There’s a couple of them around here now, speed has generally been reduced from 80km/h to 60km/h. It’s in Oslo, the capital, so there’s quite heavy traffic on these roads.
I thought you got better mileage (how is that not less polution?) by running your car at the highest gear (as long as you don’t put the pedal to the metal, of course). Make some sense of this!
Higher speed = More air resistance, friction, energy lost as heat = More energy required to make the car go faster = More gas needed.
Are you sure? Everyone I know that’s into cars tells me that gas use and gas pedal, uhmh, leverage(?) is pretty much proportional. Now, with the gas pedal pressed slightly down in fifth gear, I should use as much gas per second as if I had it pressed slightly down in first gear, but I’ll stay on the road fewer seconds.
That’s how I’ve understood it, anyway.
Air resistance goes up as square of speed. Can’t argue with that.
Efficiency may drop off at low speeds because gasoline engines are inefficient when working at very low power output (much lower than its maximum power). This is more true for cars with oversized engines. But even then, I suspect most cars are more efficient at 60 km/h than 80 km/h. I know mine is.
Also, city traffic involves repeatedly accelerating to the speed limit and slowing down again. Obviously it takes more energy to accelerate to 80 km/h than 60 km/h. 78% more energy, to be exact. (Kinetic energy is proportional to square of speed, and 80[sup]2[/sup]/60[sup]2[/sup]=1.78)
On a related note, during WWII, speed limits were lowered not to save fuel, but to save rubber. Just thought I’d throw that one out there.
The 80 to 60 k/h seem, which is about 55 to 45 mph (someone out there can check the math, I’m estimating here), seems too low, I think cars in general get the best mpg at about 60 mph, SUV’s at a lower speed however.
Most vehicles get their best gas mileage in the 45-60 mph (70-95 kph) range. So I’d say these speed limits are a “feel good” measure that is actually counterproductive to the intended effect.
Cites:
When they did the same thing here in the US during the oil embargo, they chose 55mph as the most efficent speed. Of course, circa 1975 cars in the US maybe considerably different then modern cars driven in Oslo.
scr4: The roads in question are highways, just close to the city, so the acceleration rate would be normal.
Your first cite says “between 40 and 60 mph”, your third cite has one graph that shows fuel economy to be pretty much flat from 30 to 55 mph, beyond which it falls off rapidly. Your second link is not working.
Anyway… It’s also possible that this measure is designed to reduce the number of cars on the road, by making trains more attractive in comparison. I know in Japan they have enacted stricter speed limits for trucks, and as a result there has been a shift towards rail freight.
They chose 55 mph as a compromise between efficiency and convenience (speed). That’s what they said on Car Talk anyway, they said cars at that time were most efficient at around 40 mph, which may or may not be true today.
This is what I though at first too, then I though it may go deeper then that. It lengthens commuting time by 33%, that could cause a change in where people live and work, causing people to live closer as the don’t want to deal with the longer commuting time.
Good point! And even better, they could take the subway, which is not affected.
Well, shoot! Why not cut to the chase and pass a law that requires they walk to work? That will reduce vehicular pollution.
It can also reduce congestion in some (not all) cases. If the speed limit on the main road is the same, or nearly so, as the secondary roads, then some will take the secondary roads to avoid the more crowded main road. This reduces congestion on the main road, and with less stop & go, those folks achieve better milage. Of course this then increases traffic on the side roads, but those may have been underutilized prior to the speed limit change.