Roundabouts

Frequently Asked Questions

Safety and efficiency

Benefit: fewer crashes and less severe crashes

Roundabouts benefit from good geometry, exhibiting only a fraction of the troublesome crash patterns typical of right-angle intersections. A typical four-legged intersection has 32 vehicle-to-vehicle conflict points and 24 vehicle-to-pedestrian conflict points. By comparison, a four-legged roundabout has only eight vehicle-to-vehicle conflict points and eight vehicle-to-pedestrian conflict points. This is an approximate 70 percent reduction in conflict points. In addition, since 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 illustrations at the top of this page show the conflict points of a standard intersection and a typical roundabout. Studies by the Insurance Institute for Highway Safety show that roundabouts provide a:

  • 90 percent reduction in fatal crashes; 76 percent reduction in injury crashes; 30 to 40 percent reduction in pedestrian crashes; and 10 percent reduction in bicycle crashes

Why is Iowa installing roundabouts?

Roundabouts can offer a good solution to safety and capacity problems at intersections. At intersections where roundabouts have been installed in Iowa to replace existing intersections, crashes of all types have been reduced. Roundabouts can also offer high capacity at intersections without requiring the expense of constructing and maintaining a traffic signal.

Aren't traffic signals safer than roundabouts for pedestrians?

It depends on the amount of pedestrians and vehicles. In many cases, a roundabout can offer a safer environment for pedestrians than a traffic signal because the pedestrian crossing at a roundabout is reduced to two simple crossings of one-way traffic moving at slow speeds. A pedestrian crossing at a traffic signal still needs to contend with vehicles turning right or left on green, vehicles turning right on red, and vehicles running the red light. The latter of these potential conflicts occur at high speeds and often result in injuries or fatalities to pedestrians.

Benefit: pedestrians cross one direction of traffic at a time

Pedestrians need only cross one direction of traffic at a time at each roundabout approach, as compared with two-way and all-way stop-controlled intersections. The conflict locations between vehicles and pedestrians are generally not affected by the presence of a roundabout, although conflicting vehicles come from a more defined path at roundabouts. In addition, the speeds of motorists entering and exiting a roundabout are reduced with good design. As with other crossings that require acceptance of gaps in traffic flow, roundabouts still present visually-impaired pedestrians with unique challenges.

Are roundabouts safe near schools?

Over 30 roundabouts have been installed near schools in the United States. None have reported any significant problems.

Benefit: lower vehicle speeds

A standard stop sign or traffic signal controlled intersection always has at least one direction of traffic stopped. A roundabout uses yield-at-entry traffic control to eliminate stopping when it is not required.

Benefit: fewer vehicle delays

A standard stop sign or traffic signal controlled intersection always has at least one direction of traffic stopped. A roundabout uses yield-at-entry traffic control to eliminate stopping when it is not required.

Are roundabouts appropriate everywhere?

No. The choice of using a roundabout versus a traffic signal is a case-by-case decision. The Iowa Department of Transportation can assist in evaluating each candidate intersection to determine whether a roundabout or a traffic signal is more effective.

Benefit: less vehicle pollution

Vehicles entering a roundabout must yield at entry, but are not required to stop if the roundabout is clear. This eliminates some stop-and-go traffic associated with stop sign or traffic signal controlled intersections. This leads to fewer vehicles idling while stopped at an intersection.

Benefit: lower maintenance costs as compared to a traffic signal

A traffic signal requires electricity 24 hours a day. In addition, the signals need maintenance by field personnel for burned out lights, loop detector replacement, etc. A typical roundabout generally only needs electricity for streetlights at night and maintenance for landscaping, if included.

Benefit: increased landscaping opportunities

A standard intersection requires a large paved area to accommodate all the turning movements. A roundabout provides opportunity to landscape the center island, providing green space within the intersection