5. Discussion of Intersection Safety Issues

Although the scope of this research study did not include the development of mitigating strategies for specific intersections in Utah, a review of mitigations was considered to be useful to the understanding of general intersection safety needs. Khisty and Lall (2003) stated "the objective of intersection design is to reduce the severity of potential conflicts between vehicles (including pedestrians) while providing maximum convenience and ease of movement to vehicles." The safety of intersection operations, therefore, is a primary objective. The authors suggested that intersection design must consider four features, the first three of which are important to intersection safety:

1. Human factors, including driver behavior and cognition,
2. Traffic considerations, including approach capacities, turning movements and conflicts, and vehicle sizes (as well as pedestrians and bicycles),
3. Physical characteristics, including lane geometry, abutting features, and sight distances, and
4. Economic factors, such as the costs and benefits of energy savings.

There are a number of approaches to minimizing the potential for conflicts among highway users at intersections, including traffic control devices, user information and guidance, geometric and cross-sectional design, land use and access management, circulation planning, illumination, collision avoidance systems, sight distance protection, and enforcement. Traffic control devices include stop signs, signals, yield signs, other regulatory signs, flashers, and roundabouts, along with special provisions for pedestrians and bicycles, such as countdown timers and pushbuttons. A byproduct of traffic control is the need for compliance, which can be enhanced by human or automated enforcement. Yet another byproduct, with traffic signals, is the creation of dilemma zones in which drivers can neither stop nor pass through an intersection before the signal becomes red. These zones can be eliminated through changes in the timing of yellow or all-red intervals, reducing speed limits, and advanced driver warnings (Fricker and Whitford, 2004). Geometric and cross-sectional design considers the physical components of an intersection and its approaches, including lane widths, turning lanes, turning bays, channeling, shoulder widths, medians and median widths, stop lines, and other elements. A roundabout, in fact, "straddles the line" between a traffic control device and a geometric design component. Land use and access management involves the location of abutting properties and fixtures relative to intersection traffic flow and sight lines. Access management also concerns the location of driveways relative to intersections, along with the turning movements allowed. Circulation planning can be used to manage the traffic flows within a district or along a corridor. The strategies might include peak period or permanent one-way street conversions, reversible lanes, and others. Illumination concerns the nighttime lighting of intersections. User information and guidance includes warning, informational, directional, and changeable message signs. While these strategies might be considered to be passive, collision avoidance systems can play an active role in preventing conflicts. The systems include autonomous vehicle, autonomous infrastructure, and combined or cooperative technologies designed to reduce driver error and improve driver performance and prevent collisions. In general terms, a collision avoidance system includes a set of sensors, processors, and warnings that alert drivers and other highway users to impending violations and other dangers. A national Intelligent Vehicle Initiative was established in 2003 to promote the development of in-vehicle technologies (Funderburg, 2004). Regarding roadway strategies, Ferlis (2002) discussed roadside information and roadside-to-vehicle communication devices that might be used in combination with intelligent vehicles. Finally, sight distance protection ensures that sight lines remain clear of obstructions; the fundamentals of sight distance provisions might be addressed as part of the intersection design, and in the planning of abutting land uses and fixtures. If none of the preceding strategies work, then grade separation may be the only solution.

The selection of an intersection safety enhancement has traditionally been based on engineering judgment. Certain strategies might be implemented on a widespread basis in some jurisdictions according to a formula or program. For example, the traffic signal warrants in the Manual on Uniform Traffic Control Devices are used by numerous jurisdictions to indicate the need for a signal. Recent research has produced sets of intersection safety diagnostics (Hauer et al., 2002) that guide the engineer toward best practices. A set of "Hauer" diagnostics exists for each of several collision types, including rear-end incidents, left-turn crashes, right-turn collisions, right-angle crashes, side-swipe collisions, loss-of-control events, pedestrian crashes, and bicycle collisions. The diagnostic approach for an "ailing" intersection involves responding to a series of questions related to a certain collision type. The "diagnosis" is to select one or a set of countermeasures as a treatment. Although the Hauer procedure is comprehensive and structured, engineering judgment still enters into the selection of countermeasures. With ongoing research in intersection safety, it should be possible to ascertain the effectiveness of various countermeasures in reducing the frequency and severity of certain collision types. Figure 5.1 below summarizes the countermeasures that Hauer et al. offer for consideration (plus a few additional ones).

Figure 5.1 Intersection Safety Countermeasures (from Hauer et al., 2002)