Read once that the average journey time will be shortened if speed limits are 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.
Read once that the average journey time will be shortened if speed limits are reduced.
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
After driving on the continent recently, UK motorways seem glacial at 70mph. When they aren't congested.
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
So if it's to reduce pollution it won't apply to EV's then?
Thought not.
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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Last edited by Franky Four Fingers; 24th September 2020 at 20:51.
M62 between Manchester and Warrington, the ‘so called temporary’ speed limit has been 50 for years.
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
60! Luxury! It's 40 mph on the end section of the M32 here (Bristol)
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?
People forget that EVs travelling faster will need charging "sooner". EVs emit their pollution at the power station.
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.
Quite right. I was heading off on a tangent.
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.
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.
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.
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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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?
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.
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.
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
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
Problem with this is that due to relativity you’ll become heavier and use more fuel.
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 ;-)
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.
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!
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 :-)