Assessment of Auxiliary Through Lanes at Signalized Intersections (2011) / Chapter Skim
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Pages 63-79
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64 CHAPTER 7. SAFETY ANALYSIS AND MODELING The objective of the safety analysis and modeling effort was to determine the safety effects of ATLs, either by examining collision data, or, if possible, by exercising a validated SSAM model.
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65 Table 21. Summary of SSAM-analyzed approaches (*
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66 the study approaches were not built as ATLs during the earliest time period, so this time period was not simulated for all sites. The following assumptions were assumed when preparing each VISSIM model: • When not provided by state DOT's, the researchers estimated turning movement counts using the available AADT and K-factors for each time of day, assuming that turning percentages would remain constant from 2000 to 2008; • AADT data were used to determine traffic growth or decline, assuming a linear rate from 2000 to 2008 for purposes of interpolation; • Signal timing plans were kept in "free" mode (fully actuated with no coordination)
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67 • Right Turns: 0 and 200 vph. The levels of each variable reflected those observed in the field.
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68 Figure 32. Collision data distribution within the ATL.
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69 Figure 33. 2006-2008 collision data trends versus maximum XT This relationship loses most of its strength when ATL utilization is used in place of the maximum X observed.
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70 Figure 35 displays the trend between rear end collisions and average ATL flow observed in the field for each of the 16 sites. This shows that ATL flow is positively correlated with total volume; this trend is intuitive and does not necessarily indicate that well-utilized ATLs are unsafe.
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71 Figure 36. Sideswipe collisions/year versus ATL total length.
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72 Table 22. Summary of collision and SSAM data (*
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73 intersections. Indeed, the last row in the table indicates that only 563 collisions were observed over all 16 approaches in nine years (an average of 4.5 collisions/year at each site)
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74 collisions appears to be a flat line. Note that the low number of observed sideswipe collisions may limit the strength of this comparison.
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75 Figure 39. Aggregated rear-end conflicts versus collisions.
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76 The figures display several linear relationships, the strongest of which is the comparison of sideswipe conflicts versus collisions when aggregated by 3-year period. This relationship was statistically significant at the 95 percent confidence level according to Pearson's Chi-square test (χ2 Figure 41 =3.6, df=2)
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77 Figure 42. Comparison of conflict and collision distribution over relative downstream ATL length.
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78 Figure 43. SSAM rear-end conflict comparison (no right turns)
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79 Figure 45. SSAM rear-end conflict comparison (200 right-turning vph)
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80 downstream length has little effect on safety. Second, shared ATLs (at least those with 200 or more right-turning vehicles per hour)
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