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4. SAFETY ATLs are primarily implemented as an operational treatment, but ATLs certainly have safety implications. ATLs may be expected to have a fewer number of some types of crashes when compared to a conventional intersection handling the same volume, because ATLs allow for smoother, less congested operations. On the other hand, ATLs may also cause an increase in other types of crashes due to the added merging area. This chapter explores these safety trade- offs based on empirical crash data and simulation-based safety models. Various design elements were observed in an analysis of crash data from 16 sites across four U.S. states over a period of 9 years. These 16 sites were also included in the data used to develop the operational models in Chapter 3 . Crash reports from within the ATL and its tapers were collected from the responsible agencies in each of the four states. Rear-end and sideswipe crashes were the crash types thought to be most closely related to ATL operation. Overall, the average reported frequency of rear-end and sideswipe crashes was 4.5 crashes per year per site. This is a relatively low frequency when compared to intersections generally identified as potentially hazardous in safety studies. This relatively low frequency indicates that these sites were probably not unsafe as designed. This research also employed the FHWA Surrogate Safety Assessment Model (SSAM) (12), which can be used in conjunction with microsimulation programs like VISSIM (13) to record simulated traffic conflicts. SSAM has the potential to allow practitioners to quantitatively examine the safety consequences of an alternative like an ATL, even if no crash prediction model is available. Although the analysis objective of this research was to correlate SSAM conflicts with the crash data taken from the 16 study sites, the crash sample size was ultimately too low to draw any significant conclusions. However, the trends in the SSAM conflict output allowed the researchers to identify several design elements that may affect an ATL's safety. Appendix A contains guidance on how analysts could use SSAM to help examine the safety of an ATL in the future. SAFETY PRINCIPLES ATLs add lane-changing activity to the through-movement lanes at a signalized intersection. This activity may lead to an increase in sideswipe crashes, especially near the downstream merge. At the same time, an increased through-movement capacity may prevent some rear-end crashes on the approach by decreasing congestion. In particular, the following ATL elements are critical to its safe operation: Downstream length. A sufficient downstream ATL length and taper is needed to allow for safe merging operation into the adjacent CTL traffic stream by providing drivers with enough distance to accelerate and find acceptable gaps in the CTL traffic. Access control. Driveways along an ATL create potential hazards for drivers who are preoccupied with merging into the adjacent CTL traffic Page 29