Like this site? Help us to make it better.

Debate about effectiveness of Cambridge 20mph zones

Only small changes being reported following first wave of changes

Critics are questioning the impact of a city council scheme to introduce 20mph speed limits on the majority of Cambridge’s residential and shopping streets. So far 58 streets in the north of the city have seen their speed limit reduced from 30mph to 20mph and the majority have seen a change of less than 1mph – average speeds on most of the streets already being below 20mph even before the changes.

The project is due to be rolled out across the city, but some are questioning the value of what will be a £600,000 investment with the biggest change seen on any road thus far less than 3mph.

Local Conservative activist Andy Bower, told Cambridge News that he felt there were other, more important issues for cyclists that were being overlooked.

“I think it’s fine that residents are being consulted, but I feel councillors have pushed the whole process without considering the facts, what the practical impact is going to be.

“The police have expressed their view time and time again that it doesn’t seem to be worthwhile. Perhaps councillors should pay attention to that advice. If you’re cycling down Mill Road the threat isn’t how fast you’re going – the real threat is people coming out of side streets without looking.”

However, it is often argued that even small reductions in driving speed can have a real benefit in safety terms. Results show that the largest speed reductions in Cambridge have been on the streets that had the highest averages.

Andy Preston, the city council’s project delivery and environment manager, said:

“Of those streets where average traffic speeds were previously above 20mph, some 93 per cent have seen a reduction. Only three of these streets have seen an increase in average speeds, but by a marginal degree...and the resultant mean traffic speeds remain relatively low.

“All streets that were surveyed in the north area now have average speeds of 24mph or less, with 56 per cent having average speeds of 20mph or less.”

Al Storer, from the Cambridge Cycling Campaign, said that the group supported the continued roll-out of the 20mph scheme, arguing that a consistent limit in residential and retail streets would help with compliance as drivers would be in less doubt about what the limit is. However, he argued that the changes would prove more effective if police showed greater support.

"The preliminary results from north area do only show small drops, however the data shows that in many streets the average was already well below 20mph so we would not expect much reduction there. It appears the largest reductions have been on the streets that had the highest averages.

"Better compliance may be achieved if the police would take enforcement of the limit seriously. Despite local residents asking for it as a priority the local policing team have taken little interest in doing so, which the regular speeders no doubt know.  We are sceptical that a 20mph limit on major through streets can be effective without enforcement and/or measures to reduce speeds and reduce through traffic."

Last year, government ministers and members of the All Party Parliamentary Cycling Group reacted with astonishment when a senior police officer representing the Association of Chief Police Officers (ACPO) at the Get Britain Cycling parliamentary inquiry said the body’s guidance was for such speed limits not to be enforced. In a statement, ACPO later clarified that “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.”

A spokesman from Cambridgeshire Constabulary said: “We are supportive of any scheme that will improve road safety and we have been active members of the multi-agency implementation group throughout this process.”

In any case, the merits of 20mph zones should perhaps not be measured purely in terms of the impact on drivers. One of Cambridge City Council’s stated aims in imposing 20mph speed limits is to provide road conditions that encourage active and sustainable modes of travel, such as walking and cycling.

Following a trial last year, Edinburgh looks set to introduce 20mph zones across the city with one of the main benefits found to have been that people felt safer. During the trial, cycling and walking journeys rose by 5 per cent and 7 per cent respectively, while car trips fell by 3 per cent. The percentage of children riding a bike to school rose from 4 per cent to 12 per cent, and from 3 per cent to 21 per cent among older primary age pupils. Furthermore, the proportion of parents willing to let their children play outside more than doubled from 31 per cent to 66 per cent.

Consultation packs for the next part of the Cambridge project will now be sent out. More information is available at

Alex has written for more cricket publications than the rest of the team combined. Despite the apparent evidence of this picture, he doesn't especially like cake.

Add new comment


HKCambridge | 9 years ago

There are a couple of other factors here to note.

Lots of streets in Cambridge are 20mph anyway. It's popular with residents. Indeed in some areas of the city there are probably more 20mph roads than 30mph ones already.

By doing a big roll-out it means that you no longer have inconsistency: all residential areas will have the same limit, so you're reducing the need for drivers to keep checking the limit or look for signs (there are exceptions for A roads, and some major B-roads are consulted as a separate question).

