PIERRE | Three years ago, the South Dakota Transportation Commission decided against using roundabouts at the Brookings interchanges between the business route of U.S. Highway 14 and Interstate 29.
Among the reasons were complaints that long semitrailers and large farm equipment might have trouble getting around the no-stop circles. Even so, state Department of Transportation officials kept on planning roundabouts at other spots. Now they plan to construct several roundabouts in 2018.
Last week, commissioners approved bids for a roundabout at Watertown in Codington County. It would be at the South Connector intersection of U.S. Highway 81 and 20th Avenue Southeast.
The commission’s surface transportation improvement plan also calls for a roundabout in Minnehaha County at the intersection of S.D. highways 42 and 17 at Wall Lake. And DOT officials plan two roundabouts at Sisseton in Roberts County along S.D. Highway 10.
Mike Behm, DOT director of planning and engineering, said roundabouts are statistically safer than traditional three- and four-way intersections. Nationally, they reduce fatalities 90 percent and reduce injuries 75 percent, according to Behm.
Public opinion about roundabouts before construction runs favorable in the 30 to 40 percent range, Behm said, but after they’re built, support rises to more than 60 percent. “We hope to see some good numbers on our roundabouts,” he said.
Behm’s information was part of an overall presentation on steps the department has taken in recent years to reduce frequency of traffic crashes by improving the safety of highways in South Dakota.
Installation of shoulder rumble strips began in 2010. So far there have been 21 percent fewer roadway-departure crashes, 24 percent fewer fatal crashes and 20 percent fewer injury crashes.
The department put center line rumble strips on U.S. Highway 12 between Aberdeen and Ipswich in 2016.
During the five previous years, six head-on crashes and one opposite-direction sideswipe occurred on that stretch of Highway 12. There have been zero crashes of those types in the year since the strips went in.
Shoulder widening on S.D. Highway 73 and S.D. Highway 20 north of Faith produced an 80 percent reduction in roadway-departure crashes. Ten similar projects have been recently completed or are underway, and 25 more are planned in the next eight years.
Four curved sections of highways received high-friction surface treatment in 2014. They’ve seen an 80 percent reduction in winter crashes. Eighteen more locations received it this year.
“This is an area where the state of South Dakota is leading,” Behm said.
Milbank’s four lanes of U.S. Highway 12 were condensed to three lanes in 2013. Since then the crashes came down 63 percent and injury crashes fell 48 percent. Somewhat similar changes now are planned for state highways in Tyndall and Tabor.
Diverging-diamond interchanges force vehicles from opposite directions to simultaneously turn left by routing them to the left sides rather than the traditional right. They produced solid results in spots such as from I-90 in Rapid City. Injury crashes dropped 41 percent.
Now diverging-diamonds are planned for I-90 exit 59 at Lacrosse Street in Rapid City for 2020 and for I-29 exit 71 at 41st Street in Sioux Falls for 2023.
Ten-foot shoulders depend on availability of revenues, so the department focuses on where the greatest effect can be accomplished, Behm said. Rumble strips won’t be installed in urban or suburban areas but will be placed in those rural areas where they can be fit, he said.
Center line rumble strips won’t be used everywhere but are considered for higher-traveled roads, with 2,500 vehicles per day as the threshold, according to Behm.