Safety

This section provides an overview of the safety performance of roundabouts and then discusses the general characteristics that lead to this performance. It does not attempt to discuss all of the issues related to safety; the reader is encouraged to refer to Chapter 5 for a more detailed discussion.

Roundabouts are generally safer than other forms of intersection in terms of aggregate crash statistics for low and medium traffic capacity conditions (1). Injury crash rates for motor vehicle occupants are generally lower, although the proportion of single-vehicle crashes is typically higher. However, bicyclists and pedestrians are involved in a relatively higher proportion of injury accidents than they are at other intersections (2).

Exhibit 2-1 presents comparisons of before and after aggregate crash frequencies (average annual crashes per roundabout) involving users of eleven roundabouts constructed in the United States (3). The decrease in severe injury crashes is noteworthy. However, the “before” situation at these intersections required mitigation for safety. Therefore, some other feasible alternatives may also be expected to have resulted in a reduction in the crash frequencies. This study yielded insufficient data to draw conclusions regarding the safety of bicyclists and pedestrians.

Good roundabout design places a high priority on speed reduction and speed consistency. Such designs require that vehicles negotiate the roundabout through a series of turning maneuvers at low speeds, generally less than 30 km/h (20 mph). Speed consistency refers to the design objective of slowing vehicles in stages down to the desired negotiating speed to be consistent with the expectations of drivers. Speed control is provided by geometric features, not only by traffic control devices or by the impedance of other traffic. Because of this, speed reduction can be achieved at all times of day. If achieved by good design, then in principle, lower vehicle speeds should provide the following safety benefits:

• Reduce crash severity for pedestrians and bicyclists, including older pedestrians, children, and impaired persons;
• Provide more time for entering drivers to judge, adjust speed for, and enter a gap in circulating traffic;
• Allow safer merges into circulating traffic;
• Provide more time for all users to detect and correct for their mistakes or mistakes of others;
• Make collisions less frequent and less severe; and
• Make the intersection safer for novice users.

For example, Exhibit 2-2 shows that a pedestrian is about three times more likely to die when struck at 50 km/h (30 mph) than at 32 km/h (20 mph), across a range of only 18 km/h (10 mph) difference in speed (4). Typical commuter bicyclist speeds are in the range of 20 to 25 km/h (12 to 15 mph). Therefore, the difference in design speed is critical to all users who are not within the protective body of a motorized vehicle. The minor additional delay or inconvenience to drivers of lower-speed roundabout designs (as compared to higher-speed roundabout designs) is a tradeoff for the substantial safety benefit to pedestrians and bicyclists. Older drivers may benefit from the additional time to perceive, think, react, and correct for errors (as may all users). It should be clarified that there has been no specific research performed on older drivers, older pedestrians, and older bicyclists at roundabouts. It should also be noted that visually impaired pedestrians are not provided the audible cues from vehicle streams that are available at a signal controlled intersection. For example, at roundabout exits, it may be difficult to discern the sound of vehicles which will continue to circulate from those exiting the roundabout. Therefore, information needs to be provided to these users through various appropriate design features to assist them in safely locating and navigating the crossings at roundabouts.

Furthermore, the operational efficiency (capacity) of roundabouts is probably greater at lower circulating speed, because of these two phenomena:

• The faster the circulating traffic, the larger the gaps that entering traffic will comfortably accept. This translates to fewer acceptable gaps and therefore more instances of entering vehicles stopping at the yield line.
• Entering traffic, which is first stopped at the yield line, requires even larger gaps in the circulating traffic in order to accelerate and merge with the circulating traffic. The faster the circulating traffic, the larger this gap must be. This translates into even fewer acceptable gaps and therefore longer delays for entering traffic.

Single-lane roundabouts

The safety characteristics of single-lane and multilane roundabouts are somewhat different and are discussed separately. Single-lane roundabouts are the simplest form of roundabout and thus are a good starting point for discussing the safety characteristics of roundabouts relative to other forms of intersections.

The frequency of crashes at an intersection is related to the number of conflict points at an intersection, as well as the magnitude of conflicting flows at each conflict point. A conflict point is a location where the paths of two vehicles, or a vehicle and a bicycle or pedestrian diverge, merge, or cross each other. For example, Exhibit 2-3 presents a diagram of vehicle-vehicle conflict points for a traditional four-leg intersection and a four-leg roundabout intersection of two-lane roads. The number of vehicle-vehicle conflict points for four-leg intersections drops from thirty-two to eight with roundabouts, a 75 percent decrease. Fewer conflict points means fewer opportunities for collisions. These are not the only conflict points at roundabouts or traditional intersections, but are illustrative of the differences between intersection types. Chapter 5 contains a more detailed comparison of conflicts at more complex intersections and for pedestrians and bicyclists.

