IDOT screenshot, roundabouts

This screenshot of the Iowa Department of Transportation website explains why roundabouts are safer than traditional four-way intersections.

CLINTON — Roundabouts are non-existent in Clinton, but that could change during the reconstruction of Manufacturing Drive and Bluff Boulevard.

The city proposed a roundabout at Valley West Court and Manufacturing Drive in a 2018 video outlining the project, but City Administrator Matt Brooke said that’s no longer under consideration.

“The new location for a potential roundabout will be at South 19th Street and Manufacturing Drive,” Brooke said, but details for the project won’t be finalized until the city receives federal funds from the Better Utilizing Investments to Leverage Development grant program.

Until then, the city is educating residents about the circular traffic management system on its website, in the city publication The Lighthouse and through an online video.

Roundabouts are safer because they have fewer conflict points than traditional intersections, and the types of crashes that occur are not right angle crashes, which result in the most severe injuries or fatalities, the video says.

A typical four-legged intersection has 32 vehicle-to-vehicle conflict points and 24 vehicle-to-pedestrian conflict points, according to the Iowa Department of Transportation.

By comparison, a four-legged roundabout has only eight vehicle-to-vehicle conflict points and eight vehicle-to-pedestrian conflict points, the DOT said. This is an approximate 70% reduction in conflict points.

Because all vehicles are traveling in the same direction and at a lower speed in a roundabout, crashes are generally rear end or sideswipe in nature. “Left-hand, right-angle (T-bone) and head-on crashes are virtually eliminated by a roundabout,” the DOT said.

According to a technical summary produced by the Federal Highway Administration, numerous studies have shown significant safety improvements at intersections converted to roundabouts.

The most comprehensive and recent study showed a 35% reduction in crashes and 76% reduction in crash injuries, the report said. Severe, incapacitating injuries and fatalities are rare. One study reports an 89% reduction in these types of crashes, and another reports a 100% reduction in fatalities, the summary said.

Roundabouts have benefits other than safety, the FHWA said. “When operating within their capacity, roundabouts typically have lower overall delay than signalized and all-way stop-controlled intersections.”

The delay reduction is most significant during non-peak traffic periods, but the benefits can result in reduced lane requirements between intersections, the FHWA said.

Because roundabouts can facilitate U-turns, they can reduce or eliminate left-turn movements at driveways between major intersections, and they can reduce vehicle speeds by their design without other traffic control devices.

Pedestrians are safer with roundabouts because of the reduction of vehicle speeds in the intersection and because of the island in the middle that allows pedestrians to cross one traffic stream at a time, the FHWA said.

According to the video produced by the Clinton Engineering Department, nearly 57% of fatal and injury crashes on U.S. highways occur at intersections, and nearly 54% of all crashes happen at intersections.

The Insurance Institute for Highway Safety says that intersections converted from traffic signals or stop signs to roundabouts have reduced injury crashes 72%-80% and all crashes by 35% to 47%.

Based on the results of a 2004 study (Eisenman et al., 2004), conversion of 10% of signalized intersections to roundabouts would have prevented nearly 51,000 crashes in 2018, including 231 fatal crashes and about 34,000 crashes involving injuries, said IIHS.

“Despite the demonstrated safety benefits of roundabouts, some crashes still occur,” IIHS said. A study of crashes at 38 roundabouts in Maryland found that four types — run-off-road, rear-end, sideswipe and entering-circulating — accounted for almost all crashes (Mandavilli et el., 2009).

Collision with the central island is also a common crash on roundabouts, IIHS said. These crashes often involved unsafe speeds. Some drivers may not have seen the roundabout in time to slow down sufficiently, IIHS said.

Design features that encourage drivers to slow down are the key to optimizing roundabout safety, IIHS said.

Building a new type of intersection will require teaching drivers how to navigate it. The Iowa Department of Transportation explains how to use roundabouts:

Using a roundabout is much the same as making a right turn from a stop sign. At a traffic signal, a right-turning driver stops at the stop bar, looks for conflicting traffic coming from the left, chooses an acceptable gap in the traffic flow and turns right onto the cross street. At a modern roundabout, the oncoming driver approaches the yield line, looks for conflicting traffic coming from the left, chooses an acceptable gap in the traffic flow and enters the roundabout with a right turn at the yield sign. Once inside the roundabout, a driver continues circling counter-clockwise until reaching the desired exit. Exit maneuvers are also right turns.