OPERATION GRIDLOCK: How are speed limits determined?

WILMINGTON, NC (WWAY) — In Operation Gridlock “Setting Limits”, we sat down with engineers to find out how a speed limit is determined and who can affect them.

Whether it is a highway or a neighborhood, there is a science behind the number posted on the side of each road. City of Wilmington Traffic Engineer Don Bennett said they use a specific standard to set the speed limit on all city roads.

“The standard would be the 85th percentile speed,” Bennett said.

What does that mean?

“You go out and measure the speed,” Bennett said. “Most drivers will drive at a speed they feel safe and comfortable at, so you look at the speed and find the speed that 85 percent of the drivers are at or below.”

A spokesman with the NC DOT said they also use that standard for state roads, but for all of those roads, there are exceptions.

“Sometimes design parameters constrict it,” Bennett said.

Randall Parkway is one of them.

“You come off of South Kerr Avenue and Independence Boulevard and it’s 45 and then you hit Randall and immediately it goes to 35,” Teresa Carroll said.

“The drivers out there see a roadway that’s four lanes wide with a median, limited conflicts and they assume they can drive faster on it than they really can,” Bennett said.

Teresa Carroll works off Randall Parkway and drives it every day.

“Continuously we see people being pulled over,” Carroll said. “You feel sorry for them, because if it were a situation where they were really speeding, you would say, ‘Well, they deserve it.'”

Bennett said the design of Randall Parkway makes it the exception.

“The rate of curvature in the roadway, how tight a curve, how much relative slope there is in the roadway, whether we have banking in it or if it’s flat,” Bennett said.

Another exception is neighborhoods.

“Whether it be a child running out after a ball, a pet that’s gotten loose that isn’t anticipated, coming around the corner and there is a garbage truck picking up refuse or a meter reader out there just doing their job,” Bennett said.

That is why he said most neighborhoods have much slower speeds.

“These conflicts are not expected,” Bennett said. “Therefore, driving slower in the neighborhoods affords you a shorter stopping distance and less chance of getting into a collision.”

“How much can people affect changing a speed limit?” we asked.

“The residents of any neighborhood generally have the most control over what the actual speed is,” Bennett said.

While most speed limits are not changing, Bennett said there is a growing need for drivers to start paying more attention to their speed.

“As our car technology evolves, we need to pay attention to our speedometer, because we’re more and more and more isolated from that exterior environment,” Bennett said.

You can find out more about setting speed limits in Operation Gridlock from the StarNews early next week.