Run-Off-Road CollisionsExecutive SummaryIntroductionThe American Association of State Highway and Transportation Officials's (AASHTO's) Strategic Highway Safety Plan identified 22 goals to pursue in order to significantly reduce highway crash fatalities. One of the plan's hallmarks is to comprehensively approach safety problems. The range of strategies available in the guides will ultimately cover various aspects of the road user, the highway, the vehicle, the environment, and the management system. The guides strongly encourage the user to develop a program to tackle a particular emphasis area from each perspective in a coordinated manner. To facilitate this, the electronic form of the material uses hypertext links to enable seamless integration of various approaches to a given problem. As more guides are developed for other emphasis areas, the extent and usefulness of this integration will become ever more apparent. AASHTO's overall goal is to move away from independent activities of engineers, law enforcement, educators, judges, and other highway safety specialists and to move toward coordinated efforts. The implementation process outlined in the series of guides promotes forming working groups and alliances that represent all of the elements of the safety system. In this formation, highway safety specialists can draw upon their combined expertise to reach the bottom-line goal of targeted reduction of crashes and fatalities associated with a particular emphasis area. Goal 15 in the Strategic Highway Safety Plan is Keeping Vehicles on the Roadway, and Goal 16 is Minimizing the Consequences of Leaving the Road. Subsequently, three emphasis areas evolved from these two goals:
The common solution to these emphasis areas is to keep the vehicle in the proper lane. While this solution will not eliminate crashes with other vehicles, pedestrians, bicyclists, and trains that may be in the path of the vehicle, it will eliminate many fatalities caused when a vehicle strays from the lane onto the roadside or into oncoming traffic. This section deals with ROR crashes. ROR crashes involve vehicles that leave the travel lane and encroach onto the shoulder and beyond and hit one or more of any number of natural or artificial objects, such as bridge walls, poles, embankments, guardrails, parked vehicles, and trees. (Because trees are the most abundant objects along the road, they are treated as a separate emphasis area.) ROR crashes usually involve only a single vehicle, although an ROR vehicle hitting a parked vehicle could be considered a multivehicle crash. An ROR crash, which typically consists of a vehicle encroaching onto the right shoulder and roadside, can also occur on the median side where the highway is separated or on the opposite side when the vehicle crosses the opposing lanes of a nondivided highway. Reducing the likelihood that a vehicle will leave the roadway through roadway design (e.g., flattening curves or installing shoulder rumble strips) prevents deaths and injuries resulting from ROR crashes. When an errant vehicle does encroach on the roadside, fatalities and injuries can be reduced if an agency either can minimize the likelihood of the vehicle crashing into an object (e.g., through object removal) or overturning (e.g., sideslope flattening) or can reduce the severity of the crash (e.g., by installing breakaway devices). Objectives of the Emphasis AreaTo reduce the number of ROR fatality crashes, the objectives should be to
Explanation of ObjectivesThe ideal objective of good design is to keep the vehicle in the travel lane. For vehicles that do cross the outside edge of pavement, a related objective is to enable the driver to safely recover on the shoulder before encountering the roadside. Motorists do not purposely move onto the shoulder unless they need to pull over to slow or stop their vehicle. However, errant vehicles will cross over onto the shoulder, with many proceeding onto the roadside, resulting in an ROR crash. The reasons for such errant events are varied and include avoiding a vehicle, object, or animal in the travel lane; inattentive driving due to distraction, fatigue, sleep, or drugs; the effects of weather on pavement conditions; and traveling too fast through a curve or down a grade. There are also a number of roadway design factors that can increase the probability that a driver error will become an ROR crash (e.g., travel lanes that are too narrow, substandard curves, and unforgiving shoulders and roadsides). Strategies can be applied to deal with the ROR crashes caused by these factors. If the motorist travels onto the roadside, the probability of a crash occurring depends upon the roadside features, such as the presence and location of fixed objects, shoulder edge dropoff, sideslopes, ditches, and trees. If the roadside is fairly flat without objects and the soil can support the vehicle tires, then the probability of a serious crash is minimal (indeed, in many cases the motorist may fully recover and no ROR crash is reported). Conversely, where the roadside is populated with a continuous line of different types of objects and features, the sideslope is too steep for the vehicle to recover or if the soil produces "vehicle tripping," then the probability of a serious crash is high. Therefore, there are strategies directed at reducing the number and density of possibly hazardous roadside features that would contribute to the likelihood of an ROR crash given a roadside encroachment. The final objective, reducing the severity of the crash, can be met by changes in the design of the roadside features (e.g., making roadside hardware more forgiving or modifying sideslopes to prevent rollovers) and by changes in the vehicle (e.g., better restraint systems or improved side protection) or by increased occupant use of available restraints. While increased use of restraints would probably provide the greatest benefit, the emphasis in this discussion is on roadway-related improvements. Exhibit I-1 lists objectives and related strategies for reducing the consequences of ROR crashes. Details of these strategies are covered in the following narrative. It should be noted that this is not a comprehensive list of all possible strategies to reduce ROR crashes. For example, roadway design or rehabilitation strategies such as building wide lanes or adding lane width on entire systems or subsystems or using positive guidance principals in new roadway design can clearly affect ROR crashes. However, these strategies are most likely employed in the design phase for new facilities or rehabilitation of long sections of roadways and are often high-cost improvements. AASHTO chose to concentrate efforts in this guide on lower-cost strategies that can be implemented quickly; these strategies can also be applied to "spots" on the roadway (e.g., lane widening on hazardous curves). With few exceptions, it is these lower-cost, quickly implementable strategies that are covered below. EXHIBIT I-1
* An explanation of (E) and (P) appears on page V-3. Target of the ObjectivesThe first objective addresses ways to communicate with the driver. However, there are other strategies for fulfilling this objective that target highway design features that could contribute to a crash (e.g., shoulder drop-offs and pavements with low skid resistance). The second objective employs strategies that focus on the highway, with more concentration devoted to nonfreeway facilities, especially to higher-speed rural roads. Higher-design facilities such as freeways have fairly wide shoulders and more forgiving, wider clear zones. Features within the clear zone are shielded from traffic by barriers and crash attenuation devices. On the other hand, there is an extensive system of mostly two-lane rural high-speed roadways that do not have these features. Crash data analyses show that this rural two-lane system is particularly vulnerable to ROR crashes and should be targeted for appropriate measures. Some of the same strategies appropriate for these two-lane, rural, high-speed roads can also be implemented on suburban and urban streets and on freeways. Vehicle design, restraint features and usage, and design of roadside features and roadside geometry are all valid targets for the third objective, reducing the severity of ROR crashes. Finally, another approach to comprehensively address ROR safety problems is to replace the independent activities of engineers, law enforcement personnel, educators, judges, and other highway safety specialists with cooperative efforts, an approach reiterated in this guide. |