Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
20 trajectories in earlier versions of RSAP were only linear, RSAPv3 models each trajectory as a series of line segments so trajectories can be straight, curved or compound curved. Since the trajectories come from reconstructed crashes, the trajectory information also includes the velocity at each trajectory point so that driver input like braking is inherently included. MODELING CRASH SEVERITY Earlier versions of RSAP as well as the programs ROADSIDE and BCAP used a severity index approach to estimating crash severity. This approach was largely based on the subjective assessment of crash severity and was found by Sicking to result in an over-estimate of crash severity.[Sicking09] Sicking and Mak, in the previous RSAP Engineerâs Manual, noted that it would be preferable to based crash severity estimates on real-world data but, unfortunately, they were not able to accomplish this during their original project.[Mak03] RSAPv3 uses a new method for estimating crash severity that is based directly on observed police reported crash data and, where available, maintenance records. The Equivalent Fatal Crash Cost Ratio (EFCCR) method uses police reported crash severities for a particular roadside feature to develop the equivalent fatal crash cost. Unreported crashes must be accounted for in this method in order to avoid over estimating the crash costs. Using maintenance data is one way to estimate the unreported crash rate. The EFCCR for each type of roadside hazard is essentially the probability of observing a fatal crash with that hazard. Once the probability is known, the crash cost is found by multiplying the non-dimensional EFCCR ratio by the appropriate fatal crash cost. VALIDATION Two validation cases were developed to validate the software in addition to other testing and documentation purposes. The validation cases are based on typical geometry and traffic data and generated results that were compared to actual historical crash data. The validation exercise is documented in Appendix B: the Engineerâs Manual. This exercise provides an improved understanding of roadside crash data statistics, modeling roadside crash data, and gives confidence that the results RSAPv3 provides mimic actual field observation. The cases examined compared quite well with the real-world data indicating that RSAPv3 is a valid approach at least for the cases considered to date. RESEARCH NEEDS It should be made clear at the outset that, RSAPv3 is currently intended for evaluating limited access highways where additional right-of-way can be acquired or rural two-lane roadways. There is currently not enough data to support the analysis of urban environments where roadsides are typically cluttered with fixed objects with little or no clear zone and frequent intersections. This limitation is, however, not a program limitation but a data limitation since these is simply very little data on the performance and characteristics of urban roadsides. RSAPv3 contains an implicit assumption that intersections are few and far between. This may be a reasonable assumption in designing a multi-lane access-controlled highway or a rural two- lane highway but it is clearly not a reasonable assumption for designing urban streets or suburban collectors where clear zones are limited and intersections are numerous. RSAPv3 is, therefore, better suited to the analysis of some types of roadways than others.
21 The encroachment frequency data re-analysis for RSAPv3 was successful and is completely described in Appendix B. The data itself, however, is limited: ⢠The Cooper data is bounded by an upper ADT value of 30,000 vpd. Additional encroachment frequency data should be collected to better understand encroachments at ADT values greater than 30,000 vpd. ⢠The Cooper data primarily represents passenger vehicle encroachments. It is not known at this time if trucks or other types of vehicles have the same encroachment characteristics as passenger vehicles. ⢠The vertical grade and horizontal curve adjustments currently implemented in RSAPv3 are based on an old study with limited data points. Unfortunately it is the only known study which differentiated between direction of travel and vertical and horizontal alignment, a fundamental variable for adjusting encroachment frequency. Additional research should be done to verify the directional influences of these adjustments on encroachment frequency. ⢠The re-analysis of the Copper data illustrated the importance of access density on the frequency of encroachments. This is an area that should also be examined more closely in the future. ⢠RSAPv3 includes a new multi-lane adjustment but the study used to develop the adjustment factor was relatively limited is size and scope and a larger data set would be useful in assessing the importance of the number of lanes on encroachments. Although RSAPv3 is a significant technical improvement over its predecessors, it still has drawbacks and limitations, some of which are the result of lack of data or lack of resources to obtain the necessary data. For example, RSAPv3 has incorporated the 800 or so field reconstructed vehicle trajectories from the NCHRP Project 17-22 into the crash module. These trajectories are used to determine the probable path a vehicle may take when it encroaches onto the roadside. All of these trajectories are for passenger vehicles, therefore, the analysis procedures assume that motorcycles and heavy vehicles follow the same encroachment trajectories as passenger vehicles. This assumption is likely not the case, however data is not available for motorcycles or heavy vehicles. Furthermore, the NCHRP Project 17-22 study was limited in the number of trajectories collected, the variety of road characteristics (i.e., posted speed limit, horizontal curve, vertical grade, etc.) for which trajectories where collected, and the roadside and median terrains. More trajectory data with more variety of road characteristics are needed to better understand how different vehicles behave under different situations during roadside encroachments. NCHRP Project 17-43 is currently underway and will increase the size of the trajectory database, however heavy vehicle and motorcycle trajectories will not be gathered under this study. The crash severity of modeled roadside crashes is determined in the cost/benefit module of RSAPv3. This research discontinued use of the SI method of determining crash severity which has been previously used in encroachment probability software programs and replaced it with an Equivalent Fatal Crash Cost Ratio (EFCCR). The development and implementation of the EFCCR is discussed in the Engineerâs Manual. Many EFCCRs were developed under this research effort, however, continued development of EFCCRs for the countless roadside hazards is necessary. The development of EFCCRs should be incorporated into ongoing research efforts and planned research efforts. Essentially, every time crash data is reviewed for roadside crashes, the possibility of developing an EFCCR should be considered. The process is straight forward