The severity of a collision is determined largely by the speed of impact and the angle of impact. The higher the speed, the more severe the collision. The higher the angle of impact, the more severe the collision. Roundabouts reduce in severity or eliminate many severe conflicts that are present in traditional intersections.

As Exhibit 2-3 shows, a roundabout eliminates vehicle-vehicle crossing conflicts by converting all movements to right turns. Separate turn lanes and traffic control (stop signs or signalization) can often reduce but not eliminate the number of crossing conflicts at a traditional intersection by separating conflicts in space and/or time. However, the most severe crashes at signalized intersections occur when there is a violation of the traffic control device designed to separate conflicts by time (e.g., a right-angle collision due to a motorist running a red light, or vehicle-pedestrian collisions). The ability of roundabouts to reduce conflicts through physical, geometric features has been demonstrated to be more effective than the reliance on driver obedience to traffic control devices. At intersections with more than four legs, a roundabout or pair of roundabouts may sometimes be the most practical alternative to minimize the number of conflicts.

Drivers approaching a single-lane roundabout have five basic decisions regarding other users. First, drivers must be mindful of any bicyclists merging into motor vehicle traffic from the right side of the road or a bicycle lane or shoulder. Then they must yield to any pedestrians crossing at the entry. Third, they must choose an acceptable gap in which to enter the roundabout. Then they must choose the correct exit, and finally, they must yield to any pedestrians crossing the exit lane.

By contrast, a driver making a left turn from the minor leg of a two-way stop controlled intersection has to yield to pedestrians and bicyclists, and judge gaps in both of the major street through movements from both directions, as well as the major street left and right turns and opposing minor through and right turns.

Signalized intersections have simplified the decision-making process for drivers, especially at locations where protected left-turn phasing is provided, by separating conflicts in time and space. However, the rules and driver decisions for negotiating signalized intersections are still quite complex when all the possible signal phasing schemes are accounted for. For signals with permitted left-turn phasing, the driver must be cognizant of the opposing traffic including pedestrians, and the signal indication (to ensure a legal maneuver). At roundabouts, once at the yield line, the entering driver can focus attention entirely on the circulating traffic stream approaching from the left. A driver behind the entering driver can focus entirely on crossing pedestrians.

Double-lane roundabouts

As discussed in Chapter 1, double-lane roundabouts are those with at least one entry that has two lanes. In general, double-lane roundabouts have some of the same safety characteristics for vehicle occupants as their less complicated singlelane counterparts. However, due to the presence of multiple entry lanes and the accompanying need to provide wider circulatory and exit roadways, double-lane roundabouts have complications that result in poorer safety characteristics, particularly for bicyclists and pedestrians, than single-lane roundabouts serving similar traffic demands. This makes it important to use the minimum number of entry, circulating, and exit lanes, subject to capacity considerations.

Due to their typically larger size compared to single-lane roundabouts, double-lane roundabouts often cannot achieve the same levels of speed reduction as their singlelane counterparts. Wider entering, circulating, and exiting roadways enable a vehicle to select a path that crosses multiple lanes, as shown in Exhibit 2-4. Because of the higher-speed geometry, single-vehicle accidents can be more severe. However, design of double-lane roundabouts according to the procedures in Chapter 6, especially the approach and entry, can substantially reduce the speeds of entering vehicles and consequently reduce the severity of conflicts. Even so, speed control cannot occur to the extent possible with single-lane roundabouts.

Pedestrians crossing double-lane roundabouts are exposed for a longer time and to faster vehicles. They can also be obscured from, or not see, approaching vehicles in adjacent lanes if vehicles in the nearest lane yield to them. Children, wheel chair users, and visually impaired pedestrians face particular risks. Bicycles are also more exposed to severe conflicts when choosing to circulate with motor vehicles.

Driver decisions are more complex at double-lane roundabouts. The requirement to yield to pedestrians still applies. The primary additional decisions are the choices of the proper lane for entering, lateral position for circulating, and proper lane for exiting the roundabout. Lane choice on approaching a double-lane roundabout is no different from approaching a signalized intersection: to turn left, stay left; to turn right, stay right. However, the decisions for circulating within and especially exiting a double-lane roundabout are unique.

Double-lane roundabouts with legs aligned at approximately 90-degree angles allow motorists to determine the appropriate lane choice for their path through the roundabout in a relatively easy manner. Double-lane roundabouts with more than four legs and/or with legs aligned at angles significantly different from 90 degrees make driver decisions more complicated. This occurs because it can be difficult on some legs to determine which movements are left, through, and right. For this reason, it is desirable that multilane roundabouts be limited to a maximum of four legs, with legs aligned at approximately 90-degree angles. If this is not possible, special advance guide signs showing appropriate lane choice should be considered.

When double-lane roundabouts are first introduced to an area, there is a need for adequate user education. Recommendations for user education material specifically related to this issue are presented later in this chapter.