How does the DOTD set speed limits in Louisiana? And, why changing speed limits doesn’t always work


A speed limit sign that is tilting due to a broken bolt January 12, 2011 in Detroit, Michigan. AFP PHOTO/Stan HONDA (Photo by STAN HONDA / AFP) (Photo by STAN HONDA/AFP via Getty Images)

BATON ROUGE, La. (KLFY) — Every state highway and thoroughfare has a speed limit set by the Department of Transportation and Development in Louisiana. But, how do decide which roads will have which speed limits?

In this episode of “On-Site with DOTD,” Monroe District Traffic Engineer Natalie Sistrunk, P.T.O.E., explains the process, noting that the limit is often set at the level at which most people are comfortable driving. A radar gun is used to document the speed of 100 motorists on a roadway.

“And we look at the speed that at least 85% of the people are driving at or below, and that’s where we set the speed limit,” said Sistrunk. “It’s called the 85th percentile.”

Sistrunk said picking a comfortable level is important, because “you don’t want some of them going way slower or some of them going way faster.” This, she said, is done out of safety.

Sistrunk said it’s a misconception that changing a speed limit will result in people changing their driving habit.

“I mean, I can go change the sign, but people are going to drive what they’re comfortable driving, based on the context of the roadway,” she said. Motorists behave differently on two-lane roads, as opposed to four-lane highways.

“I can’t just go stick up a sign that says ’25’ and expect people to drive ’25.’ They’re going to drive what they’re comfortable driving, so that’s where we set that speed limit.”

Watch the entire video below:

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