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TRAFFIC OPERATIONS
The operational characteristics of an ATL are similar to that of a CTL, as
described in the signalized intersection chapters of the HCM 2010 (2), with one
critically important exception: lane utilization. The HCM 2010 does not currently
account for the lane utilization impacts associated with limited-length lanes.
An adjustment to the HCM procedures is needed to more accurately reflect
the amount of traffic that is anticipated to use the ATL. These guidelines present
two approaches for estimating the amount of traffic that will use an ATL:
· Statistical model. Directly estimates the amount of through traffic that
will use the ATL using a deterministic approach. This model is used in
conjunction with the HCM 2010 signalized intersection procedure
(Chapter 3).
· Microsimulation. Through the modification of lane choice parameters to
more accurately reflect actual lane usage, microsimulation software can
be applied to predict the performance of an ATL (Appendix A).
SAFETY
Adding an ATL may decrease an intersection's safety due to the potential for
additional sideswipe crashes compared to an intersection without an ATL.
However, because an ATL will reduce congestion it may result in fewer rear-end
and other congestion-related crashes. It is not clear whether the trade-off
between increases in some crash types and decreases in others will generally
result in net positive or negative changes in crash frequency. It is clearer, though,
that the types of crashes that may increase with an ATL would be less severe
than typical crashes at major signalized intersections, on average. Overall, the
expectation of a net positive safety impact from an ATL is not unreasonable. The
analysis of crash data certainly did not highlight any unusual safety concerns at
the ATL sites investigated.
GEOMETRIC AND TRAFFIC DESIGN
Many fundamental geometric and traffic design principles of CTLs apply to
ATLs:
· The geometric design of the ATL should meet drivers' expectations;
· Signing and pavement markings should be applied to reinforce the
messages conveyed by the geometric design of the ATL;
· Adequate sight distance should be provided to adequately accommodate
advance decision making and emergency stops; and
· Driveways and other impedances should be located outside of the
intersection influence area (which for ATLs includes the entire effective
ATL length including upstream and downstream tapers).
The unique geometric and traffic design elements of ATLs relate to the
determination of their length and the use of signs and pavement markings. The
upstream ATL length should be sufficiently long to accommodate the maximum
back of queue on the approach (could be in the CTL or ATL) to ensure that the
ATL remains accessible throughout the cycle.
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