Identification of Vehicular Impact Conditions Associated with Serious Ran-off-Road Crashes (2010)

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11.1 Background Single-vehicle, ran-off-road crashes are a major cause of serious injuries and fatalities along our nation’s highways. Approximately 12,000 motorists lose their lives each year as a result of these crashes. Most of the efforts to reduce this car- nage have been focused on designing more forgiving roadsides by removing or relocating hazards and designing better safety features to mitigate the severity of those hazards that cannot be removed or relocated. The fact that the total number of single- vehicle, ran-off-road crashes has remained relatively stable and even declined in recent years while the number of vehicle miles traveled has increased steadily indicates that these efforts have been successful. The safety performance of roadside features is evaluated primarily through full-scale crash testing. The purpose of this testing is to observe and evaluate the performance of safety features under impact conditions that are either similar or more severe than those associated with real-world crashes resulting in serious injuries and fatalities. Important crash test parameters, such as impact speed and angle, point of impact, and vehicle orientation have been selected based on find- ings from limited studies of ran-off-road accidents (1, 2, 3). Although full-scale crash test data provides a small window into the nature of ran-off-road crashes, it does not provide sufficient data to identify the impact conditions associated with serious injury and fatal crashes. The research program described herein is undertaken primarily to identify appro- priate impact conditions for use in full-scale crash testing guidelines. However, knowledge of the characteristics of ran-off-road crashes has many more applications than just selecting impact conditions for full-scale crash testing guidelines. Many of the decisions related to design guidelines and policies could bene- fit significantly by better information on the impact conditions of ran-off-road crashes. For example, while the concept of multiple performance levels is embraced by the roadside safety community, highway designers are having difficulty determin- ing when and where to use various roadside safety devices. The multiple-performance-level concept involves selecting a road- side safety feature to match the range of expected impact con- ditions in the area where it is to be installed. Under this design philosophy, roadside safety features are developed to meet one of several different performance levels or impact capacities. Lower capacity—and presumably less costly—safety devices are installed at sites where the risks of high-energy impacts are lower. Although the multiple-performance-level concept has been largely embraced by the roadside safety community, a significant amount of uncertainty remains regarding how per- formance levels should be defined and where the various performance-level designs should be installed. Detailed data on ran-off-road crashes could provide a sound basis for deter- mining appropriate performance levels for different classes of highway included in the study. Safety performance evaluation criteria, such as occupant impact velocity (OIV) and ridedown acceleration (RA), are used as surrogate measures of the risk of injury for vehicle occupants during full-scale crash tests. OIV is a theoretical estimate of the speed at which the head of an unbelted occu- pant would strike the dash board. RA is calculated as the max- imum 10 ms average vehicle acceleration measured after occupant impact occurs. These measures are intended as indi- cators of the risk that an occupant will be seriously injured during an impact with a roadside safety device. Unfortunately, these measures of occupant risk have never been successfully linked to actual injuries. The difficulty associated with estab- lishing this link is the lack of available data where both the actual injuries and occupant risk measures can be determined. Detailed accident investigations that provide calculations of the occupant risk parameters and include crash injury infor- mation should provide the basis for determining the merits of the current safety performance evaluation procedures. Another measure of occupant risk includes occupant com- partment deformation and intrusion. NCHRP Report 350 (4) C H A P T E R 1 Introduction

