Is it time to look again at 70mph Limits?

[fishwomp]

Only if you look at the tailpipe of one car (which is probably ULEZ compliant anyway). Tyre particulates increase with speed.

I think I can hear the moving of goalposts. Yes, tyre particulates exist, as does brake dust.. but that is not the primary problem that speed limits are aiming for, I haven't seen data taking about those in the context of 50mph zones.. That ULEZ car might also be using regenerative brakes so dust is less an issue.

Lowering speed limits smooths out traffic flow which reduces wasteful idling.

Does it? I am not disagreeing that this might help 50mph vice 70mph, but driving through a quiet village at 20mph seems to involve a significant amount of breaking on entering the zone, accelerating out of it, and a very fuel-inefficient speed within it. Tell me that that is not polluting more.

Horses for courses - if there are places 20mph is less polluting, go for it, but they are probably the places people already go that speed during the busy times and of course, that's 20mph at midnight as well, not just rush hour..

 

[Krokodil]

Does it? I am not disagreeing that this might help 50mph vice 70mph, but driving through a quiet village at 20mph seems to involve a significant amount of breaking on entering the zone, accelerating out of it, and a very fuel-inefficient speed within it. Tell me that that is not polluting more.

In stop-start traffic you'll be driving more efficiently if you go at a lower speed that allows you to keep going rather than doing 30mph-stop-30-stop etc.

== Doublepost prevention - post automatically merged: 21 Aug 2025 ==

20mph Speed Limit and Air Pollution

Questions to the Mayor: What analysis has been made of possible improvements to air quality as a result of reducing speed limits to 20 mph in London?

www.london.gov.uk

An evaluation of 20mph zones in London, carried out by Imperial College, showed slowing traffic had no net negative impact on exhaust emissions. However, in 20mph zones vehicles moved more smoothly, with fewer accelerations and decelerations, than in 30mph zones. This smoother driving style reduces particulate emissions from tyre and brake wear - which still represents a significant cause of air pollution from zero-emission vehicles.

 

[fishwomp]

In stop-start traffic you'll be driving more efficiently if you go at a lower speed that allows you to keep going rather than doing 30mph-stop-30-stop etc.

== Doublepost prevention - post automatically merged: 21 Aug 2025 ==

20mph Speed Limit and Air Pollution

Questions to the Mayor: What analysis has been made of possible improvements to air quality as a result of reducing speed limits to 20 mph in London?

www.london.gov.uk

Who on earth does 30mph in stop start traffic...

Crucially, this was analysis of London. Not too easy to do consistent 30 mph there..
Rather different to most of Wales. The point was that on the one hand "pollution" in Port Talbot, 70->50mph and on the other, I contend that my own car telling me I am having crap MPG at 20mph Vs when doing 30!

 

[61653 HTAFC]

This appears to have been a dual carriageway in the past, with the right hand lane now hatched out.

Pedantry alert: it's still a dual carriageway- dual carriageway status refers to the directional lanes being divided by a median, not to the number of lanes per direction.

 

[Harpo]

Pedantry alert: it's still a dual carriageway- dual carriageway status refers to the directional lanes being divided by a median, not to the number of lanes per direction.

I made the mistake in the past of assuming two lanes in one direction means 70mph.

 

[cactustwirly]

I made the mistake in the past of assuming two lanes in one direction means 70mph.

No if there is no central reservation then it's still 60
There are roads with a central reservation and only a single lane in each direction which would technically be a dual carriageway and 70 would apply

 

[Shrop]

Pedantry alert: it's still a dual carriageway- dual carriageway status refers to the directional lanes being divided by a median, not to the number of lanes per direction.

You're absolutely right! I should have said it once appeared to have had two lanes in the direction of flow . There's an update too, I re-visited the location on Wednesday and noticed that it's now been changed to 30mph! I'd say that's still unreasonably slow for a straight piece of wide road with no frontages and virtually no pedestrians, especially when there are plenty of urban single carriageway locations with numerous frontages, plenty of pedestrians, parked cars etc which are 40mph ...

 

[Topological]

No if there is no central reservation then it's still 60
There are roads with a central reservation and only a single lane in each direction which would technically be a dual carriageway and 70 would apply

This example always amuses me, a 20mph dual carriageway.

I do wonder if they should put some signs to point out that 70 is not the speed limit...

 

[brad465]

This example always amuses me, a 20mph dual carriageway.

I do wonder if they should put some signs to point out that 70 is not the speed limit...

View attachment 186844

Reminds me of the Pilgrims Way between the A229 at Blue Bell Hill and Burham village. It's dual carriageway, but it's effectively two country lanes with a line of trees separating them, with a 40mph speed limit in this case:


(Image of Google Street View of eastern end of Pilgrim's Way dual carriageway section)

 

[HSTEd]

Perhaps we could make an argument for a dual carriageway with a wire barrier as a general replacement for current single carriageway roads, on account of their superior safety performance?

I believe a basic dual carriageway with a rope barrier isn't actually that much wider than a "Super 2" type road.

 

[cactustwirly]

Perhaps we could make an argument for a dual carriageway with a wire barrier as a general replacement for current single carriageway roads, on account of their superior safety performance?

I believe a basic dual carriageway with a rope barrier isn't actually that much wider than a "Super 2" type road.

They're the same width, pretty sure the A34 has a mix and that's almost motorway standard

 

[Bletchleyite]

Perhaps we could make an argument for a dual carriageway with a wire barrier as a general replacement for current single carriageway roads, on account of their superior safety performance?

You mean one lane each way and a rope barrier, thus preventing overtaking? The big problem with this (though the Dutch seem to like them) is that a broken down vehicle closes the road and it's impossible to overtake a slow vehicle.

 

[HSTEd]

You mean one lane each way and a rope barrier, thus preventing overtaking? The big problem with this (though the Dutch seem to like them) is that a broken down vehicle closes the road and it's impossible to overtake a slow vehicle.

I was actually thinking about either a two lane each way dual carriageway with somewhat narrower lanes, or some sort of "D1M", a single lane each way with hard shoulder.

 

[SargeNpton]

Perhaps we could make an argument for a dual carriageway with a wire barrier as a general replacement for current single carriageway roads, on account of their superior safety performance?

I believe a basic dual carriageway with a rope barrier isn't actually that much wider than a "Super 2" type road.

I guess you're not a motorcyclist if you are advocating wire rope barriers on any road.

The support posts are very good for broken bones and the ropes can cause strangulation or even decapitation.

 

[styles]

I guess you're not a motorcyclist if you are advocating wire rope barriers on any road.

The support posts are very good for broken bones and the ropes can cause strangulation or even decapitation.

This was my thought. I'd rather come off my bike into a crash barrier than a wire.

There are bits of the A9 where wire is used but my guess is it's been a bit of a last resort.