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Passive Taper: The ATL taper upstream of the intersection that allows vehicles to enter the ATL. Active Taper: The ATL taper downstream of the intersection that requires vehicles to merge. XT: The demand-to-capacity ratio for the through movement assuming the ATL is not in place. APPLICATION Similar to CTLs, ATLs are implemented to increase the stop-bar capacity on approaches at signalized intersections that represent a "choke point" along an arterial street. They can be applied in urban, suburban, or rural environments on either the major-street or minor-street approach. ATLs are often applied in lieu of a CTL when: · Construction of a CTL is not feasible; · The capacity added by the ATL adequately accommodates current or projected traffic demand through the intersection bottleneck; and · Sufficient length is available to accommodate upstream storage and downstream merge activity. CONFIGURATION TYPES These guidelines address four types of ATL configurations as shown in Exhibit 2-1: · One CTL with a shared ATL · One CTL, one ATL, and an exclusive right-turn lane · Two CTLs with a shared ATL · Two CTLs, one ATL, and an exclusive right-turn lane Page 8
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Exhibit 2-1 ATL Configuration Types One CTL with Shared ATL One CTL, One ATL, and Exclusive Right-Turn Lane Two CTLs with Shared ATL Two CTLs, One ATL, and Exclusive Right-Turn Lane Each of the configurations shown in Exhibit 2-1 consists of a right-hand lane addition upstream of the intersection and a right-hand merge downstream of the intersection. Results from a web survey conducted as part of this research effort found that 85 percent of ATL applications had both a right-hand lane addition upstream and right-hand merge downstream. The remaining sites included right-turn drop lanes downstream that generally ended at a commercial driveway entrance or left-hand merges. The guidelines presented in this document do not address ATLs with right-turn drop lanes and left-hand merges given the relatively few known applications of that configuration. Page 9