I vividly remember the national speed limit in the US in the early '70s. While it's fair to say that this saved some fuel, and probably reduced some accident fatality statistics, I think it had a lot of bad effects:
1. The interstate highways were all desigined for a minimum of 70 MPH cruise: Wide lanes, wide-radius banked turns, and huge run-off shoulders and medians. It was bloody obvious to John Q Public that 55 MPH was artificially low for these roads, and the sensation of crawling along on a huge slab of concrete made for a lot of angry people. The result was that people were, to some degree, incited and entrapped into breaking the law, which didn't do societal order a lot of good. Once the enforcement softened up in the late 80s, but before nationwide 55 was rescinded, most people habitually sped at least 15 mph over the limit.
2. A lot of municipalities, counties, and states were suffering from reduced tax revenue during the early 70s oil shock/inflation cycle. The 55 MPH limit and speed traps were seen as a godsend to boost the public coffers, and a lot of people got fined for doing less than 5 mph over the limit. The problem with this is that police are actually supposed to keep the peace and protect the public, not act as a revenue stream. The national (versus backwater town in the middle of nowhere) police culture of ticket quotas and aggressive speed limit enforcement based on revenue collection versus limiting unsafe vehicles and unsafe operators dates from then, and it has done us no favors at all. When was the last time you heard of a LEO ignoring somebody doing 16 mph over the speed limit in order to pull over a car with an inop headlight or a driver who merged without signaling? I thought so.
3. By mandating that no vehicle in the USA was ever expected to exceed 60 MPH, several generations of cars were built with gearing that was inefficient at any speed above 60 MPH. It wasn't until CAFE regulations and rising speed limits coincided in the mid 80s that four-speed automatic transmissions with lockup torque converters became common. Ditto five-speed manuals in the few cars that shipped with a clutch. Likewise, aerodynamic development was slower than it needed to be because the marginal value of a change is less at 55 MPH than at a higher speed.
Using a big, crude stick was a stupid way to get people to burn less fuel. Carrots would have worked just as well, and wouldn't have had the undesirable consequences. Compare what happened in the solar/renewable power and insulation businesses because of the tax credit schemes that were set up at about the same time.
