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Thread: Motorway Speed limits to be reduced.

  1. #1
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    Motorway Speed limits to be reduced.

    Just saw online that Highway’s England are going to be running a trial on parts of four different motorways reducing the speed limit to 60mph to help reduce pollution.
    I can’t help but feel that this is an excuse to bring in speed reductions on all motorways in due course.
    Cars are safer and cleaner than they have ever been, yet the motorist is getting squeezed in some form or other more and more.

  2. #2
    Read once that the average journey time will be shortened if speed limits are reduced.

  3. #3
    Id like to see the evidence that a car pollutes less at 60 than it does at 70. My guess would be that because the vehicles are going slower to start off with the need to brake and accelerate is reduce which overall would possibly reduce emissions.
    However at 60 it can’t just be the fact the car is polluting less than 70

  4. #4
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    After driving on the continent recently, UK motorways seem glacial at 70mph. When they aren't congested.

  5. #5
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    Quote Originally Posted by Franky Four Fingers View Post
    Id like to see the evidence that a car pollutes less at 60 than it does at 70. My guess would be that because the vehicles are going slower to start off with the need to brake and accelerate is reduce which overall would possibly reduce emissions.
    However at 60 it can’t just be the fact the car is polluting less than 70
    Frankie , love to disagree with you but just outside Chester where we live there are two National speed limits that enter a “ reduced emissions zone” these are 4 or 5 mile stretches with avg speed cameras on masts and big “Pollution Kills” signs. 50 mph all the way.
    We just have to suck it up. Nothing to do with road casualties, just pollution!

    Steve

  6. #6
    Craftsman
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    So if it's to reduce pollution it won't apply to EV's then?





    Thought not.

  7. #7
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    On the 50mph average speed section of the m4 between Port Talbot and the Jersey Marine turn off my trip computer shows a 10% plus decrease in fuel consumption compared to doing 70mph either side of it. Surely if I'm burning 10% less fossil fuel my car is emitting 10% less pollution?

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  8. #8
    Quote Originally Posted by higham5 View Post
    Frankie , love to disagree with you but just outside Chester where we live there are two National speed limits that enter a “ reduced emissions zone” these are 4 or 5 mile stretches with avg speed cameras on masts and big “Pollution Kills” signs. 50 mph all the way.
    We just have to suck it up. Nothing to do with road casualties, just pollution!

    Steve
    Ok I won’t argue with that however my point was an engine doing 50 won’t necessarily pollute less than a vehicle doing 60. As I said I’d suspect it was the fact the need to slow down and then having to accelerate is reduced due to the lower speeds to start off with.
    Last edited by Franky Four Fingers; 24th September 2020 at 20:51.

  9. #9
    Master Lammylee's Avatar
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    Motorway Speed limits to be reduced.

    M62 between Manchester and Warrington, the ‘so called temporary’ speed limit has been 50 for years.

  10. #10
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    Quote Originally Posted by higham5 View Post
    Frankie , love to disagree with you but just outside Chester where we live there are two National speed limits that enter a “ reduced emissions zone” these are 4 or 5 mile stretches with avg speed cameras on masts and big “Pollution Kills” signs. 50 mph all the way.
    We just have to suck it up. Nothing to do with road casualties, just pollution!

    Steve
    Do those cameras work? I often (used to) drive through the stretch towards Ewloe when going to our head office. Most people stuck to 50 but the odd
    Person would bomb through.

  11. #11
    Grand Master Velorum's Avatar
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    Regardless of whether or not it helps reduce pollution if they up the number of speed cameras then it will be a good income generator
    ''I must acknowledge, once and for all, that the
    purpose of diplomacy is to prolong a crisis"

  12. #12
    60! Luxury! It's 40 mph on the end section of the M32 here (Bristol)

  13. #13
    Master mondie's Avatar
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    As a rabid car lover I cannot wait for driverless cars for motorway journeys. There is a scientific basis to lower speed limits because drag increases with the square of velocity. However, and as already said above, cars are cleaner than ever and electric cars are becoming more comon so I do not understand the logic of this decision.

    Perhaps air pollution limits in built up areas are being reduced quicker than car pollution levels?

  14. #14
    Master Templogin's Avatar
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    People forget that EVs travelling faster will need charging "sooner". EVs emit their pollution at the power station.

  15. #15
    Master mondie's Avatar
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    While true I certainly haven't forgotten, but it is irrelevant if the justification for lower limits is local pollution in built up areas such as the resitrictions on a havily populated stretch of the A55 in North Wales mentioned earlier.

  16. #16
    Master Templogin's Avatar
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    Quite right. I was heading off on a tangent.

