mph limit where many drivers have been caught is on wide road on a steep hill on the outskirts of Ormskirk and brakes must be applied to keep to the limit. It is also right outside the police station!

I have colleagues who present speed awareness courses for those caught in 20 mph limits. The 20mph limit is often the hardest to stick to and possibly the most abused as from information the police sent to me some drivers are caught travelling at nearly twice the speed limit.
Last edited by RobC on Mon Aug 10, 2015 7:38 am, edited 1 time in total.
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Postby RobC » Mon Aug 10, 2015 7:35 am


Some interesting Police figures here. On Mill Lane on the 31st March, 30% of drivers exceeded the 20 mph limit, one doing 43mph. 50 people were caught speeding in under an hour which is £5000 in fines.
On the 2nd April, 94 drivers were caught exceeding 20 mph in 2 locations in 3 hours, that's another £9400 in fines!

Site: Mill Lane Upholland (20mph)
Date: 31/03/2015
Time: 13.45 - 14.42
Throughput: 156
Offences: 59
Speed offences: 50
Fastest Speed: 43
Seatbelts: 9
Mobiles: 0

Site: Blackgate Lane Tarleton (20mph)
Date: 02/04/2015
Time: 10.35 - 11.55
Throughput: 263
Offences: 47
Speed offences: 34
Fastest Speed: 33
Seatbelts: 13
Mobiles: 0

Site: Burscough St Ormskirk (20mph)
Date: 02/04/2015
Time: 12.25 - 14.00
Throughput: 744
Offences: 76
Speed offences: 60
Fastest Speed: 36
Seatbelts: 16
Mobiles: 0
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Postby akirk » Mon Aug 10, 2015 9:40 am


gjohns wrote:Alasdair I'm not trying to be difficult but can you point me to the evidence that a Zone has a legal speed limit?


read this Govenment paper summarising all things speed limit...
http://www.parliament.uk/briefing-papers/SN00468.pdf
it lays out all the detail / links you might want

note - it refers to zones as 20mph speed limit zones which might be a more helpful description

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Postby trashbat » Mon Aug 10, 2015 10:00 am


RobC wrote:Some interesting Police figures here. On Mill Lane on the 31st March, 30% of drivers exceeded the 20 mph limit, one doing 43mph. 50 people were caught speeding in under an hour which is £5000 in fines.
On the 2nd April, 94 drivers were caught exceeding 20 mph in 2 locations in 3 hours, that's another £9400 in fines!

I don't know whether it has any relevance beyond speed limit planning, but one of these wouldn't pass the 85th percentile test, and another comes close. This is commonly used to determine whether most people would find a speed limit reasonable for the context and conditions.
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Postby RobC » Mon Aug 10, 2015 10:29 am


trashbat wrote:
RobC wrote:Some interesting Police figures here. On Mill Lane on the 31st March, 30% of drivers exceeded the 20 mph limit, one doing 43mph. 50 people were caught speeding in under an hour which is £5000 in fines.
On the 2nd April, 94 drivers were caught exceeding 20 mph in 2 locations in 3 hours, that's another £9400 in fines!

I don't know whether it has any relevance beyond speed limit planning, but one of these wouldn't pass the 85th percentile test, and another comes close. This is commonly used to determine whether most people would find a speed limit reasonable for the context and conditions.



I can only presume that a speed limit using the 85th percentile test is established by a sample of more than an hour an a half on one particular day?
Whilst the 85th percentile test may be used to set up speed limits, I rather doubt if the 85th percentile test could be used in defence in a court of law. In my experience, judging by the speeds 85% of drivers drive at in 20mph speed limits, many if not most 20 mph speed limits musn't be reasonable!

Also the figures given may not be a good sample as perhaps only the unwary have been caught and many who might normally have been going faster may have seen the speed trap and slowed down so the percentage speeding would probably have been higher normally.
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Postby exportmanuk » Mon Aug 10, 2015 12:52 pm


The 85th percentile test seems to have been abolished in favor of political expedience and pandering to aggressive lobbying from certain organisations who seem to think we should all be walking or driving at no more than walking pace

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Postby gjohns » Mon Aug 10, 2015 2:06 pm


Thanks Alasdair
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