20mph speed limit rollout by Lewisham Council (effective September 2016)


Says whom?


Transport planners around the world.
Probably the evidence from most streetscape redevelopments occurring in the last 20 years on the western hemisphere. Even London is starting to grasp this.


It doesn’t cause gridlock if everyone overtakes the one driver travelling at 20mph.

But overtaking on narrow London roads is very dangerous.

This anecdote is an interesting counter-point:

The 20mph experiment is interesting from a scientific point of view. There are all sorts of side effects that emerge. If it is shown empirically to be counter-productive, we must set aside our ideology about “preferential treatment of cars” and restore 30mph limits for the public good.


So no actual motorists, more groups with an interest in making money then.

I am sure if I had the time and inclination I would be able to find academics, transport planners and streetscape developers who disagree with you but such is life.

Can I ask why a blanket 20 mph limit is a good idea when, as Chris has already said, academics, streetscape developers etc were quite happy with 30 mph limits? We also seem to have forgotten that cars themselves are safer, my bunny hugging Volvo stops automatically when any myopic iPhone users make a bold dash across the road without looking.

Once again the many are disadvantaged because of the few.


We’re not citing this stupid article again, are we? The article doesn’t mention one piece of data which doesn’t have a glaring hole in it. It probably takes some skill to take facts and obfuscate them so effectively.


Is it stupid because you dont like it and it doesnt reflect you views or something else?


No, it’s stupid because it provides near irrelevant data which have been derived from relevant, but not provided, data.


Irrelevant? The number of accidents has fallen across Manchester, but fallen less in 20 mph zones than in 30 mph zones. It’s a small dataset, but it certainly supports the arguments of those of us who warned about the negative consequences of 20 mph limits.


If you can find better data that refutes this, perhaps share it?


I’m not going to look for better data, but the obvious questions about this stat are:

  • how many accidents there were in each zone? (so we can avoid making inferences based on a small sample)
  • were the accidents in the 20mph zone less severe? (one of the major arguments for 20mph vs 30mph)

The article also argues that the new speed limit has had marginal impact on speeds. If this is true then why should we expect anything more than a marginal impact on accidents anyway?

Not having useful data available does not mean we should infer things from useless data.


Irrespective of the arguments for and again the 20 mph speed limit, I love this statement more than I can say. I think it should be engraved at the top of any screen with internet access.


Good questions, both of which Manchester Council must have considered before choosing to halt the rollout of 20 mph limits.

As I’ve argued elsewhere:

  • The confusion of non-standard limits means driver attention is diverted from pedestrians and other road users - instead, drivers are looking out for signage, speed bumps and cameras.
  • Introducing unsuitable blanket limits led the police to de-prioritise enforcement (as shown in posts above). A lack of enforcement will worsen a spectrum of dangerous driving behaviours (of which speed is only one).
  • Disparate driving styles on already dangerous urban roads. A handful of drivers doggedly stick to 20 mph limits, and everyone else is frustrated and delayed behind them. This leads to dangerous overtaking, rising levels of driver anger (just human nature) and awkward confluences of 20 mph and
    30 mph zones.


I can imagine there is some truth to your 1st and 3rd bullets but I haven’t read that enforcement of 30mph has been de-prioritised and can’t see how de-prioritising 20mph enforcement would lead to worse behaviours than under a 30mph limit.

And of course your arguments need to be weighed up against the argument that many drivers will now drive more slowly and this will reduce the incidence and severity of accidents.


You asked why 20mph zones might see increased accident rates - I mentioned police enforcement because the police chief guidance clearly de-prioritises enforcement within 20mph zones due to the inappropriate limits. Reduced enforcement will lead to more dangerous driving behaviour.

I wasn’t talking about enforcement within 30mph limits.


Are you referring to the ACPO guidance article you posted?

I took the article to mean that they will chose when to enforce the new 20mph limits, but not that this would mean enforcement becoming more relaxed than pre Sep-2016 (even in ‘20mph zones’).

If someone could put together a list of which SE23 roads “feel and look like a 20mph zone rather than a zone with a 30mph or higher limit” I’m sure many of us would find it useful.




I see your interpretation, but don’t believe that they are suggesting only enforcing any kind of driving standards “where the limit is obvious to drivers”


Seems the 20mph blight continues to spread to other boroughs. Quite irritating when switching from one borough to another, where the neighbouring borough doesn’t need to post obvious speed signs as mentioned above, 30 is the assumed limit.
Lovely to see places like Perry Vale where the average hasn’t dropped, embellished with “Thank you for driving safely” signs. More money, thanking people for nothing. Well done Lewisham.

Still seems to me like more thought is needed on these zones. Especially when the speed activated matrix on Brockley Road STILL flashes up with 30mph in the middle of a 20mph.

Can’t wait for the next phase to start, the traffic calming measures. Thank heavens for cycling.


Cannot wait for 20mph in Forest hill. It scares the life out of me when lorries turn on South Circular road to the left, opposite the WHSmith on high spreed. Sometimes i step back hoping it may save me if a lorry flips over, but of course there are hardly any chances of survival if happens that a lorry looses control and flips over. Some drivers are mad, not all of them, very few of them but because of those few people have to suffer.


That is a Red Route and will remain 30mph. The rest of Forest Hill other than the red routes are already 20mph.


Shame, why do people need 20mph on quiet roads when cars are driven like mad on the main roads where most accidents happen?