M4 40mph speed limit for pollution

[WindyCommon]

I drove to Cardiff last night. At around 11pm there was little if any traffic on the M4.

And yet the variable speed limits were set to 40mph, with signs saying that this was to reduce pollution.

Is this a frequent occurrence, and is it effective?

[Chrisgr31]

WindyCommon said:

I drove to Cardiff last night. At around 11pm there was little if any traffic on the M4.

And yet the variable speed limits were set to 40mph, with signs saying that this was to reduce pollution.

Is this a frequent occurrence, and is it effective?

Dont most cars have a better MPG at 50mph, in which case wouldnt that be better for pollution?

[Chris32345]

WindyCommon said:

I drove to Cardiff last night. At around 11pm there was little if any traffic on the M4.

And yet the variable speed limits were set to 40mph, with signs saying that this was to reduce pollution.

Is this a frequent occurrence, and is it effective?

Yes it is effective just look at your fuel use at 50 Vs 70

[thecremeegg]

WindyCommon said:

I drove to Cardiff last night. At around 11pm there was little if any traffic on the M4.

And yet the variable speed limits were set to 40mph, with signs saying that this was to reduce pollution.

Is this a frequent occurrence, and is it effective?

Whatever it is, it's infuriating! That late at night the amount of traffic is negligable

[WindyCommon]

Chris32345 said:

WindyCommon said:

I drove to Cardiff last night. At around 11pm there was little if any traffic on the M4.

And yet the variable speed limits were set to 40mph, with signs saying that this was to reduce pollution.

Is this a frequent occurrence, and is it effective?

Yes it is effective just look at your fuel use at 50 Vs 70

Having experienced the zero-tolerance of the M4 “smart motorway” speed policing, I stuck religiously to 40mph through the 10 mile stretch of this. 10 miles at 40mph = 15 minutes. For the preceding 100 or so miles, I’d driven at around 80mph. 10 miles at 80mph = 7.5 minutes. So I spent twice as long polluting the precious air of Gwent as I might have done otherwise.

Whilst the science may support this in absolute terms, any benefit is probably marginal, I suspect that the true motivation is anything but climate-related. Rather it is a thinly veiled layer of taxation and public sector employment.

I am a regular traveller on this 10 mile stretch of the M4 as I have family and roots in Cardiff. It is infuriating - constantly subject to roadworks and over-zealous policing. It is materially harder to traverse than the preceding 90 or so miles of the M4. We have come to expect delays and difficulty here.

I worry that this is purely an outcome of local / special-interest politics, and the ridiculous over-presence of the public sector in this part of the UK.

Why is this ok? Why should this stretch of motorway be so different/difficult? What justification is there for this? And how/why do local motorists and residents put up with it?

[Geffg]

Just another thing of motorists getting blamed and penalised for any issue they want. Lower the limits for the environment, lower the limits to protect the pedestrians ( god forbid they take responsibility for themselves near a road ) etc. Pedestrianise everywhere so people can walk without being near a car, slow cars down so when they don’t look where they’re going they don’t get injured. Motoring is just demonised now and made to feel that you should be ashamed of yourself for even owning or needing a car. So annoying and I’m glad I was able to enjoy motoring years ago. It’s now sometimes a chore due to the amount of cameras making sure you don’t make the slightest mistake no matter how complicated they make junctions and roads with bus lanes, cycle lanes, box junctions, red line areas, etc, etc, then they can make more money from us disproportionately to the offence. Unfortunately I have to drive as part of my job and a lot of it in city centres which is getting ridiculous to try and do my job with so many restrictions to parking etc.

[Graveworm]

Chris32345 said:

Yes it is effective just look at your fuel use at 50 Vs 70

Chrisgr31 said:

Dont most cars have a better MPG at 50mph, in which case wouldnt that be better for pollution?

According to the Energy Saving Trust depending on gearing and car it's between 55 & 65 FOR ICE cars so 40 or even 50 is not the best for most cars, and 20mph limits are definitely an own goal unless they actually encourage less use of cars. Electric vehicles, of course, are different. Vans, buses and HGVs. are complicated
as here
ICE:

Electric

Edited by Graveworm on Saturday 22 February 14:28

[WindyCommon]

Does highest fuel economy = lowest emissions?

Remember we are measuring aggregate emissions over a set distance, in this case 10 miles.

[Graveworm]

WindyCommon said:

Does highest fuel economy = lowest emissions?

Remember we are measuring aggregate emissions over a set distance, in this case 10 miles.

Yes(ish) almost a smooth curve.

[otolith]

This is sod all to do with fuel economy or CO2 emissions. It's about emissions of oxides of nitrogen.



https://racfoundation.wordpress.com/tag/speed-limi...

[Graveworm]

otolith said:

This is sod all to do with fuel economy or CO2 emissions. It's about emissions of oxides of nitrogen.



https://racfoundation.wordpress.com/tag/speed-limi...

