Friday, 8 March 2013

20mph Speed Limits, Their Design and The Police

Following a comment by a high ranking police officer, that the police do not enforce 20mph speed limits, i thought I would give some of my own views on the issue, explain the process we have for making 20mph speed limits and give some design thoughts. sorry, but this will be a long post.

As Easy As Riding A Bike reported comments made by ACC Mark Milson this week at the All Party Parliamentary Cycling Group Inquiry on 4th March.

ACC Wilson, of West Yorkshire Police said:

"We are not enforcing 20mph speed limits at this moment in time"

The Association of Chief Police Officers (ACPO) hastily issued a press release soon after which stated;

"In most cases, 20 mph limits will follow Department of Transport guidance and include features such as speed bumps or traffic islands designed to slow traffic. ACPO guidelines include thresholds for enforcement across all speed limits to underpin a consistent policing approach. However it is for local police forces to apply a proportionate approach to enforcement of 20mph limits based on risk to individuals, property and the seriousness of any breach. Where drivers are exceeding the speed limit through wilful offending, we would expect that officers will enforce the limit and prosecute offenders."

This has now been followed up by the ACPO lead for roads policing, Chief Constable Suzette Davenport (Gloucestershire). In a letter to the Inquiry as follows;

Clarification of the ACPO position on 20 miles per hour speed limits

I write further to the recent All‐Party Parliamentary Cycling group evidence session on ‘Get Britain Cycling’. ACC Mark Milsom represented the ACPO roads policing portfolio to address the group’s questions from a policing perspective.

"Following a very specific line of questioning on the subject, we believe the police service position on the issue 20 mph speed limits requires further clarification. For accuracy, we would be grateful if you would reflect this correspondence in written evidence for your eventual report.

We can clearly state that it is incorrect to say that police officers are not enforcing 20mph speed limits.

20mph zones are predominantly introduced in residential areas where road safety has been raised as an issue by those who live locally. The approach of neighbourhood policing teams in every community is built around ensuring that local crime and disorder issues and concerns are identified, so that a police force delivers an appropriate policing response. This applies to enforcement of 20mph zones as to any other area of policing.

Police and Crime Commissioners are now responsible for setting strategic policing priorities for each police force and in areas where 20mph zones are a local concern, may include enforcement within local policing plans.

In most cases, 20 mph limits will follow Department of Transport guidance and include ‘road calming’ features such as speed bumps or traffic islands designed to slow traffic. Wherever possible, we agree with the Department of Transport that 20mph zones should be ‘self‐enforcing’ through the use of such features. The guidance states:

Successful 20 mph zones and 20 mph speed limits are generally self‐enforcing, i.e. the existing conditions of the road together with measures such as traffic calming or signing, publicity and information as part of the scheme, lead to a mean traffic speed compliant with the speed limit.

To achieve compliance there should be no expectation on the police to provide additional enforcement beyond their routine activity, unless this has been explicitly agreed.”
ACPO speed enforcement guidelines (attached to this letter) include thresholds for enforcement across all speed limits, intended to underpin a consistent policing approach. Within that framework local police forces will take a responsible and proportionate approach to enforcement of 20mph limits based on their assessment of risk to individuals, property and the seriousness of any breach. Where drivers are regularly and wilfully breaking the law we would expect that officers will enforce the limit and prosecute offenders.
I trust that this sets out our position clearly. Please do not hesitate to contact us for further information."

ACPO's speed enforcement guidelines can be downloaded here, but there is a table which essentially gives a guide to what speed thresholds will be enforced and what will happen and I have reproduced it below;

I am going to concentrate on the 20mph speed limit here, the rest is another debate, but essentially, my impression of this is that you are fine to drive at just under 25mph or 25% above the speed limit. If you are caught above this, then you will get a fine and 3 points.

If you are caught at 35mph or 75% about the speed limit, then you have the summons to appear before the magistrate. My view has to be that speed limits are just that. They are not targets, they are maximums. 

I accept that speedometers are not 100% accurate and there is room for error, but anyone with a driving licence can surely manage to not put themselves into a fixed penalty situation? What about 35mph - I am not sure that is a speeding offence any more. It must be more serious and this speed has been reached by choice, surely?

