Q: Why does the speed limit on Brown Road in Powdersville change from 45 mph to 35 mph and back again?

Amy in short: That rural road may be subject to a variety of factors affecting the speed limits, including but not limited to sign theft. How speed limits in general are established is a balancing act of safety, design and cost.

A bit more: To determine exactly what's going on on Brown Road, which runs between I-185 and State 20 in southern Greenville County, an official said he'd have to physically drive the road and look for the speed limit signs. But the changes could be due to a few factors.

"A lot of time, speed limit signs get stolen or knocked down," which can make it seem as if the speed limit is changing or is different going in one direction than the other, said Eric Dillon, district traffic engineer with the state Department of Transportation.

Or it could be the changes are purposeful and related to the design of the road, which includes an interstate underpass and a hairpin turn near the road's terminus.

When setting a speed limit, Dillon said officials conduct a speed study using a radar gun to get a sample of vehicle travel during peak times and calculate the 85th percentile — essentially the speed at which about 85 percent of drivers feel comfortable traveling. (The 85th-percentile thing is an industry standard referenced in the federal Manual on Uniform Traffic Control Devices.)

They also take into consideration the design of the road and other hazards along the roadway to determine the most appropriate speed limit.

"You try to set a speed people will feel comfortable going through an area, but then again we try to watch out for people because they're not aware of conditions like driveways that don't have sight distance or curves that are bad or congested areas," Dillon said.

Traffic engineers seek to avoid changing the speed limit unnecessarily along a stretch of road or setting the speed limit unreasonably low.

"I'm a big fan of not setting up speed traps," Dillon said.

The first part of setting a speed limit comes in designing the road itself, particularly when it comes to making curves safe to drive at the road's established speed.

"They've got to know that speed limit up front or what they're going to set the design speed to before they can even design the roadway," Dillon said. "Once they set that, we try not to go over that amount."

But sometimes higher speed limits are associated with higher costs in surprising ways.

Dillon described one such hypothetical situation: "If we design the road at 50 mph, I've got to raise the bridge another 10 foot in the air to do this curve of the roadway for that design speed. It's going to take up four acres of property, and I've got to buy two houses and three gas stations. Can we lessen this design speed and flatten this curve out a little bit so we don't get into these properties and save some cost?"

State law (Section 56-5-1520) establishes maximum speed limits for various types of roadways — 70 mph on interstates, 30 mph in an "urban district," etc. But the law empowers the Department of Transportation to set and alter those speed limits either up or down as may be deemed "reasonable or safe."

Once established, speed limits are typically only reevaluated in the event of a road's being rebuilt or a complaint, Dillon said.

Got a question? Send it to Q&Amy by emailing me at or calling 864-298-3822. You can find me on Twitter at@QandAmyNews.

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