It’s factually accurate, economically sound, and will make drivers furious. I’m all for it.
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).
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.
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
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)
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.
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.
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)
The plural of anecdote is anecdota
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.
Not this driver 😉
Cutting low speed limits will do very little to fuel consumption, but cutting 70mph limits to 60 would be significant.
I wonder how this would be sold to EV drivers.
I live near Toronto. Speed limits are irrelevant.
It has recently changed to 20 mph in most towns in Scotland and it’s much nicer.
Instead of “30 really means 40” it has become “20 really means 30” in many people’s minds.
Plus the people actually doing 20 mph of course makes being a pedestrian a much safer environment. Anecdotally I find people to be a lot calmer now as well.
We all need a slower pace of life in the hyper fast modern age, I’m all for the 20 mph changes!
I’m trying to think of places in my city where 30 limits would be better as 20 and not really coming up with anything… Pretty much the entire city centre is already 20, and it’s only arterial routes in and out of the centre that are 30, and they are much broader with much less general chaos.
Pretty much the entire city centre is already 20
I think you’ve answered your own question. Where pedestrian footfall is highest.
So is my city atypical or is this switch a bit of deck-chair rearranging?
Sounds atypical. Most towns and cities in the UK had 30mph roads, until quite recently Wales and Scotland switched most of theirs to 20. Scotland still have plenty of 30 zones though (outskirts and industrial estates) whereas I think Wales have been a bit more strict with the switch (although I may be wrong on that, that’s just the impression I got from skim reading about it when it happened).
England I would assume has regions which are doing the same bit by bit but there’s been no blanket nationwide push for it by the government as far as I’m aware.
As I said, my city does have 30mph roads. But they’re main routes which to me don’t seem reasonable to turn into 20mph zones: they’re wide and have far fewer hazards than the smaller streets.
I used to have a 20 mile commute four times a day on dead flat roads. I bought a super-efficient car (skoda fabia greenline} and played with the speeds and gears to my hearts content to find the scientifically most efficient speed to use the least fuel. The result was that the slowest speed the car could maintain in the highest gear was most efficient. That was 30mph in 5th. I would usually average over 100 miles per gallon of diesel. Spent a lot more time in the car than i would have at 60mph, but audiobooks are great. This doesn’t add much to the discussion here. 60mph is much better than 70mph, and i’ve driven in wales with the 20mph limit in all urban areas and it doesn’t seem to cause any problems - i’m totally in favour.
Going slower will just make congestion worse, cars spending more time on the road means more cars on the road at the same time.
This just sounds like a cynical way to make more money from speeding fines.
If the average speed of congested traffic is already below 20mph, how do you figure?
Traffic that isn’t congested could become congested. Also half of congested traffic will be above average.
That assumes that the congestion is caused by the speed at which the cars travel, which it basically never is.
LOL, so raising speed limits will end traffic? and we’ve been wasting money building new highways this whole time!
