Safety Benefits of Modern Roundabouts

Safety Features

When designed correctly, modern roundabouts can have multiple advantages over other types of intersections.

With modern roundabouts, direct left-turns are eliminated by making the driver circulate around the central island to the desired destination leg of the intersection. With traffic signals, left-turns must cross against opposing traffic at or near 90° angles. The circulating movement of the modern roundabout eliminates the potential for high speed, right angle and left turn/head on collisions.

Collisions at modern roundabouts occur less frequently because they have fewer conflict points than conventional signalized intersections. A conflict point is defined as a location where collisions could occur as traffic (vehicles, bicyclists, and pedestrians) cross paths. Concerning vehicle-to-vehicle collisions, the number of conflicts is reduced from 32 points with conventional two-lane intersections (signalized or unsignalized) to 8 points with the modern roundabout, or a 75 percent reduction. For vehicle to pedestrian collisions, the number of conflicts at the intersection is reduced from 24 points with the conventional signalized intersection to 8 points with the modern roundabout, or a 67 percent reduction.

Historically, crash rates at intersections tend to decrease once a conventional intersection is converted to a modern roundabout. In March of 2000, the Insurance Institute for Highway Safety (IIHS), a non-profit research and communications organization, completed a study in which they evaluated the changes in motor vehicle crashes following the conversion of 24 intersections from stop sign and traffic signals as entry control to modern roundabouts. The settings, located in eight states, were a mix of urban, suburban, and rural environments. Overall, the study found a 39 percent reduction for all crash types. The study also estimated there was a 76 percent reduction for injury crashes for 20 of the 24 intersections (there was not enough data for the other four intersections to draw any conclusions).

In December of 2002, The Maryland State Highway Administration (MSHA) also released its own evaluation of roundabouts in their state. Maryland has the highest number of modern roundabouts in operation within the United States. After studying 30 intersections converted to modern roundabouts, they documented the following statistics:

• The average annual crashes fell from 4.05 crashes per year with a conventional intersection to 1.11 crashes per year, a 73 percent reduction.

• Crash severity also decreased, from 2.31 injury crashes per year with a conventional intersection to 0.35 injury crashes per year, an 85 percent reduction.

• The mean total crash rate for conventional intersections was 1.36 reported per million vehicles entering, while the conversion to modern roundabouts was 0.27 crashes per million vehicles entering.

A benefit/cost analysis was also conducted by MSHA for 15 single lane roundabouts. Their findings concluded:

• At locations where roundabouts had been installed, there was a 60 percent decrease in the total crash rate and a 100 percent decrease in fatal crash rates.

• There was an 82 percent reduction in the injury crash rate and a 27 percent reduction in property damage only crash rates.

• The benefit/cost effectiveness analysis indicated that for every dollar spent on the projects, there was a return of approximately eight dollars to be realized through crash reductions.

Collisions at signalized intersections are typically more severe than those that occur at modern roundabouts. This is because collisions at conventional intersections tend to occur at higher rates of speed and at higher angles. Collision types expected at signalized intersection include high speed, 90° (“T-bone”) collisions, head on collisions involving a vehicle attempting to turn left across oncoming traffic, or rear end collisions. Since the internal operating speed for the modern roundabout ranges between 15 and 25 MPH, crashes typically involve lower speed, low angle merging (sideswipe) and rear end collisions. Vehicles will typically travel at higher rates of speed, up to 55 MPH, through a signalized intersection. These speeds can be even higher if a vehicle is attempting to “beat” a red light in order to save time. Collisions at lower rates of speed translate into less severe injuries to drivers, passengers, and pedestrians. Lower rates of speed also translate into less property damage.

When designed correctly, modern roundabouts tend to be safer for pedestrians of all ages as opposed to conventional intersections. Although there are few studies on pedestrian safety at modern roundabouts conducted in the U.S., a study performed in the United Kingdom (Lalani, N. 1975. The Impact on Accidents of the Introduction of Mini, Small, and Large Roundabouts at Major/Minor Priority Junctions) found that of 38 intersections converted to modern roundabouts, there was a 46 percent reduction in pedestrian collisions. The study also found a 70 percent reduction in fatal and serious pedestrian collisions at these locations. Other international studies have shown similar results.

