Introduction

Many studies have found that one of the benefits of roundabout installation is the improvement in overall safety performance. Several studies in the U.S., Europe, and Australia have found that roundabouts perform better in terms of safety than other intersection forms (1, 2, 3, 4). In particular, single-lane roundabouts have been found to perform better than two-way stop-controlled (TWSC) intersections in the U.S. (5). Although the frequency of reported crashes is not always lower at roundabouts, the reduced injury rates are usually reported (6). Safety is better at small and medium capacity roundabouts than at large or multilane roundabouts (1, 7). While overall crash frequencies have been reduced, the crash reductions are most pronounced for motor vehicles, less pronounced for pedestrians, and equivocal for bicyclists, depending on the study and bicycle design treatments (4, 6, 7). Crash statistics for various user groups are reported in Section 5.3.

The reasons for the increased safety level at roundabouts are:

• Roundabouts have fewer conflict points in comparison to conventional intersections. The potential for hazardous conflicts, such as right angle and left turn head-on crashes is eliminated with roundabout use. Single-lane approach roundabouts produce greater safety benefits than multilane approaches because of fewer potential conflicts between road users, and because pedestrian crossing distances are short.
• Low absolute speeds associated with roundabouts allow drivers more time to react to potential conflicts, also helping to improve the safety performance of roundabouts.
• Since most road users travel at similar speeds through roundabouts, i.e., have low relative speeds, crash severity can be reduced compared to some traditionally controlled intersections.
• Pedestrians need only cross one direction of traffic at a time at each approach as they traverse roundabouts, as compared with unsignalized 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 (and thus pedestrians have fewer places to check for conflicting vehicles). In addition, the speeds of motorists entering and exiting a roundabout are reduced with good design. As with other crossings requiring acceptance of gaps, roundabouts still present visually impaired pedestrians with unique challenges, as described in Chapter 2.

For the design of a new roundabout, safety can be optimized not only by relying on recorded past performance of roundabouts in general, but primarily by applying all design knowledge proven to impact safety. For optimum roundabout safety and operational performance the following should be noted:

• Minimizing the number of potential conflicts at any geometric feature should reduce the multiple vehicle crash rate and severity.
• Minimizing the potential relative speed between two vehicles at the point of conflict will minimize the multiple vehicle crash rate and severity (it may also optimize capacity). To reduce the potential relative speed between vehicles, either the absolute speeds of both vehicles need to be reduced or the angle between the vehicle paths needs to be reduced. Commuter bicyclist speeds can range from 20 to 25 km/h (12 to 15 mph) and designs that constrain the speeds of motor vehicles to similar values will minimize the relative speeds and improve safety. Lower absolute speeds will also assist pedestrian safety.
• Limiting the maximum change in speed between successive horizontal geometric elements will minimize the single vehicle crash rate and severity.