What is ACPO?

Before I go on, it is worth explaining what ACPO is. It is essentially a professional institution for senior police officers at ranks of Assistant Chief Constable or higher (Commander in the Metropolitan and City Police) plus senior police staff equivalents (a total of 334 members). It is kind of akin to the professional engineering institutions I link on my homepage. Like ACPO, these institutions represent a profession of members rather than a representing people like a union or staff association would.

Like the professional engineering institutions, one of ACPO's roles is to give guidance and leadership to its members. Roads policing and therefore the speed enforcement guidance is one of these things it provides. Of course, guidance is not the law and any speed above the limit is technically an offence. 

However, the government has essentially co-opted this guidance as the de facto way in which the law is enforced. ACPO does not make the law, it suggests practical ways in which it can be enforced and stuffing the courts full of people driving at 23mph in a 20mph Zone is clearly daft.

Of course, ACPO should not be making policy - that is the Government's job and I wonder if ACPO have too much of a say.

The joke of all of this is that the police are suggesting (despite the back pedal) that they are not too interested in supporting 20mph speed limits unless the guidance is slavishly followed. I cannot see what the problem is, if we are looking at urban areas which are residential, high street and business park type arrangements, whether 30mph or 20mph, there are still the same number of roads for the police to enforce the law on, or is it that they don't bother too much with speed enforcement full stop? Are they worried about the "war on the motorist"? Is ACPO and the Home Office too cosy? Perhaps they are too busy playing poker (sorry, this is our current office joke about the police who do suggest that everything is down to the council to deal with!).

What is a 20mph speed limit?

A bit of history first (here is a good general background to speed limits). 30mph as the default urban speed limit was created by the Road Traffic Act 1934. Before that, the default urban limit changed from a bloke with a red flag walking in front of the vehicle to 20mph, then no limit and then the RTA1934. So, 30mph has been the default for nearly 80 years!

Confusingly, we have two kinds of 20mph speed limits. We have 20mph Zones and 20mph limits. 

Limits are provided with terminal signs (the signs as you travel in and out of the limit as expected elsewhere) and smaller repeater signs. Department for Transport Guidance, "Setting Local Speed Limits" (which I have commented on before) suggests that if traffic is already at 24mph or below (just under the ACPO fixed penalty threshold!), then limits are more likely to be successful as drivers will essentially slow down to the lower limit. 24mph will be the 85th percentile speed (i.e. 85% of traffic will be travelling at 24mph or lower) which is speed highway engineers select to cover most situations, although the latest guidance is looking at average speeds which will be a lower value.

20mph Zones are generally used to treat an area (but not exclusively) and rely on traffic calming (this links to a really old piece of guidance!) to force traffic speeds down to 20mph (or at least 24mph, so people don't get nicked). I have designed 20mph Zones and seen many go in. The problem I still have with them is as they often still allow through traffic, heavy traffic calming is needed which is no fun as a cycle user.

How to make a 20mph limit or Zone

This a pretty simple process. So, your current urban speed limit is 30mph, a roads with this is also known as a "restricted road". It is 30mph because it is lit, you only need signs where you enter or leave this speed limit and in the absence of anything to the contrary a road is 30mph by default. This is contained in S81 of the Road Traffic Regulation Act. Compare with an unlit road, where the National Speed Limit applies (60mph for cars). Any change from that arrangement requires a Traffic Regulation Order.

Any change from the default 30mph urban speed limit requires a Traffic Regulation Order made under S84 of the RTRA1984 and this includes 20mph limits and Zones. To implement a TRO, a highway authority gives public notice of its proposals, consults certain organisations and anyone else the highway authority sees fit. This is covered in the snappily titled The Local Authorities' Traffic Orders (Procedures)(England and Wales) Regulations 1996 (sorry Scotland!). The Act is what you do, the Regulations are how you do it.

After a period of notice and consultation, the highway authority can then decide if it wishes to proceed with implementing the TRO. There will be a committee (or board - such as TfL) decision or political sign-off procedure for the highway authority. 

