Roundabouts – Benefits, Safety Issues and Potential Treatments

A roundabout is a form of intersection in which traffic circulates in one direction around a circular central island, and all entering traffic is required to give way to traffic circulating on the roundabout.

Benefits of Roundabout Junctions:

• Reduced conflict points as compared to perpendicular junctions.
• Reduced queuing associated with traffic lights.
• Facility for U-turns within the normal flow of traffic, which often are not possible at other forms of junction.
• Environmental benefits. Entering vehicles only need to give way at relatively low speeds, and do not always perform a full stop. As a result, by keeping a part of their momentum, the intersection performs more efficiently from a traffic flow perspective. In addition, engines will require less effort to regain the initial speed, resulting in lower emissions. Research has also shown that slow-moving traffic in roundabouts makes less noise than traffic that must stop and start, speed up and brake.
• Safer option of junctions, as low speeds are required for traffic entering the roundabouts. Approaches are designed so that vehicles enter the circulating carriageway with limited vehicle path radius naturally slow down. Also, by limiting the entry path curve by introducing horizontal deflection to the approaches, vehicular entry speeds can be reduced, which provides drivers more time to react to potential conflicts and reduces crash severities.
• There are fewer vehicular conflict points and less potential for high severity conflicts, such as right-angle, left turn, and head-on crashes because of the roundabout’s design and because all drivers are going in the same direction.
• They are effective during power outages. Unlike traditional signalized intersections, which must be treated as all-way stop or require police to direct traffic, roundabouts continue to work as normal.

Safety Issues

• Misunderstandings of rules and not every driver knows roundabout rules. In some countries/areas new to roundabouts, people have never learned the rules (yield and driving directions) of roundabouts. They might drive wrong directions and not yield to other vehicles.
• Poor judgement of gaps by drivers entering a high-speed flow of circulating traffic, especially when there are multiple lanes.
• Rear-end collisions between vehicles waiting to join the roundabout may increase.
• Sideswipe collision during changing lanes or entering/exiting the center circle.
• Pedestrian/cycle collision by not yielding to pedestrians and cyclists.
• As circulating widths increase, the ability to control speed into and through the roundabout becomes less predictable.
• Painted (or low height) islands become less visible and hence tend to be neglected by the drivers.
• Potential conflict at a multi-lane roundabout. At the exit point, it is possible that a motorist in the outside lane may desire to continue circulating, while the motorist in the inside lane desire to exit.

Good Design Practices/Treatments

• Properly designed roundabouts control the angle at which traffic enters the intersection and the speeds of vehicles entering and going through the intersection by creating geometric curvature with center and splitter islands. This feature results in safer intersections than other at-grade intersections where vehicles can enter the intersection without slowing their speeds.
• Newer designs may also include raised platforms or humps on the approach that have been successfully used to slow the approach speed of vehicles, reducing the need for geometric curvature, and sometimes significantly reducing construction costs.
• A key element of safe roundabout operation is to ensure that the central island or splitter islands provide sufficient deflection from the straight-ahead movement to ensure slow vehicle speeds through the intersection. Where sufficient deflection is not possible (for instance due to restrictions in road space), raised platforms have been used successfully instead.
• With splitter islands, pedestrians are required to cross only one direction of traffic at a time at a roundabout and contend with slower moving vehicles because of the splitter islands.
• Flat/low height islands (i.e., marking) may not work. Center and splitter islands should be physically raised to provide readability.
• Decoration and vegetation at center and splitter islands must not obstruct driver’s sight distance of approaching or circulating traffic. However, it should be sufficiently high to obstruct the straight through view of the road ahead and concentrate drivers’ awareness on the roundabout.
• The center island and the splitter islands must be large enough to force approaching vehicles to reduce their speed in order to enter the intersection. Too small center islands and splitter islands may not work to reduce the speed of approaching vehicles during passing through the intersection because turning along the center island is not required. This defeats the purpose of a roundabout.
• A key factor in determining the size of a roundabout, both the central island and the width of the circulating carriageway, is the safe negotiation of the design vehicle for all movements.
• Humps and platforms can be used to reduce speeds, especially where there is not enough deflection on approach.
• Pedestrian and cyclist facilities can be included in the intersection design.
• Education may be needed to ensure road users know how to navigate roundabouts, especially when first introduced; and to enforcement to ensure compliance. A small center island and a lack of length of splitter islands will also make extra circulating lanes.
• Lane lines must be provided with appropriate widths. Sometimes lane lines of roundabouts are missing. Approaching vehicles will miss the courses they should drive in the intersection and consequently crashes between vehicles will be caused. The lane boundaries must be provided as per the approaching roads. This can also guide vehicles to turn along the center island appropriately and reduce their speed.
• Signage for indication of a roundabout ahead in a clear and consistent way throughout the network is very important. The variation in the use of signs and markings reflects either the lack of knowledge, the lack of attention to detail, or the lack of clear guidance for the implementation of road signs and road markings. Similarly, the variation of road markings also causes driver’s misbehaviors.
• The performance of some congested roundabouts can be improved with traffic signal control by balancing entry flows and/or a continual flow of traffic on the circulating carriageway to prevent long queues causing long delays and blocking back into preceding junctions. Signals are able to keep the circulatory traffic flow fluid and hence balance and improve the roundabout capacity.
• The number of pedestrians (and cyclists) can increase crash risks and delays because traffic is governed by yield-control entry at a roundabout, especially at intersections with a low volume of pedestrians. Providing specific crossing points and routes around the intersection separate from motorized traffic can improve pedestrian and cycle safety at roundabout intersections.
• Traffic rules and design of roundabouts must coordinate with other transportation modes to avoid increasing crash risks in arterial roads with cycle lanes and public transportation lanes.
• The following sign is normally used at a ‘standard’ two-lane roundabout. The potential disadvantage of this sign is that it fails to convey the circular nature of the junction and could confuse the motorists into thinking that a left turn just past the splitter island is allowable.

?Configuring the arrows in a ‘fishhook’ arrangement add further clarification. The sign below illustrates the basic design of the fishhook, which continues to covey that there are two entering lanes and to indicate the movements allowed from each but adds the idea of circulation.