M4 40mph speed limit for pollution
Discussion
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?
Imho this was the main reason for "smart" motorways, to reduce the car speed to get pollution down in high pollution areas. Big EU fines apparently albeit no idea if that has gone away cos #brexitAnd 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?
TX.
Edited by Terminator X on Sunday 23 February 01:47
Terminator X said:
Imho this was the main reason for "smart" motorways, to reduce the car speed to get pollution down in high pollution areas. Big EU fines apparently albeit no idea if that has gone away cos #brexit
It was definitely the main reason for some introductions of variable speed limits. The other reason, particularly where they have also turned the hard shoulder into a live lane, was dealing with congesting and delays and making journey times more predictable while penny-pinching on the roads budget.As for Brexit - we'll keep the standards, but we may need to set up an independent body to hold government to account.
https://researchbriefings.parliament.uk/ResearchBr...
usn90 said:
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?
No. There are average speed cameras through Port Talbot, nowhere else.On the M4.
Do you actually get “done” for going 1MPH over the displayed limit?
The only operational fixed cameras are Eastbound on the gantry immediately after the Brynglas tunnels, where the J25A slip merges, and Westbound on the second gantry approaching J26.
Pica-Pica said:
Muddle238 said:
Is the pollution speed limit enforceable to those with electric vehicles?
If so, why?
Yes. Because a speed limit is a speed limit.If so, why?
Muddle238 said:
Is the pollution speed limit enforceable to those with electric vehicles?
If so, why?
Yes. It also applies to cars with bigger engines, that can make use of longer gearing and have reduced emissions at higher speeds than the catch-all limit applied. If so, why?
You could also ask why someone with a 200+mph supercar should be subject to the same 70mph speed limit as a Nissan Leaf.
donkmeister said:
Yes. It also applies to cars with bigger engines, that can make use of longer gearing and have reduced emissions at higher speeds than the catch-all limit applied.
You could also ask why someone with a 200+mph supercar should be subject to the same 70mph speed limit as a Nissan Leaf.
Why is 2000rpm in 5th gear more efficient than 2000rpm in 4th gear?You could also ask why someone with a 200+mph supercar should be subject to the same 70mph speed limit as a Nissan Leaf.
Geffg said:
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.
I couldn't read beyond your bizarre dislike of pedestrian safety.And the lack of paragraph spacing.
handpaper said:
No. There are average speed cameras through Port Talbot, nowhere else.
The only operational fixed cameras are Eastbound on the gantry immediately after the Brynglas tunnels, where the J25A slip merges, and Westbound on the second gantry approaching J26.
+ Eastbound 2nd to last gantry before the Brynglas Tunnels - so you've got 2 going east and 1 going west...The only operational fixed cameras are Eastbound on the gantry immediately after the Brynglas tunnels, where the J25A slip merges, and Westbound on the second gantry approaching J26.
Evanivitch said:
donkmeister said:
Yes. It also applies to cars with bigger engines, that can make use of longer gearing and have reduced emissions at higher speeds than the catch-all limit applied.
You could also ask why someone with a 200+mph supercar should be subject to the same 70mph speed limit as a Nissan Leaf.
Why is 2000rpm in 5th gear more efficient than 2000rpm in 4th gear?You could also ask why someone with a 200+mph supercar should be subject to the same 70mph speed limit as a Nissan Leaf.
The fact is that bigger-engined cars are (usually) geared to use their considerable low-rpm torque when cruising, which reduces engine friction losses, pumping losses, allows more distance per engine revolution. So, 70mph for me is around 17-1800rpm in top. Some cars have even longer gearing. But in my OHs. S2000 it's around 3.5-4k rpm. So, with the sweet spot of petrol ICE emissions being between 2-3000 rpm, that corresponds with as much as 80mph in mine at 2000rpm, and 40mph in hers at 2000rpm. For a given distance her engine turns twice for each rotation of mine.
Clearly those are fairly extreme examples but it illustrates that what reduces emissions in one car can increase emissions in another. As the point I was responding to was "why should electric cars have to be limited?" I thought that was pertinent
WindyCommon said:
I think that your choice of denominator matters here. Lower emissions per minute, or per mile?
If the limit is on a 10 mile stretch of road, it is always a 10 mile distance. The amount of minutes it takes will vary depending on velocity.So therefore just measure it as emissions/mile for whichever speed
I've done a little reading on this subject. Here are two interesting charts from an EU study:
Figure 3 shows that NOx emissions for diesel cars reduce if mean speed is reduced from 110km/h.
But figure 4 shows that NOx emissions for petrol cars (like mine...) increase if mean speed is reduced from 110km/h.
If reduced NOx emissions are targeted, lower speed limits for petrol cars are counter-productive.
Figure 3 shows that NOx emissions for diesel cars reduce if mean speed is reduced from 110km/h.
But figure 4 shows that NOx emissions for petrol cars (like mine...) increase if mean speed is reduced from 110km/h.
If reduced NOx emissions are targeted, lower speed limits for petrol cars are counter-productive.
Edited by WindyCommon on Tuesday 25th February 21:18
The idea someone earlier came up with that going slower means it takes longer to do their journey, and so they must be polluting more...
If you're doing 70mph and you get 35mpg, you will travel 35 miles on one gallon of fuel.
If you're doing 50mph and you get 50mpg, you will travel 50 miles on one gallon of fuel.
So say you travel 700 miles:
At 70mph, it'll have taken 10 hours and you'll have used 20 gallons of fuel.
At 50mph it'll have taken 14 hours and you'll have used 14 gallons of fuel.
It took longer, the car engine was running for longer, but you used less fuel going slower to cover the same distance and thus less pollution to cover that distance.
The time taken is not the important factor here.
(But please someone correct me if I'm somehow wrong.
)
If you're doing 70mph and you get 35mpg, you will travel 35 miles on one gallon of fuel.
If you're doing 50mph and you get 50mpg, you will travel 50 miles on one gallon of fuel.
So say you travel 700 miles:
At 70mph, it'll have taken 10 hours and you'll have used 20 gallons of fuel.
At 50mph it'll have taken 14 hours and you'll have used 14 gallons of fuel.
It took longer, the car engine was running for longer, but you used less fuel going slower to cover the same distance and thus less pollution to cover that distance.
The time taken is not the important factor here.
(But please someone correct me if I'm somehow wrong.
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