The highway authority then "seals" the TRO which is literally the olde worlde seal of the Council stuck on the original paper version of the TRO which is then dated as coming into force and signed by an elected person (often a mayor) and a designated officer (often a senior council solicitor). The highway authority then advertises a further notice that the TRO is coming into force on the correct date and the new speed limit can be enforced (hopefully the scheme will be built and the speed limit signs uncovered on the right day!). There are processes to challenge decisions, but this is even more dull than this post so far!

Design of 20mph limits and Zones

Chapter 3 - Mmmmm purple...
This is the more interesting and creative bit of course. If I wanted to play with TROs all day, I would have become a solicitor! 

20mph limits are straight forward. You stick the right signs (sometimes lit) in the right places (page 105 onwards in Chapter 3 of the Traffic Signs Manual will help). 

You will also need the odd repeater sign or road marking and there you have it (a tiny bit more involved, but that's my day job). The whole process is simple, not particularly costly and that is why many highway authorities such as Portsmouth, Camden and Islington are putting in blanket 20mph speed limits.

20mph Zones were often used around schools and will have road humps or other physical features to slow the traffic. For example, if simple round-top humps are installed every 60 metres, I will pretty much guarantee traffic speeds will be within 20mph. The trouble is, if this is a through route, then it will still be traffic dominated, horrible for cycling and humps cost about £3k a go.

Below is a diagram of an estate which had a 20mph Zone installed several years ago and yes, I was involved as a local authority engineer. It is a little stylised, but the important elements are the trunk road (red) running east to west at the top, the local A-road (green) running east to west at the bottom, the local A-road (green) running north to south (west of the estate) which connects the trunk road to the other A road and the local road to the west of the estate which connects the trunk road to the east-west local A-road. The junctions with the trunk road are both signalised and all movements permitted. 

The local A-roads in London are part of the London Strategic Road Network (SRN) and the trunk roads in London are part of the Transport for London Road Network (TLRN). You will see a roundabout at the confluence of the two local A-roads and this is the edge of the local town centre.

In terms of traffic, the trunk road is busy going west in the morning and busy going east in the afternoon. The roundabout at the edge of the town centre is busy much of the day as traffic heads towards it in the morning and away in the evening. The road to the east of the estate is busy heading away from the trunk road in the morning and busy towards it in the evening.

The estate within the four roads around the edge is the 20mph Zone and speeds are controlled with road humps which pepper the area. There is also a primary school in the middle of the estate and being a faith school, its catchment has many pupils driven in from an area beyond the trunk road and A-roads, but many local children walk to the school too.

In taking the scheme forward, the entire estate was consulted along with the school, the emergency services (no bus routes involved) and the decision to proceed was taken by a committee. There was good local support for the scheme, but almost immediately after installation, some residents started complaining that people were still speeding, they wanted more humps, higher humps etc. A few tweaks were made to add some humps, but it has been operating for several years unchanged. 

The scheme was originally proposed because of casualties within the estate as a whole and long-term, there have been no other casualties. So, it was generally successful from that point of view and was the kind of thing we did a few years ago.

The problem I have with the scheme looking back, is that because of the congestion on the main roads and the road to the east, the area is still a rat-run during the weekday peaks and indeed at weekends (shopping traffic). It was a rat run before the scheme, it is just that people now rat-run a bit slower and are not having crashes which hurt people any more.

In my Rethinking the Little Things post, I discussed limiting vehicle movements to stop rat-running and so I have looked at the "traditional" layout with this in mind to achieve the same reduction in speed, but without stuffing the area full of humps. 

This plan shows the same estate with several roads closed to through traffic, but with cycle users still catered for. There are either loops or turning heads for refuse/ delivery lorries, although they may be a little more inconvenient for them to use.

I have kept a few (about a third) of the humps in order to manage the behaviour of the inevitable few residents and visitors who will speed, but the older style layout will be more self-enforcing as the rat-runners have been removed and those using the roads have a reason to be there.

Of course, humps are not the universal answer. We could put entry treatments in on the main road junctions or speed tables across whole junctions in the zone. We would also want a few 20mph logos painted on the road surface. Essentially, a simpler and better scheme could be provided for the same cost and it would not only deal with casualties, but remove rat-running and make the area more civilised for walking and cycling. I would also argue that the estate is subjectively more safe and this would be very useful to those walking the school who may have more fear of traffic.

