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8 were significantly different from previous findings. However, Compile descriptive statistics on vehicle trajectory and it was later found that the scales on some of the diagrams used impact conditions for the impact angle reconstructions might be distorted. In Develop mathematical models for the distributions of order to fit the scale diagrams onto a web page, the longitudi- impact speeds and angles nal and lateral scales were compressed differently, thus lead- ing to incorrect impact angle estimates. Plans are underway to These proposed studies and data collection plans are over reconstruct these cases again to correct the errors and reana- 10 years old, but they still are applicable today and of great lyze the revised data. interest to the current study. It should be mentioned that in order to properly establish Miaou proposed a method to estimate vehicle roadside the distribution of impact conditions, the data source needs to encroachment rates using accident-based models (22). Miaou be either the population (i.e., all ran-off-road crashes) or a rep- concluded that the proposed method could be a viable resentative sample. Some databases, such as the LBSS, are sam- approach to estimating roadside encroachment rates without pled on the basis of a comparative analysis and are not suitable actually collecting the encroachment data in the field, which for determining impact condition distributions. can be expensive and technically difficult. A pilot study was conducted by Daily et al. (23) to examine the feasibility of this approach. Data were collected on 56 km (35 mi) of tangent sec- 2.3 Data Needs for Study tions of rural two-lane highways in Idaho, including detailed of Ran-off-Road Crashes roadside, crash, and traffic data. Encroachment rates were esti- There have been a number of studies that looked into the mated from the collected crash data and found to be in the data needs for studying ran-off-road crashes. A study by Mak same order of magnitude as previous research. It was con- and Sicking identified issues and gaps in the state of the knowl- cluded that this approach is feasible, although it is limited by edge needed to improve the cost-effectiveness analysis proce- the current state of knowledge with respect to data on the tra- dure and to develop data collection plans for those issues and jectories of vehicles involved in ran-off-road, fixed-object acci- gaps that could be addressed with crash data. The research pro- dents. An experimental plan for future research that would posed five studies and developed data collection plans for those produce improved estimates of encroachment rates was devel- studies: oped, but not recommended for immediate implementation. While this study has no direct bearing on the current study, Validation of encroachment frequency/rate it could be of interest in future data collection efforts. Data on Determination of encroachment frequency/rate encroachment rates are over 25 years old and may be outdated Effect of roadside conditions on impact probability and in light of the significantly changed conditions in the interven- severity ing years, including improvements made to the safety design of Distributions of impact conditions highways (e.g., clear zone concept and improved barriers and Relationships of impact conditions, performance limits, terminals) and vehicles (e.g., front and side airbags, antilock and injury probability and severity brakes, and crush management), and other safety counter- measures (e.g., mandatory seatbelt law, tightened blood- These study plans were reviewed by a panel of experts and alcohol-content law). If a major data collection effort is to be their comments taken into consideration. The recommended implemented in the future, encroachment data may be one of study on the distributions of impact conditions focuses on the objectives. impact speed, angle, and vehicle orientation in addition to A list of suggested data elements for use with the current vehicle size, weight, and the nature of the roadside object/ NASS CDS program was proposed by Eskandarian et al. in feature. The plan for this study includes the following tasks: a study to assess the compatibility between vehicle design characteristics and roadside safety hardware (24). These data Select sample roadway segments for each of the six highway elements pertain to the design characteristics, pre-impact con- types ditions, and impact conditions of struck features and assess- Set up data collection protocol, including sampling plan, ment of impact performance of features. While the suggested accident notification scheme, data collection forms, etc., data needs pertain mostly to the issue of compatibility between and familiarize and train investigators with the protocol vehicle design and roadside safety features, the information through a small pilot study would be helpful to establishing the data needs for the data Investigate in-depth a representative sample of single- collection effort under the current study. vehicle, ran-off-road accidents on these selected roadway Under the recently completed NCHRP Project 17-24 on the segments potential use of EDR data for roadside safety evaluation, the Reconstruct the sampled accidents to determine impact authors examined the data needs for roadside safety analysis conditions and assessed whether the data needs can be satisfied with EDR