The item that Plus 3 Golfer mentioned, that increasing the speed limit only changes the mean speed of traffic by 25% of the change in speed, is exactly what Utah is seeing. They increased the speed by 5 mph, from 75 to 80 but are only seeing the average speed of traffic increase by 1.5 mph. To further combat this, they are largely doing away with the cushion (10% or 5 mph) that the highway patrol has previously given speeders -- instead they are pulling people over if they are above 81 or 82 mph. Instead, they are finding traffic tends to run at a more uniform speed.
It is an interesting point about the study done when the speed was lowered to 55, compared to the studies Utah is doing. It is true that Utah's studies are very limited -- by design. Basically, Utah is requiring a study to be done, each time, before permanently raising the speed limit of a particular stretch of highway. As such, the study is only for a limited section of road and takes into account the actual speeds drivers are traveling. Additionally, the US study was much broader, including not only rural Interstate Highways but two lane roads, urban driving, etc. One other issue is the technological differences in the 30 years since the US study was done -- cars typically can drive higher speeds more safely than 30 years ago, particularly with advances such as Electronic Stability Control and, as they become more common, lane keep assist, adaptive cruise control, and other automated driving technologies.
Of course, the flip side of this is that Utah is talking crashes being reduced, I haven't seen them address fatalities. And there are some interesting arguments about what the speed limits should be, particularly in western states where Interstate highways run through long stretches that are very sparsely populated. For example, you have about 200 miles of I-15 in Utah with minimal towns (maybe 10), and only one of those towns has a population above 3,000 and none with even 5,000. This was a major issue when the 55 mph speed limit was passed, with drivers in the Western States claiming it ignored the empty stretches of highway the frequently drive by forcing them to the same standard used on the much more populous East Coast. It is an interesting debate with some strong opinions on both sides.
Of course, with technology advances, it seems like it would make a lot of sense to have variable speed limits, where the speed limit is posted on electronically controlled signs, with speeds that vary, based on weather conditions and even time of day. One of the things I've found odd is how states, like Texas, are getting rid of night time speed limits, at the same time they are raising speed limits to speeds that are unsafe to drive at night.