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Highway crashes have dropped annually on some stretches of Interstate 15 in Utah since that state raised the speed limit to 80 mph in late 2008, studies show.

Idaho and Wyoming followed suit and increased their top speeds on portions of interstate highways to 80 mph last July. They are waiting until their laws are in effect for a full year before analyzing the impacts.

Highway safety questions popped up after four Montana legislators said they will sponsor bills next year to raise daytime speeds on portions of Montana interstates to 80 mph from the current 75 mph. One lawmaker is looking at 85 mph.

In Utah, the speed limit increased to 80 mph in three phases in 2008, 2013 and 2014, according to the Utah Department of Transportation. At present, 36 percent of interstate highway miles in Utah have speed limits of 80 mph.

The Utah department has done several studies to measure the impact of higher speed limits on highway safety along certain stretches of Interstate 15.

A 2009 Utah study found drivers complied better with the 80 mph speed limit than with the previous 75 mph limit. There was a 20 percent reduction in drivers exceeding the 80 mph limit, the study showed.