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5. GEOMETRIC AND TRAFFIC DESIGN
This chapter describes the typical design approach for an ATL and provides
guidance for determining the upstream and downstream ATL lengths, tapers,
and layout for signs and pavement markings. It requires as input the results from
a traffic operational analysis (Chapter 3) and safety assessment (Chapter 4).
The guidance in this chapter is intended to supplement the national
resources on intersection design highlighted in Chapter 1, including the
AASHTO Green Book (1) and MUTCD (3), as well as local agency design
standards and policies.
DESIGN APPROACH
Prior to beginning a design for an ATL, it is important to recognize (a) the
relationship and interaction among traffic operations, safety, and design of the
ATL and (b) physical, environmental, or right-of-way constraints of the proposed
ATL location that may preclude achievement of an ideal ATL design.
Understanding the Relationship among Operations, Safety, and Design
The relationships among the operations, safety, and design of an ATL are
dynamic and may require an iterative approach in the design process. For
example, providing advanced and overhead signs may attract more traffic to use
the ATL, which in-turn would require longer ATL lengths both upstream and
downstream of the intersection. The following list describes a few examples of
how traffic design parameters influence operational and safety performance of
ATLs:
· Upstream ATL length. If the upstream lane is too short and becomes
blocked, through traffic is unable to access it. Longer upstream ATLs are
more inviting and encourage through traffic to use the ATL.
· Downstream ATL length. Downstream lanes that are too short tend to
discourage drivers who do not feel there is sufficient distance to
comfortably merge into the CTL downstream. Short downstream lanes
may also require drivers to merge while still accelerating, which could
increase the chances of a crash. Downstream lanes that are too long
increase the exposure area for conflicts and may result in unexpected
merges far beyond the intersection.
· Signing and pavement markings. Signing and pavement markings that
encourage use of the ATL as a through lane are likely to result in an
increase in its use. Similarly, signing and pavement markings that provide
clear guidance in advance of the downstream merge can encourage safe
merging behavior. However, cluttered or confusing signing and
pavement markings may negatively affect safety by causing drivers to
"tune out" and ignore the messages.
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Understanding the Effects of Constraints
A constrained site is one where the length of the upstream or downstream
ATL is limited by physical, environmental, cost, and/or right-of-way constraints.
Exhibit 5-1 illustrates a site where the upstream ATL length is unconstrained
and Exhibit 5-2 shows a constrained site where the upstream ATL length is
limited.
Exhibit 5-1
Unconstrained Site Unconstrained Upstream ATL
Direction of Flow
Exhibit 5-2
Constrained Site
Constrained Upstream ATL
Direction of FLow
As shown in Exhibit 5-2, access to the ATL is blocked in the constrained
scenario, which increases the back-of-queue in the CTL.
ATL sites with constraints on the downstream end of the ATL could also
experience similar effects. If a lane is perceived by drivers as being too short,
many will avoid the ATL and continue to use the CTL.
For these reasons, it is likely an ATL that is constrained, either upstream or
downstream, will experience less use than what is predicted in the operational
method described in Chapter 3. In these situations, the practitioner must apply
judgment in assessing the anticipated safety and operational effects of the
constrained ATL in order to determine the net benefit gained by the ATL. In
some cases, the practitioner may find that a constrained ATL does not provide
sufficient distance downstream of the intersection to accommodate merge
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maneuvers given prevailing speeds on the approach and driver expectations. In
other cases, the practitioner may find that while ATL use at a constrained site is
less than desired, it is appropriate because it provides additional capacity benefit
to address the congestion problem.
Preliminary Assessment
ATL design is influenced by many factors, including project type, area type,
local agency operational and design policies, and facility characteristics.
Regardless of the development stage of the project, the first step is to gather
available evaluation data to initiate a preliminary assessment that will guide the
alternatives development process.
A preliminary assessment begins with an understanding of the context of the
corridor for the adjacent land uses and existing adjacent intersections for the
typical users they serve. There are a variety of factors that influence intersection
configurations, including the level of anticipated pedestrian and bicycle
activities, as well as the presence of driveways and their spacing relative to the
intersection. The ultimate objective is to understand how adding an ATL will
compare to the base condition under current and forecast conditions.