There are negative impacts. The rat-running traffic will be forced to use the congested main roads (which could affect walking and cycling on those), getting in and out of the estate roads may be more difficult and this could create casualty problems elsewhere.

There is detailed guidance in S6.1 of the DfT guidance "Setting Local Speed Limits (linked above) on what constitutes traffic calming and this is far more relaxed that was set out previously. The new guidance allows use of signs and road markings (stating "20") as features in a 20mph Zone and very few physical measures are now needed - but as my example sets out, they may still be needed.


The use of 20mph speed limits and zones have been controversial with considered and objective reporting in respected technical journals such as the Daily Mail and The Sun plus those representing "Britain's beleaguered drivers", the Association Alliance of British Drivers

Actually, with ABD, I cannot find out how many members it has as it doesn't turn up in any search on their website (which is frankly deranged and misses the point on 20mph as a speed limit), but Wikipedia reports that it may not be particularly high. 

The Chartered Institution of Highways & Transportation (CIHT) of which I am a Fellow, on the other hand proudly states that it has over 12,000 members on its website home pageCIHT recently responded to the revised guidance on setting speed limits I linked to above and in response to it being published, welcomed the guidance, but suggested that because of the limited experience of installing blanket 20mph limits, this needed to be kept under review. 

Although I would like CIHT to be a bit bolder sometimes, its views are formed from debate by technical boards and through its (+12,000!) membership and so I would suggest it provides more thoughtful and measured observations than the headline chasing press and unrepresentative nutters such as the ABD.

Some of the press coverage linked to above cries out at casualties increasing in areas with a 20mph limit - duh! If we take a town with 7 20mph Zones and 1 injury in each of them, that is 7 injuries at 20mph. If the town then triples the number of 20mph Zones to 21 and the casualty rate stays the same (1 per zone), then casualties in 20mph zones triples to 21! Of course, if the rate drops to 0.5 per zone (on average), then with 21 zones, we have 10.5 injuries at 20mph which is still more than when we started, but a good result - I hope you understand my clumsy explanation!

With more people walking and cycling on streets with 20mph limits, quality of life will improve and when we get more experience and data, I am convinced they will prove effective in the longer term. But, we need to get the engineering right and close those rat runs!


  1. In Cambridge, we've had two fairly major streets and a couple of rat runs decalred 20mph limits.

    No traffic calming- Mill Road in particular probably achieves the <24mph on the basis that at peak times it is bumper-to-bumper stationary, often in both directions. It has a reasonably busy bus route (though only single decker) that is especially useful for elderly residents travelling to the city centre or the hospital. Off peak, especially after dark, cars blat along it at 40mph+.
    The other major route, Maids Causeway, is currently having the signs redesigned to make them a bit clearer. Which seems a tad hopeless without enforcement.
    Neither have any traffic calming, and the same is true of the rat runs off Mill Road- heavily used by taxis and minicabs to and from the station. I once had it pointed out to me whilst cycling that I'd been doing 25mph down one of the rat runs. The taxi driver (who I was berating for breaching an ASL) had been attempting to overtake me and didn't understand the idiocy of the point he was making.

    Anyway, the one possible positive is that the county are moving to make almost the entire city barring the "ring road" and main radial routes 20mph, and the Police Commissioner surprisingly is backing this. But Mill Road isn't being made a nicer place for pedestrians by the limit- that can only happen if restrictions are made on the traffic using it. Cambridge already has many rising bollard restrictions- some for buses and taxis, some just for buses. A Bus (and bike) only restriction at the bridge would cut traffic dramatically. A Bus, bike and taxi restriction would be a little less effective but still have an impact- and it would slow the damn taxis down a bit. It would of course have a fairly major effect on the rest of the network, and christ knows the county council don't have a masterplan for this- it was one the reasons given for their piss-poor "safer" junction they approved this week.

  2. (I should start my own blog for these rants...)

    1. (I have just Google mapped!) Both of those roads connect A-roads together and so it is possible to "bypass the bypass" and Maids Causeway is so wide! Used together with several other streets, you can see nice cross-city rat runs.

