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The exhibit shows the minimum, average, and maximum ATL utilization per site, which are also distinguished by the number of CTLs. It is clear from the data that downstream length plays little, if any, role in enticing drivers to use the ATL. From a design perspective, the downstream ATL length should be long enough to enable drivers starting from a stopped queue in the ATL to accelerate to a safe merging speed. It should also allow drivers traveling through the intersection during the green phase to find a suitable gap for merging into the adjacent CTL traffic stream. Still, as ATLs are inherently an interim capacity improvement at an intersection, the ultimate length may be limited based on available right-of-way, environmental constraints, and construction costs. ATL design elements are discussed further in Chapter 5. Other Factors Affecting ATL Use The following operational and design characteristics should also be considered during ATL design, although their effects were not fully quantified in the statistical lane-use models because of the limited number of observations: Downstream congestion. A bottleneck downstream of the ATL merge area due to the presence of a signalized intersection, a lane drop, or heavy driveway traffic onto the roadway may cause queued traffic to spill back onto the ATL and affect its operations. Posted speed. The higher the posted speed limit, the greater is the speed differential between queued vehicles in the CTL that begin to accelerate when the signal turns green and vehicles arriving on green that may pass more easily in the ATL. This situation may encourage greater ATL use, but likely requires a longer downstream length for safe merging. Sight distance at the intersection approach . Drivers feel more comfortable using an ATL when they can see there are no obstructions in the merge area. DATA COLLECTION REQUIREMENTS Conducting a traffic operations analysis for an ATL requires the same input data as needed for a signalized intersection analysis performed using the HCM 2010 method. These data include 15-minute peak-period flow rates and heavy vehicle percentages, geometric data, and signal timing data. If driveways are present in the ATL, the driveway volume should be estimated and added to the right-turn movements at the intersection. Left turns are assumed to operate from one or more exclusive turn lanes and to not influence the operation of the adjacent CTL or ATL. Such was the case at each site visited for this research. The following bullet items summarize the data that must be measured in the field or estimated in order to predict the through-movement approach volume that will use the ATL: Through-movement demand flow rate on the approach, in vehicles per hour Page 23