You can look at that as less speeding, as the UDoT has, or simply the fact that people didn't automatically start going 10 mph over the limit, having found a speed that satisfies 85% of them. This shouldn't be that surprising – even when Montana had stretches of highway with unlimited speeds, it was only a few folks, mostly out-of-staters, that raced at max velocity. And most folks on unlimited sections of the Autobahn aren't even going 150 mph.
According to the Deseret News, a UDoT official said that accidents haven't increased, either, nor did they expect an increase in fender benders. What he said has increased, though, is speed differential, which we don't quite understand. Apparently, the difference between the fastest and slowest vehicles has widened. We can only imagine he was speaking of the 2 mph difference between the slightly higher lower average speed of 83 mph vs the 81 mph from before. Even though one Utah legislator wasn't happy about the increased limit, saying speed kills, the proof appears to be... well, not necessarily.
[Source: Deseret News | Image: Ken Lund via CC2.0]