Modern roundabouts are safer for pedestrians because the splitter islands located at each road approach provides refuge for pedestrians in the middle of each crossing. Therefore pedestrians only need to cross one direction of traffic at a time as opposed to crossing the entire road width all at once. This means pedestrians can focus on traffic coming from one direction as opposed to two or more directions.

Crosswalks at modern roundabouts are typically set at least one full car length back from the yield line. Therefore pedestrians do not have to cross in front of drivers that are looking for a gap in traffic. Experience has shown that crosswalks located one car length behind the yield line allows drivers to focus on the crosswalk prior to focusing on the circulating roadway. This simplifies the driver’s decision making tasks by separating the two actions.

Because properly designed roundabout geometry reduces entry and exit speeds, a pedestrian is more likely to survive a collision with a vehicle at a modern roundabout than at other intersections. Slower speeds means less distance is needed to bring a vehicle to a stop. Per a study conducted by the United Kingdom Department of Transportation, a pedestrian’s chance for survival is drastically reduced as collision speeds increase beyond 30 MPH. Vehicle speeds in excess of 40 MPH are typical at traffic signals, especially if a driver is attempting to “beat” a red light. Since roundabout entry and exit speeds are designed for lower speeds, the likelihood of survival and recovery are greatly increased.

Modern roundabouts are also becoming popular for intersections at or near public schools throughout the United States. Here in Michigan, modern roundabouts are located near seven different public schools, with two more locations slatted for construction in 2007.

Locations where roundabouts currently exist at or near Michigan public schools

School	Intersection	Location
Bennett Woods Elementary	Bennett Rd. at Hulett Rd.	Okemos, MI
Beck Centennial Elementary	25 Mile Rd. at Hayes Rd.	Utica, MI
Gaylord High School	Old US-27 at Livingston Blvd.	Gaylord, MI
Harvest Elementary	Campus Parkway at Community Dr.	Saline, MI
Middle School at Parkside	Fourth St. at West Ave.	Jackson, MI
Saline High School	Campus Parkway at Suncrest Dr.	Saline, MI
Stoney Creek High School	Tienken Road at Sheldon Rd.	Rochester Hills, MI

Locations where roundabouts are proposed for construction at/near public schools in 2007

High School	Intersection	Location
Chippewa Valley H.S.	Romeo Plank Road at 19 Mile Road	Clinton Twp., MI
Skyline H.S.	M-14 at Maple Road	Ann Arbor, MI

There is no before crash data to provide for the Campus Parkway roundabouts since they were constructed as part of the school development. However, since opening to traffic in 2004, there has only been one reported minor crash at the Campus Parkway / Suncrest Drive roundabout. No injuries were reported.

At Old US-27 at Livingston Blvd. near Gaylord High School, there were 20 crashes reported between the years 1994-2003, with five of those incidents involving serious injuries. The community lobbied to have a traffic signal installed at the intersection, but sufficient traffic warrants were not present to support its installation. The Otsego County Road Commission elected instead to install a modern roundabout to address safety concerns and congestion. The roundabout was opened to traffic prior to school resuming class in the fall of 2006. To date, there have been no crashes or injuries reported at the intersection.

A modern roundabout was also constructed near Bennett Woods Elementary School in Okemos, MI, at the intersection of Bennett and Hulett Roads by the Ingham County Road Commission. When asked about the effectiveness and safety of the modern roundabout, Mr. Jeri Mifflin, Principal of the school, stated, “It definitely has improved the flow of traffic and has not proved to be a safety concern that several parents feared.”

Similar testimonials can be heard at other locations across the U.S. where School Boards, faculty, and parents questioned the use of modern roundabouts near public schools but later supported their use once they were opened to traffic. Although data is limited, the results indicate that inexperienced and newer drivers understand and navigate modern roundabouts very well.