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114 Chapter 4 â Understanding Results 4.1 Performance Measures Transportation projects are generally assessed with a variety of quantitative and qualitative performance measures. Safety performance is one important performance measure, and the results of NCHRP Project 17-89 enable the safety performance of existing and planned PTSU facilities to be assessed. Other key performance measures on typical PTSU projects include operational performance, cost, right-of-way needs and environmental impacts, and the timeframe to implement improvements. PTSU facilities may be evaluated as alternatives to a no-build scenario, a conventional widening (i.e., the addition of general purpose lanes), or both. The PTSU Informational Guide In Part I of NCHRP Web- Only Document 309, Volume 1, provides additional discussion of performance measures. 4.2 Crash Frequency and Severity The CPMs in Chapter 2 of this document estimate the annual frequency of FI crashes and the annual frequency of PDO crashes for freeway sites. They may be summed together to compute total crash frequency. Crash frequency predicted by the CMPs represents the expected long-term annual average crash frequency of a site. The observed crash frequencies at a site naturally fluctuate from year to year. A major finding of this project is that PTSU presence is associated with higher FI crash frequency and higher PDO crash frequency. Table 1 of the PTSU Informational Guide provides specific FI AF values for common PTSU design and operation scenarios, and a similar table could be constructed for PDO AF values. Although the table indicates that PTSU presence generally is associated with higher crash frequency, the magnitude of the crash frequency increase is smaller when turnouts are present, the width of the portion of the shoulder used for travel is 12 feet instead of 11 feet, and the proportion of time that PTSU operates in a typical day is low. Severity distribution functions (SDFs) in Chapter 2 of this document provide the proportion of K (fatal), A (incapacitating injury), B (non-incapacitating injury), and C (possible injury) severities within the frequency of FI crashes predicted by the CPM. A major finding of this project is that an increase in the proportion of time PTSU operates is associated with a decrease of the proportion of FI crashes that are K or A severity levels. As a result, Table 2 and Table 3 of the PTSU Informational Guide present severity distributions for common PTSU scenarios and the associated changes in crash frequency, severity, and cost if PTSU were implemented relative to a âno PTSUâ condition. In many scenarios, the crash cost decreases despite the FI crash frequency increase due to the reduction in crashes with K and A severity levels. If an existing freeway is being analyzed and historical crash data are available, the empirical Bayes (EB) Method should be used to improve the reliability of the estimated average crash frequency. The EB Method combines model predictions and site-specific crash data in proportion to the level of uncertainty that can be attached to each value. Appendix B of the HSM Supplement described the EB method, and Chapter 2 of this document includes overdispersion parameters for each CPM. These parameters are necessary to apply the EB Method. The EB Method is applicable to a proposed PTSU project on an existing freeway when the number of through lanes on the freeway will not change but the shoulder will be opened to traffic for some period of the day. As discussed in Chapter 1 of this document, use of locally developed calibration factors is necessary to obtain reliable crash frequency values. If a locally developed calibration factor is not available, computations of AFs for different scenarios (with and without PTSU, with or without rumble strips, etc.) can provide an understanding of the relative percent differences in crash frequency of difference scenarios. Table 1 of the PTSU Informational Guide, for example, shows the computed values of the PTSU Operation AF for FI crashes, and the values in this table indicate changes in crash frequency as
115 variables used in the computation of the AF change. A reliable estimate of the crash frequency itself cannot be determined though this approach. 4.3 Comparison to Current Highway Safety Manual As discussed in Chapter 1, Chapter 18 of the HSM Supplement (1) contains CPMs for freeways, and NCHRP Project 17-89 produced a second set of freeway CPMs. Many freeways without PTSU could be analyzed with either model. However, the CPMs were developed using data from different years at different sites in different states. As a result, if a site is analyzed with both CPMs, the estimated average crash frequency is unlikely to be the comparable unless both CPMs are calibrated using data for the same jurisdiction. The use of similar AFs in the HSM and NCHRP Project 17-89 CMPs results in similar effects associated with changes in variable values (i.e., changes in lane width, rumble strip presence, etc.), and the trends of the safety performance functions in the two sets of CMPs are similar as well. The NCHRP Project 17-89 Final Report (2) provides additional information on these points. If both models are locally calibrated, it will increase the comparability of the results from the two sets of CPMs. If local calibration factors for both sets of CPMs are available, analysts should select the set of CPMs that are most appropriate to use for a given project. Guidance on CPM selection is provided in Chapter 1. 4.4 References 1. American Association of State Highway and Transportation Officials (AASHTO). 2014. Highway Safety Manual Supplement. Washington D.C. 2. Jenior, P., J. Bonneson, L. Zhao, W. Kittelson, E. Donnell, and V. Gayah. 2021. NCHRP Web-Only Document 309: Safety Performance of Part-Time Shoulder Use on Freeways, Volume 2: Conduct of Research Report, Transportation Research Board, Washington, D.C.
