“The character of I-15 in some of those passes has changed with improvements we have made,” Miles said. “We have more climbing lanes and more passing zones to handle trucks. That allows us to take a look at the speed limits.”
UDOT also proposes to add another seven miles total of 80 mph areas on I-70 at small, scattered locations, Miles said. Those changes are mostly to ensure that speed zones are the same in both directions of I-70, and will resolve some discrepancies where they now differ.
Miles said he expects the changes to occur within a month, after consultation with the Utah Transportation Commission and local governments.
He said UDOT constantly reviews speed limits by studying data such as engineering changes, crash history and how fast traffic actually moves in different sections.
UDOT studies in areas proposed for higher limits show that 85 percent of motorists often are traveling faster than the posted limit. That creates problems with maneuvering around slower-moving traffic, which may be trying to obey posted limits.
With the higher speed limits, more traffic is expected to travel at similar speeds.
“This will increase safety,” Miles said. “The closer we sign the system to the way it operates, the better off we all are.”
Past studies have shown that increasing speed limits to 80 mph has increased average speeds by only 1 to 2 mph, and actually decreased them in some areas – suggesting they reflect what drivers intuitively feel is a safe speed.
Utah Transportation Commission member Naghi Zeenati quipped as one such study was reported that the higher speed limits allow “most of us to drive legally at what had been illegal.”
After speed limits were raised in recent years, highway deaths have climbed in Utah. But Miles said the higher speed limits are not to blame.
Most increases in fatalities have occurred on arterial roads and city streets, he said, not freeways.
Also, UDOT spokesman John Gleason notes that the number of vehicle miles traveled in the state increased amid cheaper gasoline prices in recent years, and higher travel usually brings more highway deaths. Miles said fatalities are up because of that by 10 to 15 percent in most states, while they have increased about 7 percent in Utah.
Such things as distracted driving and failure to use seat belts are bigger reasons for increasing deaths, Gleason said.
After experimenting with 80 mph speed zones for several years on stretches of I-15 in southern Utah, the Legislature in 2013 passed a law allowing UDOT to raise limits to 80 mph wherever studies show it likely would be safe.
When Utah first adopted 80 mph limits, Texas was the only other state with such fast speeds on any roads. Now, Idaho, Wyoming, Montana and South Dakota all have opened up the throttle.