Assuming that a car's engine runs somewhere near the optimum rev band much of the time, it seems to follow that if these limits were observed it would take cars 50% longer to cross a zone at 20mph than at 30mph, thereby polluting similarly more.
The amount of polution produced varies with the car and the way its being driven, surely nothing can be or should be assumed.
Accidental injury might be reduced, but at the cost of more damage to health. So which is preferable?
It well proven that being hit at 30 mph will cause far more serious injuries than being hit at 20 mph.
What we need is proof that the polution has been increased, I wonder, do local authorities do any before/after studies
Round our way the only 20mph zones are near schools at certain times so its not an issue for me personally, I don't drive at those time of day in those areas. But what needs stopping are the drivers with the chipped diesels that produce more smoke than an ocean liner. They would never pass an MOT so they need taking off the roads and crushing even if they are nearly new and valuable. The drives know exactly what they are doing.
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Assuming that a car's engine runs somewhere near the optimum rev band much of the time,
A rash assumption. Few people would drive in a different gear to maintain a steady 20mph to the one they would use to drive at 30mph.
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Having a really accurate instant fuel usage readout on my lorry has been an eye opener., you don't get such accuracy in a car because under power figures unless on a long incline the in gear times are too short to judge by.
For example cruising across our yard at 10 mph in the correct gear the vehicle is doing roughly the same mpg as at 30 mph, which isn't so far different from a nice gentle 50mph cruising speed...not trailing throttle just maintaining speed on a level road i am talking about, i suspect friction or lack of it is helping the slow speed economy.
A completely different kettle of fish when pulling hard on an incline mind, where in the lower gears say 4 to 7, less than 1mpg is quite normal, quadrupling up the scale to 3.9 mph which is a typical fuel consumption under full power in 12th, top gear.
So, IMHO maintaining a steady 20mph will produce very little different pollutants than at 30, for one thing it's easier on the vehicle and unless its got more gears than you can shake a stick (what on earth cars need 9 gears for i cannot understand, except to brag about) then it will most likely be able to cruise at 20 in the same gear as 30.
I agree about the overfuelled lowered idiot mobiles about, as well as the ill maintained MPV's typically Zafiras, some of them the whole back of the car is covered in soot and it's like a battle cruiser laying smoke when they floor it, you can taste the muck if stuck behind them as you quickly search for the recycle air switch.
Edited by gordonbennet on 28/09/2018 at 14:51
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I agree about the overfuelled lowered idiot mobiles about, as well as the ill maintained MPV's typically Zafiras, some of them the whole back of the car is covered in soot and it's like a battle cruiser laying smoke when they floor it, you can taste the muck if stuck behind them as you quickly search for the recycle air switch.
This has been one of my main complaints to some people who complain about the quality of the air, as some own these motors that spew black smoke out but will not do anything about it
it seems they are more than capable of complaining about bad air, but wont pay out to have the smoke problems fixed, as for 20mph limits I don`t see many sticking to it, unless there are steep speed humps about, but then some bought 4x4s to counteract them
wasn`t there comments about accidents increased after 20mph was applied?
Edited by bolt on 28/09/2018 at 16:00
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I agree about the overfuelled lowered idiot mobiles about, as well as the ill maintained MPV's typically Zafiras, some of them the whole back of the car is covered in soot and it's like a battle cruiser laying smoke when they floor it, you can taste the muck if stuck behind them as you quickly search for the recycle air switch.
This has been one of my main complaints to some people who complain about the quality of the air, as some own these motors that spew black smoke out but will not do anything about it
it seems they are more than capable of complaining about bad air, but wont pay out to have the smoke problems fixed, as for 20mph limits I don`t see many sticking to it, unless there are steep speed humps about, but then some bought 4x4s to counteract them
wasn`t there comments about accidents increased after 20mph was applied?
Re the black smoke. What's going on with that then? How do they get away with that? Is it remapped engines that can somehow revert to standard fuelling to pass the MOT?
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For example cruising across our yard at 10 mph in the correct gear the vehicle is doing roughly the same mpg as at 30 mph, which isn't so far different from a nice gentle 50mph cruising speed...not trailing throttle just maintaining speed on a level road i am talking about, i suspect friction or lack of it is helping the slow speed economy.
