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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
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