Traffic Circles & Roundabouts Expert Article

Although they are similar, there are distinct differences between roundabouts and traffic circles. In this article the highway engineering experts at Robson Forensic discuss both roadway features including the implications on vehicle crash investigations. 

Traffic Roundabouts

Many people are confused when they hear the term roundabout. In fact, most people believe a roundabout is nothing more than a new name for a traffic circle. Although they are similar, there are distinct differences between a roundabout and a traffic circle. Traffic circles create confusion for motorists as there is little positive guidance or direction; consequently, people are often terrified when confronted with having to traverse a traffic circle.

The traffic circles of the past had numerous safety and operational problems due to their design and traffic rules. Traffic circles often have stop signs or traffic signals within the circular intersection. To enter the traffic circle, vehicles enter the traffic circle in a straight line. Vehicles that are in the circle have the right of way; consequently, vehicles approaching and attempting to enter the traffic circle must yield to vehicles already in the circle. This can cause congestion when many vehicles attempt to enter the circle at the same time. Traffic circles are also designed to allow vehicles to enter and circulate at higher speeds. Traffic circles are typically much larger than a roundabout.

The modern day roundabout is designed to provide more positive direction to motorists as they approach, enter and exit the roundabout. The entrances to the roundabout are tapered so that vehicles don’t enter the roundabout at 90 degrees. There is more direct and efficient signing to reduce motorist confusion. Traffic speeds in the roundabout are also lower when compared to a traffic circle. The modern roundabout is a circular intersection with specific design and traffic control features that makes it different than a traffic circle or rotary. With a roundabout, the entry points are yield controlled (traffic already in the circle has the right of way while traffic wanting to enter the circle must yield). This yield-at-entry rule prevents traffic from locking-up and allows free flow movement.

The approaches to the roundabout also contain channelizing devices (typically small raised islands) that direct traffic into the roundabout at the proper speed and angle. These channelizing devices constrain vehicle speeds at the roundabouts entry points. The geometry of a roundabout results in the slowing of traffic while maintaining traffic flow. Generally, a roundabout will be designed for traffic speeds around 20 mph or slower. In a roundabout, all directions of traffic are usually kept open and safely flowing. Because of this, there is very little (if any) stopping or idling from waiting vehicles. In addition, modern roundabouts are smaller than traffic circles. A traffic circle is typically 4 to 5 times larger in diameter than a roundabout.

Does Your Case Involve a Roundabout or a Traffic Circle?

1. Modern roundabouts follow the “yield-at-entry” rule in which approaching vehicles must wait for a gap in the circulating flow before entering the circle. Many of the old traffic circles in the United States required circulating vehicles to grant the right of way to entering vehicles. Some traffic circles may also use stop signs or signals to control vehicle entry.
2. Modern roundabouts involve low speeds for entering and circulating traffic, as governed by small diameters and deflected (curved) entrances. In contrast, traffic circles emphasized high-speed merging and weaving, made possible by larger diameters and tangential (straight) entrances.
3. Adequate deflection of the vehicle entering a roundabout is the most important factor influencing their safe operation. This is done by adjusting the geometry of the entrance alignment, splitter island, center island, and exit alignment to ensure that through vehicle paths are significantly deflected. Roundabouts typically limit the speed of vehicles to 20 mph or less.

Safety Data on Roundabouts 

Roundabouts can be used as alternatives to signalized intersections and stop sign controlled intersection. The safety advantages of roundabouts over signalized and stop sign controlled intersections are significant. Some of the most common types of crashes, associated with signalized or stop sign controlled intersections, are nearly eliminated when using a roundabout.

According to ongoing studies from FHWA and the National Cooperative Highway Research Program, roundabout injury crashes are reduced about 75 percent and fatalities by 90 percent or more compared with those at conventional intersections. In addition, because roundabouts reduce vehicle delays and the incidence of having to stop and accelerate, there are reductions in fuel usage. According to the Insurance Institute of Highway Safety, “…constructing roundabouts in place of traffic signals can reduce fuel consumption by about 30 percent.”

Traffic Rules Relevant to Roundabout Crash Investigations:

• Drivers approaching the roundabout YIELD to circulating traffic within and watch for pedestrians and bicyclists.
• Approaching traffic should not stop if the way is clear.
• Drivers are responsible to watch for pedestrians and bicyclists as they exit.
• Left turns are completed by traveling around the central island.

Highway Engineering Investigations

The highway experts at Robson Forensic examine crash sites, use field measurements, scrutinize crash scene photos, and review infrastructure design documents to determine if highway factors or road conditions contributed to the cause of motor vehicle crashes.

For more information contact the author of this article or submit an inquiry.

Featured Expert

Gordon Meth, P.E., PTOE, PTP, RSP2I

Civil Engineer & Highway Engineering Expert

Gordon Meth, P.E., is a civil engineer with over 30 years of design and analysis experience in traffic, highway, and site/civil engineering. He has particular expertise in intersection design and… read more.