  17. #17
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    Quote Originally Posted by Chinese_Alan View Post
    Do those cameras work? I often (used to) drive through the stretch towards Ewloe when going to our head office. Most people stuck to 50 but the odd
    Person would bomb through.
    Alan the answer is I dont know but the only people who speed through are the Irish plates heading for the ferry. Its a case of do you feel lucky. The ones on the A583 Wrexham stretch are reinforced by little lazer picture vans! Drove through one today.

  18. #18
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    Not a fan.of this trend but it does seem to make sense; most ICE cars seem to be designed/tuned to be most fuel efficient at around 55-60mph - so that must logically coincide with when they are the least polluting.

    If average journey times decrease then that would be an added bonus, a win-win.

  19. #19
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    I thought it was just the SR-71 that became more fuel efficient the faster it went.

    Presumably, it would be exempt from certain taxes, too.

    If it could do L. A. to D. C. in 60 minutes, London to Bath would be a doddle.

  20. #20
    Quote Originally Posted by Ruggertech View Post
    On the 50mph average speed section of the m4 between Port Talbot and the Jersey Marine turn off my trip computer shows a 10% plus decrease in fuel consumption compared to doing 70mph either side of it. Surely if I'm burning 10% less fossil fuel my car is emitting 10% less pollution?
    I might be missing the point here but your consumption goes down 10% while your velocity goes down ~30%, therefore if it takes you 30% longer to get there you engine is on 30% longer thus producing more pollution?

  21. #21
    Master thieuster's Avatar
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    Mid-march we went from 120 km/hr (±75 mph) to 100 km km/hr (± 62 mph) here in The Netherlands.

    It's a total disaster!

    Driving hasn't become easier, simply because it's slower nowadays. All cars now have more or less the same speed. As if you're driving along in a 62 mph traffic jam! One needs to be ultra-alert. Lorries start overtaking as well on dual carriageways, because the speed difference on the two lanes is more or less gone. That causes long 'tails' of cars behind the lorry - and that 'tail' goes on for miles after the lorry has gone, because all cars accelerate to 62 mph and stay close to each other.

    So the argument: 'it's safer' has gone out of the window. The number of accidents hasn't come down simply because they're not longer caused by speed difference, they're now caused by not keeping enough distance.

    I still think that the French and (mandatory) Italian system works best: 90 km/hr for lorries on the inner lane, on the middle lane 110 km/hr for those who want to drive calmly/save fuel and 130 km/hr on the lane for overtaking.

    M
    Last edited by thieuster; 25th September 2020 at 06:42.

  22. #22
    Quote Originally Posted by eldrich View Post
    I might be missing the point here but your consumption goes down 10% while your velocity goes down ~30%, therefore if it takes you 30% longer to get there you engine is on 30% longer thus producing more pollution?
    I don’t think you’ve missed the point at all, if you assume a journey of 100 miles and you could achieve a constant speed of 60 mph your time will be approx 15 minutes more to complete that journey than if you were doing 70 mph. So I drive from my house to Newquay in Cornwall which is approx 200 miles door to door and I’ll be on the roads approximately 30 minutes longer, it makes little sense to me.

  23. #23
    Quote Originally Posted by thieuster View Post
    Mid-march we went from 120 km/hr (±75 mph) to 100 km km/hr (± 62 mph) here in The Netherlands.

    It's a total disaster!

    Driving hasn't become easier, simply because it's slower nowadays. All cars now have more or less the same speed. As if you're driving along in a 62 mph traffic jam! One needs to be ultra-alert. Lorries start overtaking as well on dual carriageways, because the speed difference on the two lanes is more or less gone. That causes long 'tails' of cars behind the lorry - and that 'tail' goes on for miles after the lorry has gone, because all cars accelerate to 62 mph and stay close to each other.

    So the argument: 'it's safer' has gone out of the window. The number of accidents hasn't come down simply because they're not longer caused by speed difference, they're now caused by not keeping enough distance.

    I still think that the French and (mandatory) Italian system works best: 90 km/hr for lorries on the inner lane, on the middle lane 110 km/hr for those who want to drive calmly/save fuel and 130 km/hr on the lane for overtaking.

    M
    Decreased pollution Menno?

  24. #24
    Grand Master snowman's Avatar
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    Quote Originally Posted by Velorum View Post
    Regardless of whether or not it helps reduce pollution if they up the number of speed cameras then it will be a good income generator
    Indeed, got to pay for Furlough somehow

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  25. #25
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    Quote Originally Posted by Franky Four Fingers View Post
    I don’t think you’ve missed the point at all, if you assume a journey of 100 miles and you could achieve a constant speed of 60 mph your time will be approx 15 minutes more to complete that journey than if you were doing 70 mph. So I drive from my house to Newquay in Cornwall which is approx 200 miles door to door and I’ll be on the roads approximately 30 minutes longer, it makes little sense to me.
    If I drive for 50 miles at 50mph and average 50mpg, compared to driving for 50 miles at 70mph and average 45mpg my car has burnt less fossil fuel for the same distance, regardless of the time taken. So I would say the point has been missed completely.