      I think the problem here is that parts of Mill Road and most of Maids Causeway have been designed for traffic efficiency with multi-lane approaches at junctions, guardrail, traffic signals etc. The bus lanes are clearly there to help get them past the congestion which must already exist (otherwise why do it).

      Slapping a 20mph limit on these roads will never slow traffic because drivers are conditioned to drive faster because of the layout (and I cannot criticise drivers too heavily here).

      The answer is to close the through routes to traffic and accept that the already congested ring road will get worse, but in time, people will start to change their travel patterns.

      I have not been to Cambridge much, but I have always arrived by car and used the park-and-ride which for relatively isolated urban areas are a good idea to keep traffic out of the core, but in Cambridge, there is still too much traffic in the core.

      I imagine there are people living several miles away from the city and cycling will not be practical and inter-urban bus services are probably poor. Of course, the M11 can bring people in quickly from a long way away, so perhaps as well as park-and-ride, park-and-cycle is an option if only the centre was more civilised. As for long-distance through traffic - well it should be discouraged and forced to use road like the M11, A14 and A10.

    2. Transport priorities for Cambridge are set by the County Council. The councillors that control the Council, by a sizeable majority, are entirely from outside the city, places poorly served by public transport.
      Thus cars versus everyone else is a party-political issue. Which is maddening.

  3. Thanks for an excellent post - I think I already knew a fair bit of what you write about limits and zones and calming etc but this is a great coherent summary of the whole scene.

    I live in Surrey, where the council, notably the cabinet member for highways John Furey, are dead set against introduction of 20 limits. Unfortunately this infects their highways engineers as well - although of course they may just be reflecting the natural need to preserve their paychecks - who argue that limits are no good without calming because they won't be policed.

    You might just as well say that a 30 limit is pointless because it isn't calmed and the police, by and large, don't enforce, just as they don't enforce any speed limits anywhere to any significant extent (in Surrey, at any rate).

    This however ignores a number of points which the 20splenty campaign makes in its material. One is to explode the assumption that without enforcement, no-one will keep (roughly) to the limit. Baloney! I do, and I see plenty of others who do as well. In fact, the surveys by the RAC and others which tell us that 2/3rd of drivers break speed limits at least occasionally also, bu deduction, tell us that one third do not breach limits, and so can be expected to confirm to a new lower limit once they have got used to it.

    They also claim an average 1.6mph drop in average speed which doesn't sound much until you consider that this average disguises larger falls where speeds are currently in the higher 20s and smaller falls where speeds are currently in the lower 20s - no shit, Sherlock!

    And then there is the 85th percentile. It is not so much the average speeds which cause collision or injury, as the outliers at the higher end. If you can squish teh speed rannge into a narrower band, then the average fall is less of an issue.

    But, from my own amateur angle, I would have to agree that permeability is the real answer. If roads are for access only, you can't go any great distance in them and speeding becomes even more pointless. You are also more likely to be there as a resident or regular visitor, and human nature being what it is most people don't sh*t on their own doorstep.

    In the past, few residential areas were designed to calm traffic through permeability, but you see it a little more often in housing estate built in the last decade. For example, in my home town of Lee on Solent, an estate completed in around 2008 is designed as a warren of culs-de-sac with foot and cycle permeability and only a couple of roads through. Anearby estate built in the 60s and 70s, Bridgemary, has no such impermeability for cars plus it has wide roads, large radius corners etc. As far as I can tell, it does make a difference.

    The City of London has, in principle, a policy that through traffic on what it has designated as "access roads" (uncoloured on its street plan in its Local Implementation Plan") is prohibited, but for the most part no physical measures are taken to enforce that prohibition so, unsurprisingly, they are swamped with through traffic rat-running, especially taxis and delivery vans.

    There are some examples however of permeability measures, and the most recent of those is the closure of the exit from Stonecutter Street onto Farringdon Road. What remains is a two-way cycle path with bollards to prevent anything wider getting through. It has so far caused great confusion, mainly I suspect due to the overwhelming use of Satnavs which have yet to be updated, and a lot of ire among the cabbie fraternity, but they'll get used to it. Meanwhile, the street environment for the many thousands of people who work there has been immeasurably improved.

