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Relationship between ATL Crashes and Total ATL Length Exhibit 4-7 compares sideswipe crashes (combined over all analysis years in the dataset, unlike the preceding exhibits) at each site to total ATL length (sum of upstream ATL length, intersection width, and downstream ATL length not including tapers). While it may be hypothesized that longer ATLs allow for safer merging, it is also possible that more exposure to merging areas would lead to more frequent sideswipe crashes. Based on the direction of the linear relationship shown in Exhibit 4-7 (again the best-fit line), it appears that the probability of sideswipe crashes increases as the length of the ATL increases. Exhibit 4-7 y= 0.0043x - 0.0532 20062008 Rear-End Sideswipe R2 = 0.3744 Crashes/Year versus ATL Total 14 Length 12 Sideswipe Collisions 10 8 6 4 2 0 0 500 1000 1500 2000 2500 3000 Upstream + Downstream ATL Length (ft) In summary, the analysis of the data from the 16 study sites showed some relationships between rear-end crashes and congestion, between rear-end crashes and flow rates in the ATLs, and between sideswipe crashes and ATL length. However, the relationships are weak and causation is unclear in all cases, so practitioners should not over-interpret the findings. In the future, perhaps calibrated crash prediction models for ATLs will be available to provide firmer guidance to practitioners considering ATLs. SAFETY EVALUATION CONSIDERATIONS The following guidance is recommended for conducting a safety evaluation of an existing ATL: Collect crash data for the ATL approach and remove non-ATL-related crashes. Closely examine rear-end and sideswipe crashes along the approach with the ATL, including the tapers. Collect crash data over as long a time as possible given that important safety-related conditions remained unchanged. Page 35

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Exercise caution in observing when the ATL was constructed and not include data from prior to the ATL opening. Use a method similar to the safety analysis of conventional intersections as described in the HSM (4) to evaluate the crash data. Review the crash data from the 16 sites examined in this chapter to understand how typical ATLs perform. Evaluating the safety implications of ATL proposals or designs is currently difficult given the lack of crash prediction models or CMFs. Until those tools are available, practitioners should be confident that, based on the data presented in this chapter, well-designed ATLs are not likely to cause safety problems. An SSAM analysis could also be used when a practitioner wishes to examine the potential safety effects of building an ATL or altering an ATL's design or operational elements. As Appendix A describes, an SSAM analysis requires a calibrated microsimulation model of the intersection that exhibits an appropriate estimate of the flow in the ATL. Ten or more simulation runs should be used to populate the sample size. During SSAM analysis of the trajectory files, only rear- end and lane-change conflicts should be examined, and a time-to-crash (TTC) threshold of 1.5 seconds is preferred to yield a larger sample size. The practitioner should then look for the relative change in conflict frequency when a design element (e.g., downstream length) or operational element (e. g., XT) is altered to draw conclusions about the safety effects of the ATL design. Page 36