A TRO to introduce 20mph costs about £1,000. As I said, 20mph is popular with residents, so under the existing situation every time someone wanted their road 20mph, a TRO had to be individually obtained for a small area. A city-wide consultation is a bigger job, but potentially more efficient, and means it never needs to be done again.

The other thing is that this is all being introduced without any traffic calming. However, once the roads are 20mph, you can introduce traffic calming on a case-by-case basis. The limit will support changing the street environment, at which point enforcement is less of an issue.

Al__S | 9 years ago

shay cycles- I think you're getting a little confused between the two sorts of average speed. The "average" being talked about here is the average speed at a point location. They use those grey boxes with two tubes extending across the road to gather this information, and as far as I understand it generally try to put them where they reckon speeds are likely to be highest.

That's a world away from individual vehicles average speeds which would require an ANPR set up.

shay cycles | 9 years ago

All the talk of averasge speeds (and better ways of looking at median and means) surely misses the point a bit.

Speed limits are not about average speeds, they are the fastest speed at which motor vehicles may travel on the limited road. Unless the road is designed to make driving on any of it at over 20mph impractical then the alternative is enforcement.

There are all sorts of reasons why traffic driving at no more than 20mph improves safety and almost none of those are to do with average speed. E.G. the car pulling out of a side road in a rush does so partly because of the speed of the traffic on the road at that point and time (not its average speed), or the severity of the injury caused to someone hit by a vehicle is related to the speed of the vehicle at that time (not the average speed).

Slowing traffic's maximum speed gives everyone more time to see what's going on around them, reduces the "need" for harder accellerations, hard braking and makes the environment of the road more pleasant as well as safer.

For 20mph limits to be truly effective they simply need to be heavily enforced - cheaper than changing the infrastruture to make people slow down.

Still what chance have we when the government decides to spend £15billion enabling traffic to travel faster on a range of main routes?

dcmarsh | 9 years ago

I have read the article with interest, and had a look at some of the raw data. Although the average speed can a useful indicator, it doesn't tell the complete story. It is the shape of the data that is far more interesting. Do we know if the quoted average was the mean or median speed? Ideally you want the distribution of people driving in urban areas to be right skewed, and talk about median speed - i.e. the majority of people drive slow, a few drive fast. The data appears to be a symmetrical shape (cambridge - north phase base data.pdf) - thus mean speed is useful. The second issue is that the summary data presented is attributes (count) data - how many vehicles drive within a certain 5mph band. this unfortunately looses the fidelity of the data. Finally, as with all, they appeared to have taken a sample over a 1 week period, and then applied old school analysis (comparing simple means and standard deviation) - which misses the point completely. As this was just a sample the correct statistical test should have been applied, e.g. a 1-sample t-test (if the data was normal). I have found the raw data - the actual speed of each vehicle - variables data e.g. . A cursory look indicates that the data is bi-modal - it has two humps at 11mph and 16mph. Thus a single mean is definitely not useful as a measure. the data needs to be split apart first and understood why there are two humps. More to follow....

Nixster | 9 years ago

Speed limits are only part of the story in reducing vehicle speeds and creating better and safer streets for pedis and cyclists. Street design and an urban environment that supports lower speeds will be more effective than a blanket 20mph limit alone.

WolfieSmith replied to Nixster | 9 years ago


Nixster wrote:

Speed limits are only part of the story in reducing vehicle speeds and creating better and safer streets for pedis and cyclists. Street design and an urban environment that supports lower speeds will be more effective than a blanket 20mph limit alone.

Absolutely. It's a shift in focus away from drivers that's needed. Our whole area is going 20mph on Thursday. One busy road has a rat-run road off it that leads to the countryside - very popular with motorists and cyclists. Instead of blocking it and retaining a cycle lane and drop kerbs it was just blocked 8 years ago with a big brick planter. I asked Sefton Council why they didn't make it accessible to cyclists at the time? Their reply: 'We asked the local residents of the road whether they wanted a cycle path or a planter and they voted for the planter.' This woolley thinking is what needs to change. In future every blocked road should be accessible by cycle path.

Many councils are still asking questions like 'Do you want a cycle lane outside your house?' and then publishing the negative responses as somehow democratic. Like asking people if they want to keep on drinking and driving and supporting the few who say 'yes!' It's looking at every little road change and asking 'How can we fit cyclists into this change?'

jacknorell | 9 years ago

"Furthermore, the proportion of parents willing to let their children play outside more than doubled from 31 per cent to 66 per cent."

Reads like a success story to me.

Latest Comments