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?

[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?

[WindyCommon]

Does highest fuel economy = lowest emissions?

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

[WindyCommon]

I think that your choice of denominator matters here. Lower emissions per minute, or per mile?

[WindyCommon]

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.

Edited by WindyCommon on Tuesday 25th February 21:18