Do we have a peer reviewed study (ie facts) that reduction of urban speed limit from 30 to 20 actually does save fuel ?
Highway limit, sure, wind resistance and all that, well proven.
30 down to 20 in a stop start environment, where in many cases you don’t even get to 30 ? Colour me skeptical - I’ve seen this often asserted but never found an actual scientificly run study to prove it.
The plural of anecdotes isn’t data, but from my personal experience my fuel consumption went up when 20 was introduced here a few years ago (I can theorise why if you want, but I can’t prove why, only state that it is measurably higher with no real change in driven miles - I track my consumption).
TFL did a study related to this which was cited when Wales set the default speed to 20.
They found that there was minimal difference in emissions between the two speeds. Any benefit of lower drag etc is countered by the fact that the engine is not geared well for 20 so generally it will be the same or only slightly better doing 20. (I am making the assumption here that lower emissions will equal less fuel usage.)
However, the greatest benefit is that there are less cars on the road because it encourages active travel and discourages driving. So comparing driving to not at all is a huge fuel difference.
From 30 to 20 specifically I have the same doubts about fuel economy specifically, but I do know there’s research that 20 is much more survivable for pedestrians when accidents occur, which is more important to me than the pinch at the pump. So, mixed bag.
Do we actually need a study to answer this given it’s basically a physics and mechanics question? The statistics on a given car/engine’s efficiency is published data if we want to work out hard numbers, but even just understanding the high level relationships kinda gives us the answer:
Firstly engines are generally more efficient the slower you run them, so you’re just gonna use less fuel anyway. Aerodynamic drag also affects anything that moves and the more something moves, the more drag it experiences.
Then you have acceleration which is also where a good percentage gets burnt, getting up to 30 from zero is going to burn through more fuel than zero to 20. Stop-start traffic would exacerbate losses here at the higher limit. Finally, limits are all day round, stop-start traffic is really only a major factor around rush hours.
Basically it’s kinda mathematically impossible for 20 to not be more fuel efficient than 30 in any normal situation.
(And we’ve not even talked about all the pollution reduction and health benefits we get from cars running slower around people)
For the level of significance, yes. The speed component of energy consumption is huge at highway speeds, but moderate at 30mph, because of the growth rate of air resistance with respect to speed. The relatively smaller contribution at lower speeds means that losses due to gearing can negate most of the benefit.
This is because while the vehicle is going slower and so incurring lower resistive losses, in an ICE car you’ll be in a lower gear, so the engine will be turning over at a similar speed, or perhaps even higher, incurring similar or higher resistive losses. These losses are a greater relative component of the total energy consumption at low speed than at high speed. (And at high speed you will not need to gear down when reducing your speed, so decreased speed will pretty much always result in a proportionally lower engine speed)
Historically it was a well known stated thing that 56mph was the sweet spot between the gearing, engine speed and air resistance. Going slower means lugging the engine if you over-gear and is less efficient because covering less ground for the engine speed, therefore fuel burnt. 20mph will not save fuel. Electric cars will maybe get better range, but they may not, because every motor has a design efficiency for a certain rpm and the designed gearing to the roadspeed from the manufacturer.
This is exactly why (in my opinion) my personal fuel consumption went up. 20 is too slow for 3rd, so I’m stuck at higher rpm in 2nd (or going at 25 and idling in 3rd)
Throw in the hills of Sheffield, and I’m driving around having to rev the balls off in 2nd, or labour the fuck out of it and kill it trying to do it in 3rd.
Less of an issue if your car is an oversized wanktank, like most. See, the bigger problem is the American influence on our vehicles; stupidly large, unnecessarily heavy, consuming more fuel to move, and doing more damage to roads and whatever mommy hits between the house in suburbia and little fucking Timmy’s football practice half a mile up the road.
Tax vehicles by a multiplicative factor of physical size, weight, and engine displacement, with a reduction for electric powertrain, or part thereof, and watch the gormless SUV craze end in a fucking hurry.
Do we have a peer reviewed study (ie facts) that reduction of urban speed limit from 30 to 20 actually does save fuel ?
Highway limit, sure, wind resistance and all that, well proven.
30 down to 20 in a stop start environment, where in many cases you don’t even get to 30 ? Colour me skeptical - I’ve seen this often asserted but never found an actual scientificly run study to prove it.
