Utah's fastest-ever speed limit — 80 mph — is now legal along two test sections of I-15, spanning a total of 34 miles.

From milepost 207 south of Nephi at Mills to milepost 188 at Scipio is a 19-mile 80 mph zone. The other 80 mph zone is from just south of Fillmore at milepost 162, to just south of Kanosh, at milepost 144 — an 18-mile span. These test sections include both southbound and northbound traffic.

The Utah Department of Transportation put the new speed-limit signs up the week after Christmas, replacing the former 75 mph maximum limit.

According to Robert Hull, UDOT's director of traffic and safety, these test zones are part of House Bill 406, approved by the state Legislature in 2008. It specified that UDOT should study I-15 from milepost 64 north of Cedar City to marker 222 south of Nephi to determine if speed limits greater than 75 mph were feasible.

Cameron Roden, spokesman for the Utah Highway Patrol, said troopers "will be keeping a close eye on speeds there." He said people who believe they are safe from speeding tickets by always going 10 mph or less over the posted limit will find less leniency for going more than 80 mph there.

"Officers will be pretty strict," he warned.

Roden also stressed that speed limits are always contingent on weather and road conditions, and winter isn't always an ideal time for going the full speed limit.

He hadn't received any reports yet on the status of speeding tickets in that area since the stretches opened several weeks ago.

Hull said the zones have adequate signage for drivers to know when the speed limit is in effect and when it is reduced back to 75 mph.

"There are concerns that drivers will continue to go above and beyond," he said. "Everyone needs to be careful."

Hull also emphasized that the public should realize these are test zones, and their success will determine the future of higher speed limits in Utah. Currently, he said, the only areas of the nation that also have 80 mph speed limits are found in western Texas.

"Some drivers may not want to go 80 mph," said UDOT spokesman Adan Carrillo.

UDOT will study all the impacts of the new 80 mph zones — especially in a few months when winter winds down. In about a year, UDOT must report its findings back to the Legislature.

Hull said road geometry and population density were key factors in determining the two locations. Curvy roadway and canyon areas were not suitable for the higher speed limits. UDOT also studied accident history for those two areas.

The 80 mph limit was based on studies last summer that showed the 85th percentile of speed in those stretches was already 80 mph.

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