When should roundabouts replace traffic signals?
We recommend British policy, which seeks to optimize the type of intersection as well as the type of intersection regulatory device.
Because cross intersections are so dangerous, do not build new cross intersections unless you have no other choice. (Signalized cross intersections are generally more dangerous than cross intersections regulated by STOP signs or YIELD signs.)
Treat a cross intersection having capacity and safety problems in one of three ways:
Convert the 4-leg intersection into a pair of 3-leg intersections, i.e. an offset intersection, if the crossing flows are not too high. Arrange the offset so that left turns on the main road back up away from the pair of tees, not between them.
Install a raised median on the centerlines of the side streets just before the cross street. Sometimes this will reduce crashes caused by drivers tending to shoot straight across without stopping.
For heavy crossing flows, convert the problem intersection into a roundabout if there is enough space to do so. The following are non-issues: imbalanced flows, steep grades, and heavy concentrations of pedestrians or trucks. In these cases, we would still recommend a roundabout if there is enough space. In built-up areas, mini-roundabouts can be built if the speed limit is 30 miles per hour or less.
The main issue is economic. Roundabouts give superior traffic and safety performance to signalized intersections and to cross intersections almost everywhere, but roundabouts cost more, and not every signalized intersection or cross intersection is in great need of correction. Therefore it behooves a highway agency to start at the top of its high-crash list and its high-congestion list and work down, often converting the problem location into an offset intersection where crossing flows are not too high.
If your signal is at a 3-leg intersection and the side-street flows are light enough, it may be best merely to regulate the intersection with one STOP sign or one YIELD sign. If your signal is at a mid-block pedestrian crossing, roundabouts cannot help. If your signal is at an intersection of five, six, or seven legs, the intersection will probably give better performance if some of the legs are redirected into a roundabout.
Exceptionally, we continue to recommend traffic signals to some very high-demand entries to roundabouts where there is ample space between roundabout entries. Where pedestrian flows are very high, we recommend pedestrian grade separations or signalized crossings near the roundabout.
Other Roundabout Policies
Several jurisdictions in North America have instituted policies whereby the feasibility of roundabouts must be evaluated for all new intersections, for existing intersections where traffic signals are warranted, or where capacity or safety problems have been identified.
New York State is one example. The Regional Municipality of Waterloo in Ontario, Canada, is another. We believe the policies of these two road owners should be followed by road owners everywhere.
The modern roundabout is a type of intersection. It should be illustrated along with other types of intersection in the Intersections Chapter of every highway design manual. It should be recommended not only as a retrofit where there are present safety and capacity problems, as New York and Waterloo have done, but also as an intersection in new construction where roads join or cross each other for the first time.
The choice of whether to use a roundabout intersection or a non-roundabout intersection (for example, a T-intersection or an offset intersection) should be made on economic grounds. The best economic choice of intersection in terms of costs and benefits should be made at each site. In general, roundabouts are too expensive for low-flow sites, but they give great safety and capacity benefits at high-flow sites. Since most intersections have low flows, most intersections should not be roundabouts.
In addition to their benefits at high-flow sites, roundabouts also give great safety benefits at high-speed sites, especially where a signalized cross intersection would be the alternative. In general, signalized cross intersections are more dangerous than roundabouts everywhere, but the safety advantage of roundabouts over signalized intersections is greatest at high-speed crossings. Roundabouts replacing signalized high-speed intersections can reduce fatalities by 90 to 100 percent.