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7 advances in electronics, many more data elements could Some earlier work relied on reconstruction of impact speed be incorporated into the EDRs and the recording period and angle by the investigating officers, such as the studies by could increase significantly. Garrett and Tharp (8), Perchonok et al. (2), and Lampela and In addition to the interest of NHTSA, the highway roadside Yang (1). As discussed previously, the use of enhanced police- safety community has also shown great interest in the EDR reported level crash data to estimate impact conditions is lim- data. A study, NCHRP Project 17-24, "Use of Event Data ited by a number of factors, such as expertise and experience of Recorder (EDR) Technology for Roadside Crash Data Analy- the investigating police officers, availability of time for the offi- sis," was conducted to review and recommend a minimum cers, and lack of officers' knowledge on the impact perfor- set of EDR data elements for roadside safety analysis as well mance of roadside safety features. Thus, while the results from as procedures to retrieve, store, and use the data (18). these studies provide some insights into impact conditions, their accuracy is somewhat questionable. While the EDR technology is very exciting and promising, Under the pole and narrow bridge studies (9, 10), impact there is still much development to be done and impediments conditions were estimated from in-depth investigations and to overcome before it can reach its potential, including: presented in the reports. Mak et al. took the data from these studies and developed statistical models for the distributions Engineering issues. There are no current standards govern- of impact speeds and angles (3). After screening, a total of ing the design and use of EDRs, such as data elements to be 596 cases were available for analysis. The authors found that included, data format, data retrieval, etc. Such standards are the gamma function provides the best fit for univariate impact needed if data are to be collected on a large scale. Also, cur- speed and impact angle distributions. Statistical models for rent EDR data elements are, as expected, focused on vehicle impact speed and angle distributions were then developed parameters with no specific consideration for information using the gamma function for the following five functional pertaining to ran-off-road crashes. classes: Institutional barriers. EDR data are intended for the data needs of vehicle manufacturers, which may be reluctant to Freeway share their proprietary designs for competitive and legal con- Urban arterial siderations. Inputs from governmental agencies and research Urban collector/local road institutions are needed in the early planning and design Rural arterial stages if the EDR data are to be expanded into the roadside Rural collector/local road safety area. Legal consideration. There are still questions pertaining to For some roadside features, such as longitudinal barriers, ownership of the EDR data, privacy issues, use of EDR data impact conditions are defined by both impact speed and angle. in tort claims, etc. Until such concerns are addressed and However, there is no known means of mathematically express- resolved, large-scale collection of EDR data appears unlikely. ing a joint gamma distribution. The authors tested various known joint (bivariate) distributions, but with no success. They then proceeded by assuming that the impact speed and impact 2.2 Impact Conditions of angle are independent of each other and estimated combined Ran-Off-Road Crashes probability distributions for impact speed and angle stratified Despite the large number of studies on ran-off-road crashes, by functional class and based on the gamma distribution. These there are relatively few studies that actually attempted to esti- impact speed and angle distributions were used in some of the mate the impact conditions. The main reason for the lack of cost-effectiveness analysis procedures, including the Texas such effort is that, in order to estimate the impact conditions, Transportation Institute (TTI) ABC model (19). The distribu- an in-depth level of crash investigation is required, including tions were adjusted to reflect the current higher speed limits detailed data on the roadway, vehicle trajectory, object(s) under NCHRP Project 22-14 (20). The revised impact condi- struck and damage sustained, vehicle and damage measure- tion distributions were used with the Roadside Safety Analysis ments, and driver and occupant injury levels. The costs associ- Program (RSAP) (21). ated with in-depth crash investigation are, as may be expected, Other sources of impact conditions include data from on- very high and there have only been a few studies that incorpo- going NCHRP Project 17-11 and the FHWA Rollover Study rated such in-depth crash data. Another limitation is that some (16, 17). A total of 559 NASS CDS cases from 1997 through of the studies, such as the LBSS data, were not based on a rep- 1999 were selected under these two studies. Supplemental field resentative sample and the resulting distributions of impact data were collected on these cases, which were then recon- conditions could be biased, probably toward the more severe structed to estimate the impact conditions. The impact speed crashes. and angle distributions developed under these two studies