2requires that “Deformations of, or intrusions into, the occu- pant compartment that could cause serious injuries should not be permitted.” This requirement is relatively subjective and has been interpreted differently by the various crash test- ing agencies. The requirements are quantified under the Man- ual on Assessment of Safety Hardware (MASH; 5) based on limited National Automotive Sampling System (NASS) data and engineering judgment. Nevertheless, a database with detailed information on ran-off-road crashes would provide the needed data to develop a link between the location and magnitude of vehicle intrusion and the severity of occupant injury. Any such link would provide an objective basis for establishing limits on occupant compartment deformation and intrusion. Vehicle stability is also used as a measure of occupant risk. Although crash data clearly shows that the risk of injury increases when a vehicle rolls over, some engineers believe that the risk of injury for occupants of vehicles that only roll 90 degrees is relatively low. Unfortunately, no data are avail- able that can be used to explore this possibility. If data on suf- ficiently large numbers of ran-off-road crashes are collected, it may be possible to test this hypothesis. Guidelines on the selection and placement of roadside safety features can also benefit from a detailed crash study such as the one described herein. Most current guidelines are based on benefit/cost analysis techniques and rely heavily on crash sever- ity estimates. These crash severity estimates are based on both the estimated impact conditions, including speed, angle, and vehicle orientation at impact, as well as the severity resulting from any given impact condition. Data collected in this study would be extremely valuable if collected in a sufficiently repre- sentative manner to allow an estimate of impact conditions associated with all ran-off-road crashes. Furthermore, if data are collected in a representative manner, detailed crash recon- structions could also provide a wealth of crash severity data with which to validate procedures for relating impact condi- tions to occupant risk. Placement guidelines provide procedures for selecting and designing safety features to accommodate the characteristics of specific sites. For example, guardrail installation guidelines rec- ommend procedures for calculating length-of-need and flare configurations based on the characteristics of the specific site where the barrier is to be located. Many facets of safety hard- ware installation guidelines are based on the expected vehicle trajectories and impact conditions at the given site. For exam- ple, procedures for selecting guardrail runout lengths included in the Roadside Design Guide (RDG; 6) are based on vehicle trajectories measured in a study of encroachments into the medians of divided highways during the 1960s (7). Vehicle tra- jectory data collected in the current study should provide a sig- nificant source of additional data regarding such information as the trajectories and the distances vehicles travel along the roadside during a crash. Guardrail placement guidelines also make recommendations regarding maximum flare rates. Increasing the flare rate raises the vehicle impact angles and thereby increases crash severity. Detailed crash data, coupled with injury severity information, should shed some light on this relationship and thereby provide a better foundation for making recommendations on maximum flare rate. Finally, guidelines on grading requirements are provided for guardrail terminals and crash cushions, including limits on slopes in front of and behind these systems. These guidelines are based mostly on data from limited full-scale crash tests without information from real-world crashes. Also, the RDG provides guidelines as to roadside slopes that merit guardrail protection. Again, these guidelines are based on limited testing and simulation. Detailed data on roadside topography for ran- off-road crashes would provide additional insight into the cur- rently accepted guidelines. 1.2 Objective The specific objectives for this study included the following: 1. Identify the vehicle types, impact conditions, and site char- acteristics associated with serious injury and fatal crashes involving roadside features and safety devices; 2. Create a robust relational database for future research; and 3. Develop an implementation plan for a long-term data col- lection effort. The first objective pertains to the collection of detailed infor- mation on serious injury and fatal crashes involving roadside features and safety devices. The data were then analyzed to identify the vehicle types, impact conditions, and site charac- teristics associated with these crashes. The second objective was to create a relational database suit- able for future research. The database consists of crash data from prior and current studies that have in-depth crash data and will include future data collection efforts as well. The third objective was to develop an implementation plan for a long-term data collection effort on detailed data for ran- off-road crashes. As discussed previously, there are many addi- tional applications for such detailed crash data beyond the current study, from performance evaluation of selected road- side safety features and devices to the formulation of policies regarding roadside safety. Thus, a long-term continuing effort to collect detailed data on ran-off-road crashes would be highly desirable. 1.3 Scope The scope of work for this study was specifically formu- lated to address the three objectives and consisted of the fol- lowing major tasks:

31. Identify the data needs for addressing the specific objectives of this study. A literature review was conducted on previous studies involving in-depth crash data collection, impact conditions of ran-off-road crashes, data needs for study of ran-off-road crashes, and reconstruction of ran-off-road crashes. 2. Develop a work plan to collect the needed data. Various data collection alternatives were evaluated and a retrospec- tive supplemental data collection approach was selected for use with the current study. An appropriate data collection protocol was developed, including the sampling plan, data collection forms and field procedures, as well as manual review and reconstruction procedures. 3. Conduct a retrospective supplemental data collection effort of approximately 400 crashes selected from the 2000 and 2001 NASS Crashworthiness Data System (CDS) data. Supplemental field data were collected to gather additional information about the crash sites and roadside features. In addition, these crashes were reconstructed to estimate the impact conditions, including speed, angle, and vehi- cle orientation. 4. Develop a relational database suitable for future research. The database was first developed with data from the cur- rent study. Similar data from previous studies, including NCHRP Project 17-11 and the Federal Highway Admin- istration (FHWA) Rollover Study, were then manually reviewed and reconstructed prior to incorporation into the database. 5. Analyze the database to address the specific objectives of this study, including identification of the vehicle types, impact conditions, and site characteristics associated with serious and fatal crashes. 6. Develop a proposed implementation plan for a long-term data collection effort. The implementation plan outlined a long-term effort to continue collecting detailed data on rep- resentative ran-off-road crashes and the flexibility to con- duct special studies on specific roadside safety features and devices. Data collection protocols for the continuous data collection and a selected special study were developed. Also, a pilot program was conducted to demonstrate the feasibil- ity of the long-term data collection effort and to iron out the details and identify any potential problems. 1.4 Report Organization This report summarizes the results of the work conducted under the study. Chapter 2 presents a summary of the literature review and other ongoing and future research and data collec- tion efforts. Chapter 3 outlines the study approach, including data collection alternatives, data collection plan, and develop- ment of the database. Results of the analyses are presented in Chapter 4. The proposed plan for a long-term data collection effort is outlined in Chapter 5. Finally, a summary of the study findings and conclusions are presented in Chapter 6. Some of the details too voluminous for the main body of the report are included as appendices. Appendix A presents the critical review of individual references. Appendix B summa- rizes the results of the analysis of the 1997-2001 NASS CDS data, including the list of 2000 and 2001 cases to be sampled for supplemental field data collection. Appendix C outlines the protocol for the supplemental field data collection and manual review used for the current study. The details of database ele- ments are shown in Appendix D. Additional tables, plots, and analysis results too voluminous for the main report are shown in Appendix E. Finally, the field data collection forms and the corresponding coding instructions and field procedures for the proposed long-term data collection effort are presented in Appendix F. These appendices are available from the NCHRP Report 665 blurb page on the TRB website (www.trb.org) by searching for “NCHRP Report 665”.