Vehicle conflicts

Single-lane roundabouts

Exhibit 5-1 presents a diagram of vehicle-vehicle conflict points for a traditional three-leg (“T”) intersection and a three-leg roundabout. As the figure shows, the number of vehicle-vehicle conflict points for roundabouts decreases from nine to six for three-leg intersections. Note that these diagrams do not take into account the ability to separate conflicts in space (through the use of separate left or right turning lanes) or time (through the use of traffic control devices such as stop signs or traffic signals).

Exhibit 5-2 presents similar diagrams for a traditional four-leg (“X” or “cross”) intersection and a four-leg roundabout. As the figure shows, the number of vehicle-vehicle conflict points for roundabouts decreases from 32 to 8 for four-leg intersections.

Conflicts can be divided into three basic categories, in which the degree of severity varies, as follows:

• Queuing conflicts. These conflicts are caused by a vehicle running into the back of a vehicle queue on an approach. These types of conflicts can occur at the back of a through-movement queue or where left-turning vehicles are queued waiting for gaps. These conflicts are typically the least severe of all conflicts because the collisions involve the most protected parts of the vehicle and the relative speed difference between vehicles is less than in other conflicts.
• Merge and diverge conflicts. These conflicts are caused by the joining or separating of two traffic streams. The most common types of crashes due to merge conflicts are sideswipes and rear-end crashes. Merge conflicts can be more severe than diverge conflicts due to the more likely possibility of collisions to the side of the vehicle, which is typically less protected than the front and rear of the vehicle.
• Crossing conflicts. These conflicts are caused by the intersection of two traffic streams. These are the most severe of all conflicts and the most likely to involve injuries or fatalities. Typical crash types are right-angle crashes and head-on crashes.

As Exhibit 5-1 and Exhibit 5-2 show, a roundabout reduces vehicular crossing conflicts for both three- and four-leg intersections by converting all movements to right turns. Again, 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 running a red light, and vehicle-pedestrian collisions). Therefore, the ability of single-lane roundabouts to reduce conflicts through physical, geometric features has been demonstrated to be more effective than the reliance on driver obedience of traffic control devices.

Double-lane roundabouts

In general, double-lane roundabouts have some of the same safety performance characteristics as their simpler single-lane counterparts. However, due to the presence of additional entry lanes and the accompanying need to provide wider circulatory and exit roadways, double lane roundabouts introduce additional conflicts not present in single-lane roundabouts. This makes it important to use the minimum required number of entry, circulating and exit lanes, subject to capacity considerations. For example, according to United Kingdom roundabout crash models, for a 10,000 entering Average Daily Traffic (ADT), flaring the entry width from one to two lanes is likely to increase injury crashes by 25 percent (8).

The number of vehicular and pedestrian conflicts points in both conventional intersections and roundabouts increases considerably when they have additional approach lanes. The designer is encouraged to graphically determine conflicts for a particular location, as this information can raise awareness of design issues and may be useful in public presentations.

The types of conflicts present in multilane roundabouts that do not exist in singlelane roundabouts occur when drivers use the incorrect lane or make an improper turn. These types of conflicts are depicted in Exhibit 5-3 and Exhibit 5-4, respectively. While these types of conflicts can also be present in other intersection forms,they can be prevalent with drivers who are unfamiliar with roundabout operation. The conflicts depicted in Exhibit 5-4, in particular, can be created by not providing a proper design geometry that allows vehicles to travel side-by-side throughout the entire roundabout (see Chapter 6). Crashes resulting from both types of conflicts can also be reduced through proper driver education.

As with single-lane roundabouts, the most severe vehicular crossing conflicts are eliminated and replaced by less severe merging conflicts. The additional conflicts unique to multilane roundabouts are generally low-speed sideswipe conflicts that typically have low severity. Therefore, although the number of conflict points increases at multilane roundabouts when compared to a single lane roundabouts, the overall severity of conflicts is generally less than alternative intersection control.