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2. ATL CHARACTERISTICS
In many ways, ATLs at signalized intersections are similar to CTLs. Both
have the same physical footprint at the intersection and both carry through
traffic. As a result, many aspects of the analysis and design process are the same
for an ATL and a CTL.
However, there are some unique characteristics of ATLs that require special
attention. These relate to the lane-change maneuver required to enter the ATL
upstream of the signalized intersection and merge back into the CTL
downstream of the intersection. These lane-change maneuvers influence traffic
operations and safety performance as well as design elements such as lane
length, signing, and pavement markings.
The purpose of this chapter is to (1) identify and define key terms used to
describe ATLs, and (2) draw attention to characteristics and user needs that are
unique for ATLs compared to CTLs.
These guidelines do not attempt to duplicate fundamental guidance related
to the traffic operations, safety, and design characteristics of through lanes at
signalized intersections as described in the resource documents identified in
Chapter 1. Rather, these guidelines focus on practices and procedures that are
unique to the analysis and design of ATLs.
TERMINOLOGY
The following list includes key terms and definitions used throughout these
guidelines to describe ATLs.
Auxiliary Through Lane (ATL): A limited-length through lane added
upstream and downstream of an intersection.
Shared ATL: An ATL that accommodates right-turning movements in
addition to through movements.
Exclusive ATL: An ATL that does not include turn movements.
Continuous Through Lane (CTL): An approach through lane that is adjacent
to the ATL and continuous at least one-half mile upstream and downstream of
the intersection.
Upstream ATL Length: The available queue storage on the approach
measured between the end of taper and the stop bar at the intersection. The
upstream ATL length should be sufficient to ensure the ATL is accessible
throughout the cycle.
Downstream ATL Length: The downstream length of the ATL measured
from the stop bar for the opposing direction and the beginning of taper. The
downstream length of the ATL should be sufficient to ensure that vehicles in the
ATL are able to merge adequately at the desired prevailing speed.
Prevailing Speed: The majority of drivers feel comfortable traveling at this
speed on a given road section, regardless of the posted speed.
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