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  26. #26
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    What is the most efficient speed to drive at?
    I remember as a child my dad saying 57mph, and I’ve always gone at that speed where practicable (where the speed limit allows!).
    Is this 57mph on the in car speedometer? Or ‘GPS 57mph’? Or some other speed?

  27. #27
    I don't think all cars use less fuel at 60mph than 70, it may be true for small cars but most large engines are or can be more efficient at the higher speed depending on gearing, obviously they can pull a higher gear due to more torque, as an example my Merc does about 28mpg at 60 yet does about 32 at 70-80, My Noble is even more efficient at the higher speeds.

    Yet our EV never achieves the claimed mileage and I don't know anyone who has an EV or hybrid who has hit the claimed numbers on a real road.

  28. #28
    Quote Originally Posted by eldrich View Post
    I might be missing the point here but your consumption goes down 10% while your velocity goes down ~30%, therefore if it takes you 30% longer to get there you engine is on 30% longer thus producing more pollution?
    Yes, you are missing the point! Fuel consumption is measured in mpg (or similar), time doesn’t come into it.
    If you travel 100 miles at 50mpg that’s 2 gallons. At 40mpg it’s 2.5 gallons.

  29. #29
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    Quote Originally Posted by Kingstepper View Post
    Yes, you are missing the point! Fuel consumption is measured in mpg (or similar), time doesn’t come into it.
    If you travel 100 miles at 50mpg that’s 2 gallons. At 40mpg it’s 2.5 gallons.
    Indeed, as you say, mpg is a measure of consumption over a distance, not time.
    Last edited by Ruggertech; 25th September 2020 at 09:04.

  30. #30
    Quote Originally Posted by eldrich View Post
    I might be missing the point here but your consumption goes down 10% while your velocity goes down ~30%, therefore if it takes you 30% longer to get there you engine is on 30% longer thus producing more pollution?
    You're not missing the point but your maths/physics is the wrong way round.

    As mentioned elsewhere in the thread, whilst rolling resistance is linear, drag goes up as a cube of speed (ie if you double the speed you have eight times the drag)

    In your scenario it's more likely that your fuel consumption will go down by 30% but your velocity goes down by 10%

    Note - Maris corrected my physics so I've updated this accordingly. My mistake is still there for all to see in Maris' quote below
    Last edited by Gyp; 25th September 2020 at 13:21.

  31. #31
    Quote Originally Posted by ach5 View Post
    What is the most efficient speed to drive at?
    I remember as a child my dad saying 57mph, and I’ve always gone at that speed where practicable (where the speed limit allows!).
    Is this 57mph on the in car speedometer? Or ‘GPS 57mph’? Or some other speed?
    Fuel consumptions used to be shown under 3 circumstances which if I recall were the urban cycle, 56mph and 70mph.

    The urban cycle was a set stop/start environment, whereas the other two were effectively steady speeds with 70mph being the UK motorway speed limit

    Of those 3, 56mph was always the best fuel consumption figure.

    It's not that 56mph is the most economical speed to drive at though, it's just the most economical speed of those three scenarios.

    But why 56? 56mph is 90kph so you only have to do the test once for each car to get accurate figures for both the UK and Europe.

    Because of <physics> a car driven steadily at 30 will use less fuel than one driven at 40, which will use less fuel than one driven at 50, which will use less fuel than one driven at 60 and so on. Because of <physics> it's quite possible that a car travelling at 90mph is using twice the amount of fuel per mile than the same car would at 60mph

  32. #32
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    Quote Originally Posted by Gyp View Post
    You're not missing the point but your maths/physics is the wrong way round.

    As mentioned elsewhere in the thread, whilst rolling resistance is linear, drag goes up as a square of speed (ie if you double the speed you have four times the drag)

    In your scenario it's more likely that your fuel consumption will go down by 30% but your velocity goes down by 10%
    Yes he is I'm sorry. As Kingstepper said above "If you travel 100 miles at 50mpg that’s 2 gallons. At 40mpg it’s 2.5 gallons."


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  33. #33
    Drive there flat out, use your engine for the least possible amount of time, less pollution, must be. Better have a smiley or someone will take me seriously

  34. #34
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    Quote Originally Posted by catch21 View Post
    Drive there flat out, use your engine for the least possible amount of time, less pollution, must be. Better have a smiley or someone will take me seriously
    I like the attitude

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  35. #35
    Problem with this is that due to relativity you’ll become heavier and use more fuel.

  36. #36
    Bernoilli's equation applies. If you restrict the flow but keep on pumping the same quantity of stuff down the tube, the pressure goes up. In practical terms that means closer vehicle spacing, more congestion and more time sitting in a traffic jams. Enjoy.

