Roundabout Safety

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 single-lane roundabout has only eight vehicle-to-vehicle conflict points and eight vehicle-to-pedestrian conflict points. A conflict point is defined as a possible point of contact between two vehicles or a vehicle and a pedestrian where their paths of travel intersect. 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 below show the conflict points of a standard intersection and a typical roundabout.

Standard 2-lane conflict points vs single-lane roundabout conflict points

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

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.

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.

Roundabouts keep traffic moving. The major delay on a person's morning or evening commute is usually the time spent sitting at traffic signals. Eliminating the need to stop and wait reduces delay.

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 then 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 then 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.

Roundabouts are pedestrian friendly. The splitter islands (see illustration above) provide a space for pedestrians in the middle of each crossing. Therefore, pedestrians only need to cross one direction of traffic at a time. The pedestrian crosswalks are set at least one full car length back from the yield line. That way, pedestrians do not have to cross in front of drivers that are looking for their gap in traffic. Experience has shown that the stopped vehicle one car length back from the yield line is more aware of pedestrians.

According to the Insurance Institute for Highway Safety, modern roundabouts reduce motor vehicle crashes. Their July 2001 Status Report noted "most serious kinds of crashes at conventional intersections are virtually eliminated by roundabouts. Crashes that do occur tend to be minor because traffic speeds are slower." The study reviewed 24 intersections around the United States that have been converted from stop signs or traffic signals to modern roundabouts. At those intersections, all crashes were reduced by 39 percent. Serious crashes were reduced by 76 percent. At the time of the study, there had been no fatalities at any of the new roundabouts. So, the study estimates that fatal or incapacitating injuries will be reduced by 90 percent at those intersections.

Modern roundabouts are usually less expensive than signalized intersections for two primary reasons:

• Expensive traffic signal equipment, as well as maintenance of that equipment, is not needed; and
• Under certain traffic conditions, the free flow movement of the roundabout is able to reduce the capacity needs of adjoining roadways; thus, fewer traffic lanes may be needed. Roundabouts usually do not require separate left- and right-turn lanes, which also helps lower costs of intersection approaches.

Since roundabouts are currently rare around the United States, all types of drivers may experience initial confusion upon their first encounter. However, as roundabouts become more common and motorists become more familiar with their operation, the initial confusion will be significantly reduced. Most people quickly learn their operation. Plus, because of the low speeds, there is generally much less risk of a crash or injury compared to a traditional intersection.

Roundabouts have design features specifically intended to accommodate trucks, buses, tractors, and larger vehicles. The main characteristic is a truck apron, a slightly raised area around the center island allowing larger trucks easier circulation in the roundabout. It is typically 3 to 4 inches higher than the paved roadway. A truck apron is used instead of increasing the normal driving width to prevent smaller vehicles from achieving higher speeds through the roundabout. With a properly designed truck apron, a roundabout is able to accommodate all types of larger vehicles.

At roundabouts, the traffic circulates counter-clockwise and moves toward vehicles at the yield line from the left. Vehicle operators should always yield at the entry to circulating traffic. In practice, that means yield to traffic from the left, similar to the action that is necessary when entering a freeway or turning right at a red traffic light/signal.

When approaching a roundabout, pay attention to pavement markings and signage indicating the lane you should be in to get to the proper exit of the roundabout. Use your turn signal to indicate to other drivers where you intend to go. Look for pedestrians and bicycle traffic crossing your lane and yield to them, if present. Begin to look at the traffic already circulating within the roundabout as you approach the yield line and choose an appropriately-sized gap in traffic. Use this gap to merge into traffic within the roundabout.

Do not stop within a roundabout. If emergency vehicles are approaching, exit the roundabout before pulling over. If a confused driver stops within a roundabout, do not pass or overtake the driver using the truck apron. Stopping within a roundabout to let another drive in is dangerous and should not be done. It creates an unpredictable situation for drivers around you and can lead to crashes.

Do not change lanes within the roundabout. This is unpredictable behavior to other drivers and can cause crashes. Roundabouts are equipped with signage and pavement markings prior to entry that indicate what lanes will convey you to which exit. In general, Multi-lane roundabouts should be approached the same way as any other intersection. To turn left, use the left-most lane and signal for a left turn. To turn right, use the right-most lane and signal for a right turn. In all situations, vehicle operators should pass counterclockwise around the central island. When preparing to exit, vehicle operators should turn on their right turn signals as soon as they pass the exit before the one that will be used.

Trucks, buses, and other large vehicles may utilize the truck apron surrounding the center of the roundabout in order to safely pass through the intersection. The truck apron is typically a slightly raised area of pavement that is designated by a different color or texture of pavement. Give large vehicles more space to navigate the roundabout, just like you give them more space to turn at intersections.

Exiting a roundabout means turning to the right, just like most freeway exit ramps. Use your turn signal and watch for crossing pedestrian and bicycle traffic ahead of your vehicle.

Watch this video for more information.

Iowa Department of Transportation

Wisconsin Department of Transportation

• Bicyclists and Pedestrians Navigating a Multilane Roundabout
• Emergency Vehicles in Roundabouts
• FHWA Roundabout Driving Rules
• Large Vehicles & Trucks Navigating a Multilane Roundabout
• Navigating a Multilane Roundabout
• Roundabout Driving Rules