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 discusses both roadway features with a specific focus on the benefits of modern roundabouts and how they offer advantages in both safety and efficiency.

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.

Three basic principles distinguish the modern roundabout from 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.

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,”

Prior to constructing the Hamilton roundabout, we approached the residents in the area about removing the all-stop control intersection and replacing it with a roundabout. The proposal was met with much skepticism. After construction of the roundabout was complete, and the public witnessed the roundabout in action, the public embraced the change. To date, the roundabout is aesthetically pleasing, has significantly reduced the number of crashes (there have been no reported crashes since the roundabout was completed in 2010). There are no delays (stops) at the intersection so there are fuel savings and reduced carbon and greenhouse gas emissions.

Below are the rules that drivers need to follow in traversing a roundabout:

  • As you approach a roundabout there will be a YIELD sign and dashed yield limit line. Slow down, watch for pedestrians and bicyclists, and be prepared to stop if necessary.
  • When you enter, yield to circulating traffic on the left, but do not stop if the way is clear.
  • A conventional roundabout will have ONE-WAY signs mounted in the center island. They help guide traffic and indicate that you must drive to the right of the center island.
  • Upon passing the street prior to your exit, turn on your right turn signal and watch for pedestrians and bicyclists as you exit.
  • Left turns are completed by traveling around the central island.

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