The latter shows the number and type of conflicts that occur at intersections with three and four legs, respectively.The number of potential conflicts for all users increases substantially at intersections with more than four legs.Providing exclusive left-turn bays that can accommodate left-turn movements can improve operations and safety while providing flexibility to accommodate varying traffic patterns.Positive offset left-turn lanes can improve sight distance for left-turning movements but may prohibit U-turns if insufficient width is available.Figure 15 shows how a skewed intersection approach can increase the distance to clear the intersection for pedestrians and vehicles. Lane arrangements, location of channelization islands, and medians should be established to facilitate pedestrian access and the placement of signs, signals, and markings.Consideration of these “downstream” issues as part of design can optimize the operation of an intersection.Geometric design has a profound influence on roadway safety; it shapes road user expectations and defines how to proceed through an intersection where many conflicts exist.
This chapter addresses the following topics: a primary goal of intersection design is to limit or reduce the severity of potential road user conflicts. The approach alignment to an intersection as well as the intersection itself should present the roadway user with a clear definition of the proper vehicle path.
On low-speed roadways or in areas with pedestrians, promotion of lower turning speeds is appropriate. The use of exclusive turn lanes, channelized right turns, and raised medians as part of an access control strategy are all effective ways to separate vehicle conflicts.
This can be accomplished with smaller turning radii, narrower lanes, and/or channelization features. Figure 13 illustrates how the addition of a left-turn lane can reduce conflicts with through vehicles traveling in the same direction. Facilitate the movement of high-priority traffic flows.
On high-speed roadways with no pedestrians, it may be desirable to promote higher speeds for turning vehicles to remove turning vehicles from the through traffic stream as quickly and safely as possible.
This can be accomplished with longer, smooth tapers and larger curb radii. Separation of conflict points can ease the driving task while improving both the capacity and safety at an intersection.