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3. OPERATIONAL ANALYSIS
This chapter presents the operational guidelines for evaluating ATLs at
signalized intersections. It is founded on a method to predict the volume of
traffic that will use an ATL. In addition to evaluating ATL and intersection
performance, the results from the operational analysis are needed to determine
the recommended minimum design lengths for the upstream and downstream
ATL described in Chapter 5.
This method supplements the Highway Capacity Manual 2010 (2)
procedures. The HCM 2010 does not directly account for the effects of short lane
additions (including ATLs) in its lane utilization factor fLU. Instead, it directs
users to apply a simulation tool to predict the through-movement volume in the
short lane. While some ATL characteristics can be modeled through micro-
simulation, the models presented here are based primarily on empirical
observations taken at a number of existing signalized ATL approaches across the
United States. A detailed discussion of how microsimulation may be used to
evaluate ATL performance is provided in Appendix A. Appendix B describes the
computational engine that incorporates the ATL lane use prediction model
presented in this chapter.
Given its unique operating characteristics and lower utilization, an ATL
should always be treated as a separate lane group, regardless of whether the lane
is shared with right turns or is exclusive. Thus, an approach that consists of a
separate left-turn lane, one CTL, one ATL, and one exclusive right-turn lane
would have four lane groups as shown in Exhibit 3-1.
Exhibit 3-1
Exclusive ATL Lane Group Diagram
While the HCM 2010 treats a shared through-right lane as a separate lane
group (which is different from how the HCM 2000 lane groups were designated),
it still combines all exclusive through lanes into a single lane group. This process
is not appropriate in the presence of an ATL because the lane utilization of the
ATL is much lower than that of a traditional CTL.
This chapter presents the following information:
· Principles of ATL operations
· Data collection required for analysis
· Analysis techniques for ATL operations
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