The plural of anecdotes isn’t data, but from my personal experience my fuel consumption went up when 20 was introduced here a few years ago (I can theorise why if you want, but I can’t prove why, only state that it is measurably higher with no real change in driven miles - I track my consumption).
TFL did a study related to this which was cited when Wales set the default speed to 20.
They found that there was minimal difference in emissions between the two speeds. Any benefit of lower drag etc is countered by the fact that the engine is not geared well for 20 so generally it will be the same or only slightly better doing 20. (I am making the assumption here that lower emissions will equal less fuel usage.)
However, the greatest benefit is that there are less cars on the road because it encourages active travel and discourages driving. So comparing driving to not at all is a huge fuel difference.
I can’t find the exact study I looked at before, but I think this might be similar: https://content.tfl.gov.uk/the-impact-20mph-limits-and-zones-in-london.pdf
If I’m not accelerating and steady at around 20 my Kia hybrid is pretty much running on battery anyway.
Whereas my 998cc, 3 cylinder, 3rd hand car requires screaming the tits off it in 2nd to get around the hills of Sheffield at 20mph. GG.
The plural of anecdote is anecdota
From 30 to 20 specifically I have the same doubts about fuel economy specifically, but I do know there’s research that 20 is much more survivable for pedestrians when accidents occur, which is more important to me than the pinch at the pump. So, mixed bag.
That’s a good reason to do it.
Do we actually need a study to answer this given it’s basically a physics and mechanics question? The statistics on a given car/engine’s efficiency is published data if we want to work out hard numbers, but even just understanding the high level relationships kinda gives us the answer:
Firstly engines are generally more efficient the slower you run them, so you’re just gonna use less fuel anyway. Aerodynamic drag also affects anything that moves and the more something moves, the more drag it experiences.
Then you have acceleration which is also where a good percentage gets burnt, getting up to 30 from zero is going to burn through more fuel than zero to 20. Stop-start traffic would exacerbate losses here at the higher limit. Finally, limits are all day round, stop-start traffic is really only a major factor around rush hours.
Basically it’s kinda mathematically impossible for 20 to not be more fuel efficient than 30 in any normal situation.
(And we’ve not even talked about all the pollution reduction and health benefits we get from cars running slower around people)
For the level of significance, yes. The speed component of energy consumption is huge at highway speeds, but moderate at 30mph, because of the growth rate of air resistance with respect to speed. The relatively smaller contribution at lower speeds means that losses due to gearing can negate most of the benefit.
This is because while the vehicle is going slower and so incurring lower resistive losses, in an ICE car you’ll be in a lower gear, so the engine will be turning over at a similar speed, or perhaps even higher, incurring similar or higher resistive losses. These losses are a greater relative component of the total energy consumption at low speed than at high speed. (And at high speed you will not need to gear down when reducing your speed, so decreased speed will pretty much always result in a proportionally lower engine speed)
Historically it was a well known stated thing that 56mph was the sweet spot between the gearing, engine speed and air resistance. Going slower means lugging the engine if you over-gear and is less efficient because covering less ground for the engine speed, therefore fuel burnt. 20mph will not save fuel. Electric cars will maybe get better range, but they may not, because every motor has a design efficiency for a certain rpm and the designed gearing to the roadspeed from the manufacturer.
It could be down to gearing? You could, for example, be sat at 2000rpm at 20 in 3rd, or 1500rpm at 30 in 4th. But this is very vehicle dependent.
This is exactly why (in my opinion) my personal fuel consumption went up. 20 is too slow for 3rd, so I’m stuck at higher rpm in 2nd (or going at 25 and idling in 3rd)
Throw in the hills of Sheffield, and I’m driving around having to rev the balls off in 2nd, or labour the fuck out of it and kill it trying to do it in 3rd.
Less of an issue if your car is an oversized wanktank, like most. See, the bigger problem is the American influence on our vehicles; stupidly large, unnecessarily heavy, consuming more fuel to move, and doing more damage to roads and whatever mommy hits between the house in suburbia and little fucking Timmy’s football practice half a mile up the road.
Tax vehicles by a multiplicative factor of physical size, weight, and engine displacement, with a reduction for electric powertrain, or part thereof, and watch the gormless SUV craze end in a fucking hurry.