American speed limits - history
There is a commonly-held belief outside the US that the maximum speed on America's roads is 55mph. In reality, this hasn't been the case since 1987.
Back in 1909, the state of Washington introduced a speed limit of one mile in five minutes (12mph) in "thickly settled areas and business districts", and at one mile in two-and-a-half minutes (24mph) for rural areas.
In these early days, states could set their own upper limits and by 1972 the majority were 70mph, although ten states had a limit of 75mph, and two (Montana and Nevada) had no upper limit at all (although these are all daytime limits for cars on designated "fast" roads; trucks, other roads and nighttime driving all had reduced limits).
But then the fuel crisis intervened, and the American government was forced to do something to reduce the country's massive fuel consumption.
The answer they came up with was to enforce a maximum speed limit across the US of 55mph. However, federal legislation is not actually able to impose such a limit, so instead the government made this maximum limit a condition for their continued funding of their highways programme. In other words, states had to impose the 55mph limit or the federal contribution to their road programme would dry up.
The Transportation Research Board issued a special report shortly afterwards pointing out that this reduction in maximum speed had, as a side effect, saved over 9,000 lives.
Montana and Nevada, with their "reasonable and prudent" upper limit, both felt very strongly that this imposition was unreasonable. Montana's view resulted in a derisory "energy wasting" speeding ticket of only $5 for those who broke the 55mph limit on roads which would otherwise not have had a fixed limit.
Nevada, on the other hand, tried a trick to encourage the government to let them get way with a higher limit. In 1986 they changed the upper limit on a stretch of Interstate to 70mph, and linked this with a requirement to wear a seatbelt. Washington immediately threatened to withdraw highway funding, and Nevada toed the line.
In 1987, under mounting public pressure, the limit was raised to 65mph.
Following the increase, accident fatalities on roads with the increased limits went up about 30%. However, like all such contentious issues, there is an alternative view, and it's possible to seek out evidence that accidents rates actually decreased when the new limit was imposed.
In 1995, the US Government withdrew the national capping of the speed limits completely, and allowed states to set their own. Many states were immediately ready to go back to the way they were, but some were more cautious. Montana reverted to its "reasonable and prudent" limit in 1995, although somewhat surprisingly, Nevada established 75mph for its rural Interstates.
But Montana's "reasonable and prudent" limit was challenged in its courts in 1998. The Montana Supreme Court ruled that the law was unconstitutional in that it was too vague (it goes against the "due process" clause in the state's Constitution). And so, Montana has had to impose a fixed upper limit, which is 75mph.
Maximum speed limits (which apply to "rural interstate highways") now vary from state to state, ranging from 60mph in Hawaii, 65 mph in many of the north-east states and Oregon, 70mph in the south-east states and west coast, and 75mph in the mid-west states. Texas is unusual in that long stretches stretches of I-10 and I-20 had the speed limit raised to of 80mph in 2006.