K-STATE HIGHWAY RESEARCH, part of a series

K-STATE RESEARCHERS STUDY SAFETY, COSTS ASSOCIATED WITH ROUNDABOUTS

MANHATTAN -- It doesn't take a math whiz to know that the shortest distance between two points is a straight line. However, that straight line may not be the safest route -- especially when it intersects with another. Going the way of a roundabout may be the best solution for controlling traffic at an intersection, according to a trio of researchers at Kansas State University.

Eugene Russell, professor of civil engineering; Margaret Rys, associate professor of industrial and manufacturing systems engineering; and Greg Luttrell, a graduate research assistant, have been studying three primary issues in relation to traffic flows and conflicts at roundabouts: safety, operation and costs. The study, funded by the Mack-Blackwell National Rural Transportation Study Center, focuses on a comparison of the lone roundabout in Manhattan -- and the state of Kansas -- at the intersection of Gary Avenue and Candlewood Drive and similar intersections throughout the city. Three other roundabouts are currently in the design phase -- two in Newton and one planned by the Kansas Department of Transportation for an exit on the Kansas Turnpike in Topeka. Lawrence plans to construct a roundabout later this year.

"We are going to try to find some other comparable roundabouts across the country and do before and after studies so we can confirm what's been stated since the early 1970s -- that roundabouts are much safer than signal controlled intersections," Luttrell said.

Not to be confused with a traffic circle, from a physical standpoint, a roundabout will have a "diverter" or "splitter" island which diverts or deflects traffic into the traffic stream of the roundabout, according to Luttrell. These islands are the primary differences that separates roundabouts, which are approximately 50 to 60 feet in diameter from their distant cousin, which measure about 10 feet in diameter and do not have a splitter island. Each has a different purpose and trying to compare the two is "like trying to compare stop signs and yield signs," Luttrell said. "They're very different."

Luttrell, a former city traffic engineer in South Dakota, said another way to look at a traffic circle is as a "replacement at a small residential intersection for a stop sign" as opposed to a roundabout which is a safer, less expensive, and easier to maintain replacement for a traffic signal.

Luttrell estimates the initial cost of installing a traffic signal at about $100,000, plus an additional $3,000 per month for electricity and maintenance.

"A roundabout has a higher capacity for handling traffic than a stop sign or traffic circle by up to 30 percent and it's anywhere from 10 to 100 percent safer, handling traffic more smoothly," Luttrell said. "There are a lot of maintenance issues dealing with traffic signals that just don't occur at roundabouts.

"Signals have to be replaced every 10 to 15 years. A roundabout should last as long as the road and that could be 20 to 50 years, so from an economic standpoint, the roundabout is going to be even better over time."

Another area enhanced by roundabouts is safety. According to Luttrell, a standard intersection has 32 conflict points or sites for potential accidents. Add pedestrians and the potential increases even more. A roundabout reduces those conflict potentials to a mere eight points.

"When you enter a normal intersection you have to watch traffic from the left, straight ahead, to your right and if you're smart, you're watching your rear view mirror for the people behind you," Luttrell said. "When you hit a roundabout you're only watching one direction; you're watching to the left because nobody's coming across the intersection and nobody's coming from the right."

Russell said roundabouts reduce the severity of accidents as well.

"The worst accident you can have is a head-on-collision; the second worst is a right angle collision," Russell said. "You're not going to have any of these because there is nobody coming at right angles to you. There may be sideswipe kinds of accidents but these are generally less serious."

The trio hope the study will also yield an environmental analysis of vehicle emissions associated with vehicle delays at traffic signals versus roundabouts.

"At a typical red light you're going to have a 20 to 30 second delay," Luttrell said. "Of course some people are going to get the green, but at a roundabout it is conceivable that no one may have to stop. It depends on who is already in the roundabout."

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For more information, contact Russell at 785-532-1588.

Prepared by Keener A Tippin II.

March 1989