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80-mph speed limit: Does it make cents?

By Bill Gephardt | Posted - Feb 13th, 2013 @ 8:27am

SALT LAKE CITY — Utah lawmakers are considering a bill that would increase the speed limit along parts of I-15 to 80 miles per hour.

If HB83 is approved, motorists could arrive at their destinations more quickly, but their fuel efficiency would decrease, costing them more money in gasoline. But there's another way to look at it when the value of time is factored in.

Fuel prices now average $3.27 per gallon in Utah. AAA calculated average cars are most efficient when traveling at 60 miles per hour. More speed means less fuel economy.

"For every 5 miles over 60 (mph), you'll lose 10 percent of your fuel efficiency," said AAA spokeswoman Rolayne Fairclough.

Maximum fuel efficiency is not the goal of the proposal before the Utah Legislature. Currently there are 80-mile-per-hour test areas along I-15. UDOT reported that in three years of testing, those areas have proven to be as safe for drivers as areas with lower speed limits. The proposed legislation would make those areas permanent and raise the speed limit to 80 mph through most of the I-15 corridor.

How does decreased fuel economy compare to how much a motorist's time is worth?

Test Case

Let's say you're driving an S.U.V., which gets 15 miles per gallon, at 60 miles per hour from Salt Lake City to St. George (400 miles). With gasoline at $3.27 per gallon, fuel will cost about $87. The trip will take 6 hours 45 minutes.

If the speed increases to 80 miles per hour, the mileage drops to under 11 miles per gallon. The fuel will cost about $122, but it will take only 5 hours to make the trip.

State Workforce Services said the average wage for Utah is $20 per hour. That means by saving an hour and 45 minutes in the drive, motorists save $35 worth of time.

Remember, the increased fuel cost for driving faster was $35, meaning the average wage earner would break even by driving faster.

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