  37. #37
    Grand Master Saint-Just's Avatar
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    Quote Originally Posted by catch21 View Post
    Drive there flat out, use your engine for the least possible amount of time, less pollution, must be. Better have a smiley or someone will take me seriously
    I only drive flat out when I think by brakes are about to fail, to limit the risks to others by reducing the time on the road.
    'Against stupidity, the gods themselves struggle in vain' - Schiller.

  38. #38
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    Quote Originally Posted by Groundrush View Post
    Bernoilli's equation applies. If you restrict the flow but keep on pumping the same quantity of stuff down the tube, the pressure goes up. In practical terms that means closer vehicle spacing, more congestion and more time sitting in a traffic jams. Enjoy.

    Not sure it's quite that simple (or linear).

    With drivers having to brake and accelerate frequently, which is the nature of most motorways, it's more like someone randomly putting and releasing kinks in a hose.

    I'm sure a more eloquent and scientific answer will be made shortly ;-)

  39. #39
    Valid reasons you can offer to the court for exceeding the new limits now include travelling to access child care and conducting an eye test while you drive. Oh and anything else dom was doing while exceeding the speed limit is perfectly fine as well, obvs.

  40. #40
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    Quote Originally Posted by Groundrush View Post
    Bernoilli's equation applies. If you restrict the flow but keep on pumping the same quantity of stuff down the tube, the pressure goes up. In practical terms that means closer vehicle spacing, more congestion and more time sitting in a traffic jams. Enjoy.
    I find when in the 50 zone, cruise control set, all is pretty relaxed. As soon as you leave it near Llandarcy the whacky races resume with mad lane changing and panic braking.

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  41. #41
    Quote Originally Posted by Groundrush View Post
    Bernoilli's equation applies. If you restrict the flow but keep on pumping the same quantity of stuff down the tube, the pressure goes up. In practical terms that means closer vehicle spacing, more congestion and more time sitting in a traffic jams. Enjoy.
    Time to buy another bike

  42. #42
    Master Tifa's Avatar
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    Quote Originally Posted by Templogin View Post
    EVs emit their pollution at the power station.
    Shhhhhhh!!!!!
    You'll spoil the suprise when they find out....

  43. #43
    Grand Master Dave+63's Avatar
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    Quote Originally Posted by Kingstepper View Post
    Problem with this is that due to relativity you’ll become heavier and use more fuel.
    Going off at a tangent, I thought that the theory of relativity meant that time slowed down the faster you went rather than your mass increase.

  44. #44
    Quote Originally Posted by Dave+63 View Post
    Going off at a tangent, I thought that the theory of relativity meant that time slowed down the faster you went rather than your mass increase.
    Believe both will occur.

  45. #45
    Grand Master Dave+63's Avatar
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    Quote Originally Posted by Tifa View Post
    Shhhhhhh!!!!!
    You'll spoil the suprise when they find out....
    We all know; the principle being that all the pollution is in one place where it can be dealt with more efficiently instead of forcing everyone around you to breathe it in.

    Now if we could all just fart in a centralised methane collector, we’d have more renewable energy to power our EVs and even cleaner air to breathe!

  46. #46
    Quote Originally Posted by Kingstepper View Post
    Believe both will occur.
    Because I get to spend more time in Greggs

  47. #47
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    Quote Originally Posted by Gyp View Post
    Fuel consumptions ...

    Brilliant, thanks Gyp!

  48. #48
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    Quote Originally Posted by jaytip View Post
    Just saw online that Highway’s England are going to be running a trial on parts of four different motorways reducing the speed limit to 60mph to help reduce pollution.
    I can’t help but feel that this is an excuse to bring in speed reductions on all motorways in due course.
    Cars are safer and cleaner than they have ever been, yet the motorist is getting squeezed in some form or other more and more.


    This is already happening in Austria and Croatia as far as I know, limit is 80 or 100(night, also due to noise pollution) where it used to be 130.
    Just drove 1200km from Germany to Croatia and those 250km through Austria seemed like eternity.

  49. #49
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    Quote Originally Posted by Gyp View Post
    You're not missing the point but your maths/physics is the wrong way round.

    As mentioned elsewhere in the thread, whilst rolling resistance is linear, drag goes up as a square of speed (ie if you double the speed you have four times the drag
    I always thought it was cubed, ie you need 8 times the power to go twice as fast, might be wrong though.

  50. #50
    Quote Originally Posted by Maris View Post
    I always thought it was cubed, ie you need 8 times the power to go twice as fast, might be wrong though.
    Oh yes, you could be right. I'll have to go check...

    <checks>

    Yes, you are of course right, it's cubed.

    https://physics.info/drag/#:~:text=T...re%20powerful.

    An even stronger argument that reducing speed saves fuel

    It goes a long way to explaining why I can get 70mpg in the lad's car and he struggles to get 35mpg :-)

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