    1. In responding to consultation which led to the latest speed limit setting advice, I suggested 20mph as being the default and any higher limit would need to be chosen and consulted on - I am happy that we have lower limit roads, but they still look like fast and open roads - on a trip (by car) to Portsmouth, I drove into the 20mph limit near the Historic Dockyard and did have people up my rear as I refused to go over 20mph. When walking round the area, there were sill some people who didn't fancy stopping at the zebra crossings. I think that we all need to keep pushing as this is all a cultural change as much as an engineering change.

    2. Hypothetically, had the limit been 30mph, what speed would you have chosen to drive at on that occasion?

    3. Good question.

      To be honest, nearer 30mph. The problem is that the streets at the Dockyard are still wide and open and invite speed (regardless of the limit).

  4. Great post, thanks.

    Rather than slicing the residential area into a number of separate vehicle accessible areas, would you consider keeping it as a single area and only provide a single entrance way from the main road network?

    The incentive would be that you'd remove potential conflict points along the main roads, the less turning vehicles the less conflicts, thus making these roads better for pedestrians and cyclists.

    Possible negatives would be the difficulty in providing turning space for lorries/maintenance vehicles.

    It probably depends on the specific case, but your example above would you see such an approach as beneficial or too tough a sell?

    1. You can do either and I know I have only shown one example. The pitfall of one access point is that the people who live nearest will be the ones who complain about all of the traffic from the estate passing their houses and possibly one junction takes a greater deal of pressure (although the pressure from rat-running traffic will have gone). It always has to be site specific. Accommodating deliveries and refuse is awkward in many streets (especially where there is parking everywhere) and possibly some limited access might be a compromise. The point is to make the estate utterly unattractive to through traffic.

  5. There are really two problems here, a) few drivers really bother to think about why there are speed limits and b) the lack of understanding of the need for enforcement leading to the culture of the Sacred Driving Licence. The solution is really political, we need people to understand and call on those in power have the laws enforced to protect vulnerable road users.

  6. I really want to see such more posts in future. Excellent post!

  7. i know this is an old topic but a police van has very likely just caught over 200-400 people doing over 25-35 in a 20 marked area as he parked it just after a corner to get max fines (they have not caught me as they was closing the Flap as i was coming past wish i had a chat with eh officer so i could get his badge number)

    as this road used to be a 30mph road (the normal avg speed of cars on this road is 35mph) on december the 24th 2015 they did it as well they have made over £20-£30,000 just been parked there as most people assume that road is 30mph (any slower people just overtake you as the road is wide as it leads to an industrial estate)

    is there anything that can be done to report the police van packed where it should not be as it should've never been parked there as its not an assigned spot (warrington they park in designated spots) as it was setup to catch as many people as it could, there are no speed cam sings on this road as well

    1. Assuming the limit is properly set out, signed and has a valid traffic order, why shouldn't the police be enforcing it? There is no legal requirement to place warning signs, the clue is in the speed limit signs!

      It is not about "making money" at all - the fines are not ring-fenced and any "profit" actually goes to the Treasury.

      Of course, parking in a position which is not safe could be raised as a complaint (although at 20mph, it would have to be pretty poorly placed). I wonder if on the flip-side, local people have been demanding enforcement against those who decide to ignore the speed limit?

      I'm sorry, but not much sympathy from me (unless there is a problem with the design, signage or traffic order!)

  8. And of course, if you can do the other things as well to make them safe routes for all cyclists of all ages and abilities, same for pedestrians, make them convenient journeys and give them the right omafietsen, the more people cycling, ergo fewer people driving, and even less threat from motor vehicles, less congestion on the main road and also making it more pleasant to cycle on the shared streets because there would be even fewer motor vehicles.

    Also, the Dutch use speed bumps on ordinary stretches of road less often in the 30 km/h zones. Often they just raise the junction to pavement level and take away the indication of who goes first, making traffic more cautious, and removing any idea that there are main streets in the 30 km/h zone. They also tend to pave roads in brick, making a rumble for cars if they come by, making the driver alert and more likely to be slower, and warning people nearby that there's a car. The parking is also in modern designs, in completely separate bays, paved in a different colour of brick, and generally designed like this: