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).
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)
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.
