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1. INTRODUCTION An auxiliary through lane (ATL) is a limited length through lane added upstream and downstream of an intersection, as shown in Exhibit 1-1. Exhibit 1-1 Typical Auxiliary Through Lane Configuration ATLs are typically applied as an intermediate-cost treatment to reduce recurring bottlenecks at signalized intersections. They can be applied to either the major-street or minor-street approach. When an ATL is present, through traffic is allowed to disperse across an additional through lane at the signalized intersection, which increases the stop-bar capacity of the approach. This increase capacity reduces delay and queuing for through vehicles. An ATL also reduces the time required for the green light phase to serve the through demand on the approach, meaning the extra time can then be allocated to other movements at the intersection. ATLs are typically applied at locations where additional through capacity is desired but construction of a continuous through lane (CTL) is not feasible. ATLs can also be applied as an interim improvement until a CTL improvement is made or can be justified. In summary, an ATL achieves a portion of the capacity benefits of a CTL for a portion of its cost and right-of-way/environmental impact. Field data and observations were collected at 22 ATL approaches across the United States to analyze ATL performance and the relationship between the traffic operations, safety, and design characteristics of ATLs. Results from this data collection effort indicate that during the peak period an average of 24 percent of through traffic used the ATL on approaches with one continuous through lane. Despite the relatively low utilization, ATLs provide significant benefits in terms of reduced vehicle delay. Analysis of the field data shows that the presence of the ATL reduced delay by an average of 100 seconds per vehicle compared to the case with no ATL in place during the observational periods (assuming all other factors remain equal). This delay reduction is because many of those approaches would have operated in an oversaturated state without the ATL in place. Similar conclusions were reached for ATLs with two CTLs. Thus, while ATLs do not achieve the full operational benefit of a CTL, their operational benefits are nonetheless substantial. ATLs are most effective under congested conditions when the demand-to- capacity ratio for the through movement approaches 1.0 without the ATL in place. If congestion levels are too low there is limited incentive for drivers to use Page 1