Tends to confirm my suggestion? If this applies to a typical car, then as it is in the 20 zone for 50% longer than a 30 (assuming driving at the permitted speed) it will leave 50% more emissions in the zone. After all, if it is stationary with the engine idling, the emissions just stack up. And if it shot through at 60, very little would be left behind :-(
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Tends to confirm my suggestion? If this applies to a typical car, then as it is in the 20 zone for 50% longer than a 30 (assuming driving at the permitted speed) it will leave 50% more emissions in the zone. After all, if it is stationary with the engine idling, the emissions just stack up. And if it shot through at 60, very little would be left behind :-(
If the same amount of fuel is being used to cover the ground, wouldn't the emissions be the same thereabouts?
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If this applies to a typical car, then as it is in the 20 zone for 50% longer than a 30 (assuming driving at the permitted speed) it will leave 50% more emissions in the zone. After all, if it is stationary with the engine idling, the emissions just stack up. And if it shot through at 60, very little would be left behind :-(
Simple fact. When I was commuting I was using the M1 for probably 75% of the distance driving as close as possible to the national limit as traffic conditions permitted and I would average about 50mpg in my Focus diesel.
Then they introduced a 50mph limit on that stretch of M1 whilst they did long term roadworks. Obviously my journey to and from the M1 on both commutes was the same but the drive on the M1 was slower. After a couple of months my average mpg had risen to over 55 mpg and you have to also consider that my non commuting driving remained exactly the same.
So its clear to me that driving at a lower speed (taking longer to cover the same distance) saves fuel and since mpg and emmisions are directly related it reduces the amount of polutants.
Once the limit was removed and it was back to driving as close to 70 mph as possible my mpg returned to a regular 50 mpg and I was kicking out more crap from the exhaust.
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Portsmouth is my nearest city and it's had 20mph limits on all but the main roads for several years. Much of Pompey is Victorian terraces with more cars than parking spaces so the roads are often one way with parked cars on both sides and room for one to drive down the middle.
If I lived on one of these roads I'd appreciate slower moving traffic from a safety aspect over any increase in pollution. Also whilst not everyone takes notice of the limit, without it more cars would be doing 30mph plus.
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viewed differently, how much longer were the journeys at the slower speed, multiply that by 10 and that is "lost" time per week
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viewed differently, how much longer were the journeys at the slower speed, multiply that by 10 and that is "lost" time per week
I'd say with some certainty that in 20 limit v 30 it's nothing like the 50% being posited upthread. In fact I'd suggest there's little real difference at all.
Similarly with 50mph roadworks. We had years of 50mph from M1 J16 to 19 because they'd just renewed the central barrier when they got funding for conversion to smart motorway. Three lanes at 70 were subject to constant slow downs to almost stop due 'concertina' braking. Average speed <50mph. With speed limit 50 for road works traffic was smoother and average speed no less and probably more.
Still prefer the A5 south of M1 J20 or 18 and often find it faster than M1 but that's because I'm much closer to A5 then M1.so less time spent on minor roads.
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Afraid your conclusion is wrong for various reasons.
1/ Above 50mph air resistance starts to build up, so the difference between 55 and 70 mph on the same road and in the same gear can be 10-15%; the faster you go the worse it gets; the same 10-15% difference is found between 70 and 80mph,etc
A particularly aerodynamic car suffers less, but still suffers; and the modern upright SUV suffers more. The difference in mine is 35% between 55 and 70mph.
2/ You cannot drive most cars at 20mph in top gear; and although it is possible in some diesel cars, it will b***** up the engine if done long term.
So effectively you are comparing apples with pears.
My main grumble with 20mph zones is, the zones are (mostly) there 365 days a year, the schools are closed for over 30% of those days.
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Is your post about speed and emissions serious?!
Edited by straggler on 03/10/2018 at 16:49
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Is your post about speed and emissions serious?!
Not clear whose post you refer to. But it seems to me that - even if GB's vehicle does the same mpg at 10mph as at 50 - at the slower speed it takes longer to cover the distance, so the emissions are greater per yard than at the higher speed. Or, by reductio ad absurdum, at zero speed there can be no argument, can there?
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