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65 CHAPTER 6 Summary of Findings 6.1 Study Approach all individual speed limit classes. Chi-square tests for inde- pendence showed that departure velocity and angle could be Data was collected under three different studies: the FHWA considered independent for all speed limit classes. Further, Rollover Study, NCHRP 17-11, and NCHRP 17-22. Each of combined velocity and angle distributions developed based these studies involved a retrospective data collection and analy- on the assumption of independence were subjected to chi- sis of historical NASS CDS cases. Supplemental site informa- square tests for goodness-of-fit. These tests showed that the tion was collected to identify characteristics of the roadway, differences between predicted and measured distributions of roadside, and objects struck during the crash. This supple- departure velocities and angles were not statistically signifi- mental information was then utilized to reconstruct each crash cant at the p = 0.05 level for any speed limit class. Thus, the in order to determine vehicle departure and impact conditions. models of departure velocity and square root of departure The data was then compiled into a relational database that can angle can be reliably used to develop distributions for a vari- be used to analyze the data. ety of speed limit classes included in the study. Further, the database provides definitive support for reduc- 6.2 Findings ing the length of guardrail used in advance of roadside obstacles. The distributions of longitudinal departure lengths A relational database of ran-off-road crashes has been devel- included in the data set correlated surprisingly well with rec- oped. The database includes detailed characteristics of the vehi- ommended guardrail runout lengths generated from Cooper's cle, trajectory, roadway, roadside, objects struck, and crash encroachment data. The only significant difference between result for 877 crashes. The data are strongly biased toward seri- the longitudinal departure length distributions and the mod- ous crashes with 15% fatal and 72% A+K crashes. The database ified runout length guidelines was associated with the use of can be used for many different purposes, including identifi- a 60 mph design speed for a full access-controlled freeway. In cation of roadway departure and roadside impact conditions, this situation, modified runout length guidelines were found and ran-off-road trajectories. The database can also be used to to be shorter than longitudinal travel distances found in the develop a relationship between impact conditions and crash data set. Therefore, it is recommended that states either use severity for some common obstacles, such as trees and poles. a design speed of 70 mph for all controlled access roadways, Although prior studies showed departure velocity to be or an additional category should be added to the guardrail most closely associated with highway functional class, this runout length table to accommodate 60 mph design speeds roadway classification was not available in the current data- with full access control. base. In the absence of highway functional class, speed limit was found to provide the best discriminator for departure 6.3 Long-Term Data Collection velocity and angle. Departure velocities were found to be accurately modeled with a normal distribution while no sin- A detailed work plan for a long-term data collection system gle common distribution provided a good fit to departure was developed and pilot tested. The plan involves implement- angles for all speed limit classes. However, the gamma distri- ing a continuous sampling subsystem and possibly a special bution was found to fit the square root of the departure angle study subsystem within the NASS CDS. The continuous sam- for all speed limit classes. The dependency between departure pling subsystem would provide a steady stream of new cases angle and velocity was found to be relatively insignificant for that would be very similar to the existing database while a
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66 special study would focus on one particular type of crash such would provide greater information regarding the causation of as W-beam guardrail impacts. injuries and fatalities during crashes involving roadside safety If implemented, the long-term data collection plan could hardware. This information will provide the foundation for provide information that would allow development of the the next generation roadside safety features designed to dra- relationships between impact severity and crash conditions matically reduce injuries and fatalities associated with ran- for a wide variety of roadside features. Further, such a study off-road crashes.