That same article incorrectly implies though that for the same speeds..
"By the same token, it reduces CO2 emissions and fuel consumption, which saves drivers money."
I wish it was all about NOX as in petrol cars an inconvenient truth is NOx per mile falls with increases in speed
It also references a Euro 5 2.0D (At least 6 years old now) which, like all diesels does have an higher NOx than petrol and a different consumption curve and Euro6 add blue etc more than halves this. (Or reduces it by 66% in petrol cars) More CO2 is still bad or at least I do seem to have read something about that...

Edited by Graveworm on Saturday 22 February 17:05

[meatballs]

Super frustrating in a Tesla I'd imagine

[otolith]

Graveworm said:

I wish it was all about NOX as in petrol cars an inconvenient truth is NOx per mile falls with increases in speed

Erm, that article says the exact opposite of that - are you looking at the data for petrol cars? Petrol cars are not the cause of the NOx problem.

[RSTurboPaul]

I don't have the misfortune to visit this part of the world often, but I'd probably end up going through it in third gear to make sure I didn't 'creep' momentarily over the limit, which would probably kind of defeat the object of their exercise...

[Graveworm]

otolith said:

Graveworm said:

I wish it was all about NOX as in petrol cars an inconvenient truth is NOx per mile falls with increases in speed

Erm, that article says the exact opposite of that - are you looking at the data for petrol cars? Petrol cars are not the cause of the NOx problem.

Yes I am looking at the data for petrol cars - which is why I said petrol cars. But the speed limit is for all vehicles. So you are happy that this one size fits all, increases everyone's journey times, with the acknowledged economic impact. It increases the output of CO2 in all cars just to reduce the NOx in non Euro 6 diesels whilst increasing it in petrol cars. CO2 long term is a bigger issue and, where most motorways are, the health risks of NOx are marginal. The hypocrisy is I don't see them increasing speed limits elsewhere, especially urban areas (Where NOx is the biggest problem), to put the Diesels closer to their sweet spot for NOx as well as reducing CO2 and fuel use for all vehicles.

Edited by Graveworm on Saturday 22 February 23:21

[usn90]

Just to sideline here, this zero tolerance you speak of
On the M4.

Do you actually get “done” for going 1MPH over the displayed limit?

[otolith]

Graveworm said:

Yes I am looking at the data for petrol cars - which is why I said petrol cars. But the speed limit is for all vehicles. So you are happy that this one size fits all, increases everyone's journey times, with the acknowledged economic impact. It increases the output of CO2 in all cars just to reduce the NOx in non Euro 6 diesels whilst increasing it in petrol cars. CO2 long term is a bigger issue and, where most motorways are, the health risks of NOx are marginal. The hypocrisy is I don't see them increasing speed limits elsewhere, especially urban areas, to put the Diesels closer to their sweet spot for NOx as well as reducing CO2 and fuel use for all vehicles.

It would be nice to have different speed limits for different classes of car, based on their emissions, but then enforcement would be difficult, speed differentials might be risky, and you would have the problem of deciding which pollutant you benchmark on. Realistically, if you have an NOx problem and the only tool you have is a speed limit reduction, that’s what you will do.

[Evanivitch]

WindyCommon said:

Why is this ok? Why should this stretch of motorway be so different/difficult? What justification is there for this? And how/why do local motorists and residents put up with it?

It's the busiest section of motorway in Wales, it's a bottle neck at the Brynglas Tunnels, it's through an area of high urban population, it's not an average speed camera zone, it's not heavily policed, all the gantry cameras are well logged on google maps and Waze.

Why didn't the huge average speed section at Reading bother you? Or the variable section at Bristol (which IME has more variation and less justification than Newport)?

[matt2911]

Evanivitch said:

It's the busiest section of motorway in Wales, it's a bottle neck at the Brynglas Tunnels, it's through an area of high urban population, it's not an average speed camera zone, it's not heavily policed, all the gantry cameras are well logged on google maps and Waze.

Why didn't the huge average speed section at Reading bother you? Or the variable section at Bristol (which IME has more variation and less justification than Newport)?

As much as it is a pain being limited at that time you are correct it’s easy to speed up between gantries.

I strongly doubt OPs time is so precious that the extra few minutes makes a significant difference to them...

With the road works OP, I’m very happy that the road works are annoying you as Wales sensibly limit them to night time’s usually rather than ruining the road at peak times. Which regardless of it inconveniencing you is better for most of us. Highways England could learn something from this.

[Evanivitch]

matt2911 said:

With the road works OP, I’m very happy that the road works are annoying you as Wales sensibly limit them to night time’s usually rather than ruining the road at peak times. Which regardless of it inconveniencing you is better for most of us. Highways England could learn something from this.

Lotta night time tree cutting right now!