I was wondering if the US speed limit was lowered nationwide to 55 (and enforced) what the savings (%) in gas consumption would be.
The U.S. did this experiment already from 1974 to 1987 (some of it lasted until 1995) and it didn’t work out that well. The fuel percent savings was 1% or less and there were other problems involved like clogging up the roads because of slower traffic. It is possible that the savings would go negative if it were reintroduced today especially on busy urban interstates if it was truly enforced.
I wonder whether there would be changes in the intervening 25 years?
If anything, cars have become more efficient at higher speeds. Average drag coefficients have gone down while the efficiency of planetary gear transmissions (more gearing, more efficient torque conversion, almost universal overdrive gearing), and other factors have increased efficiency at highway speeds, and the difference per mile between 55 mph and 65 mph is negligible. Last year I took a road trip up Interstate 5 and back down the California/Oregon coastline. On the way up I maintained an average running speed of ~80 mph, and back down the average speed was ~60 mph. The difference in mileage was insignificant, and in both directions mileage was better (by over 1 mpg) than my normal mostly-highway commute.
In terms of pollution, the longer the car is running, the more is emitted, so inching through gridlock is nearly as bad as running at high speed. In terms of efficiency, you can achieve much greater efficiency gains by running the mixture slightly lean compared to a stoichiometric combustion ratio (which all modern fuel injection systems do), making certain that tires are properly inflated and balanced, using the air conditioner instead of opening windows, et cetera. Setting speed limits at 55 mph doesn’t even demonstrate a lower number of highway accidents or severity, though modern crash protection systems and mandatory seat belt laws don’t make this a head to head comparison.
So there are no real benefits to reducing speed limits back go 55 mph, and possible detriments to efficient traffic flow.
Stranger
Little problem,
Were you driving in the US when “55” was the law? Here in Californian on the freeways the vast majority of vehicles were travelling over 55 and in many cases well over 55. The highway patrol simply couldn’t pull everyone over so they only ticketed people who were driving recklessly or really fast.
Just like they do today. How many people do you see actually driving 65 ( the current speed limit)?
That’s the point, actually. People tend not to drive a certain absolute speed, but a certain number of miles above the speed limit.
If the speed limit is 55, then the majority of speeders might go 65 and a few at 75. If the speed limit is 65, then the majority of speeders might go 75 and a few at 85. But it doesn’t happen that the majority go 75 when the speed limit is 20 mph lower.
Two reasons. One is that drivers are herd animals and the vast majority don’t go much faster or much slower than the others. Having even a small percentage go the speed limit will affect the rest. The other is that you are much more likely to be stopped for speeding at +20 mph than at +10 mph and the fines in some states go up as the overage goes up.
The footnotes on that Wiki page for the low rate of savings go to places like the Heritage Foundation, which has a track record on reality similar to the Discovery Institute, which supports creationism. Every quote on that page attacks the law with no opposing cites, and its hard to imagine that none exist from more neutral organizations.
Or even a different page on Wikipedia:
Now it’s true that reimposing a 55 mph limit is a political impossibility today without a crisis much larger than the gas “crisis” of the 70s. And it’s true that trips at lower speeds take much longer and that imposes a price that has to be considered. It also seems to be true that today’s cars are engineered to get better mileage at higher speeds than before, although they could easily be engineered to be more effective at a lower speed if need be.
The real problem is that the question is unanswerable because we don’t know how peoples’ driving habits would change if the speed limit were lowered, especially if the reason was overwhelmingly compelling. We don’t know whether and how the cars they would buy would change, and we don’t know how cars themselves would change. I don’t believe you could predict any of these today and if the cause were a true crisis every prediction would go out the window.
Lowering the speed limit saves lives.
And all of those people are out there burning our gas! (Greedy bastards!)
Sorry, but the NHTSA numbers don’t bear that out. In fact, adjusting for the number of driver-miles travelled, fatalities have dropped by about 16% for passenger automobiles since 1994. Of course, the stats don’t tell the whole story; both automotive safety and medical trauma technology have increased, likely contributing to the decrease in fatalities, but regardless there is not an appreciable increase in fatal accidents at higher speeds.
You speak as if gas is a community resource, and the overuse of it is a tragedy of the commons. This may or may not be the case, depending upon your point of view, but I daresay that neither OPEC nor Shell Oil or British Petroleum consider this to be the case, and at any rate the amount of oil you personally consume in the operation of a motor vehicle is dwarfed by the amount used to transport goods and produce, produce energy, and provide various government and private services. Unless you are literally living off the fruit of your personal labors from the land and forging your own tools, you are a part of a global community of oil consumers whose overall dependence upon oil as an energy medium is undeniable, and to single out one particular set of consumers for what you may view as excess consumption is disingenuous. Overuse of finite fresh potable and irrigable water is actually a more pressing resource issue.
Stranger
(Pssst… Old joke. 
)
Okay – here’s a follow question. What would happen for motor-vehicle fuel consumption if automatic transmisson computers were programmed to shift at 1500 rpm? When I shift at 1500, rather than 3000 (manual transmission), I demonstrate nearly 10% increase in in-town milage.
I take exception to the claim that people drive X mph over the limit. In rural New Mexico, the speed limit is 75 on the interstates, and with the exception of the Santa Fe/Albuquerque stretch, the vast majority of the traffic is moving at 70-80 mph, with a fair number of drivers a few mph under the limit. When the limit was 55, traffic ranged from 50-70 mph, but heavily skewed toward the upper end.
Likewise the limit in town was increased to 65 mph, and compliance soared. It is not uncommon for light traffic at night to be moving at 60 or so.
Yes, you still have the odd speeder, but not like the 55 mph days, when nearly all the traffic was running at 65 or so.
I would also point out that in the 55 mph days, there were several groups advocating for higher speed limits. I’m not aware of any such groups now.
I once calculated that for each life claimed to have been saved by the 55 mph speed limit, an extra 57 years were spent driving. That was assuming only one occupant per car. Those would be 365x24 hour years, not 2000 hr “man years.”
On that basis, any fatality over the age of 20 or so that was prevented, actually cost more life than was saved.
Speed limits where people don’t drive within a reasonable percentage of the posted speed are artificial and arbitrary. That why when there’s not a regulation in place that says otherwise, speed limits are set to what 80% to 90% of the people actually drive at. So you have good speed limits. On I-94 in Detroit, for example, the posted speed is 55 (law says it can’t be higher), but yet everyone drives the same 70 to 80 that they do where the posted speed is 70.
The Basic Rule is the best rule. And it is usually appplied to over the speed limit as well as under. It should be the universal rule of the road. If you drive in excess for the road conditions, and have an accident, your are in violation. If you are driving the normal highway speed with the rest of the traffic, you are not in violation. You are expected to maintain control of your vehicle at all times, or you are at fault.
From here:
Designated and posted speeds are not the final word in Oregon. All travel on public streets and highways is subject to the Basic Rule. The Basic Rule is both a safety valve and an acknowledgment that drivers are able to act independently, reasonably, and with good judgment.
The Basic Rule states that a motorist must drive at a speed that is reasonable and prudent at all times by considering other traffic, road, and weather conditions, dangers at intersections and any other conditions that affect safety and speed. The Basic Rule does not allow motorists to drive faster than the posted speed, nor does it set absolute speeds designated for all conditions. The Rule expects drivers to be responsible for their own actions.
Sudden braking at 25 mph on ice or snow in a speed zone posted at 30 mph can cause the driver to lose control of the vehicle. In this case, the ice or snow has made the posted speed unreasonable. The posted speed is in violation of the Basic Rule.