Practitioners should understand the range of intersection applications for the
variety of design environments. In most cases, the evaluation of an ATL will fall
under one of three possible scenarios:
· A new intersection on a new facility. New road connections with new
intersections are provided as part of a typical road network expansion.
Traffic demand forecasts may indicate the need for additional through
capacity at signalized intersections.
· A new intersection on an existing facility. Introducing a new
intersection to serve either a new road connection or access to a new
development may require an ATL at a proposed signal to meet
operational requirements for the facility.
· An existing intersection on an existing facility . Traffic growth along a
facility may trigger the need to add capacity at an existing intersection.
Adding an ATL could provide operational relief for an intersection that
is not meeting the desired operational performance.
These guidelines focus on an intersection configuration that includes an
upstream lane add and a downstream right-hand merge as illustrated in Exhibit
5-3.
Exhibit 5-3
Upstream ATL Length Downstream ATL Length ATL Configuration
Right-hand lane add Intersection Right-hand lane
(passive taper) merge (active taper)
Direction of Flow
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Assessing Need for an Exclusive Right-Turn Lane
As part of the ATL design process, the practitioner should evaluate the need
for an exclusive right-turn lane. The decision on whether to construct a separate
right-turn lane should follow local agency practice. It should consider the
anticipated volume in the design year, safety effects, and the operational effect it
may have on usage of the ATL.
Exhibit 5-4 contains a flowchart that illustrates the possible combinations of
lane assignments for an ATL approach. These guidelines focus on one-CTL and
two-CTL facilities with intersections that have either a shared through/right
outside lane or an exclusive right-turn lane. Exhibit 5-5 shows the following
range of options that can be considered for improving a one-CTL facility without
an exclusive right-turn lane:
· Add a right-turn lane
· Add an ATL with a shared through/right lane
· Add an ATL with an exclusive right-turn lane
Exhibit 5-4
Design Approach Flowchart Existing Intersection Configuration
One-CTL Two-CTL
Shared Through/Right ExclusiveRight
Potential Improvements
Add Exclusive Right
Add ATL with Convert Right to
Shared Through/Right ATL with Shared
Through/Right
Add ATL with Add ATL with
Exclusive Right Exclusive Right
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Exhibit 5-5
Potential Improvements for
One-CTL Configuration
Considering the Effect of Driveways
Motorists entering and exiting driveways add friction to an outside lane on a
multilane facility and discourage the use of the lane. The same is true when
driveways are located within an ATL. A driveway in the upstream ATL portion
may cause the ATL to operate as a typical right-turn lane, while a high-volume
driveway immediate downstream of the intersection would discourage motorists
from using the ATL through the intersection.
Where possible, driveways should be located outside of the ATL and its
tapers. However, situations may arise where driveway access may be needed
within the ATL. In this case, the practitioner must apply judgment to determine
the anticipated safety and operational effects of the driveway on ATL
performance. While the operational model described in Chapter 3 does not
account for driveway effects, it is assumed that the presence of driveway activity
will result in lower use of the ATL compared to a condition where no driveways
are present. The presence of the driveway is also expected to increase the
potential for rear-end and angle crashes.
Applying Existing Guidance
The AASHTO Green Book does not provide guidance for ATLs at signalized
intersections but includes information about auxiliary lane applications.
However, the auxiliary lane discussions within the Green Book refer to high-
speed facilities and free-flow conditions.
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The MUTCD provides permanent signing and pavement markings guidance
associated with an ATL design, especially the downstream portion of an ATL.
Exhibit 5-6 shows the current MUTCD guidance for signing and striping.
Exhibit 5-6
Current MUTCD Signing and
Striping Guidance
As illustrated in Exhibit 5-6, the MUTCD identifies one permanent sign
located at the merge of the ATL and one optional sign to guide drivers leaving
the intersection (MUTCD Figure 3B-14):
· At the merge of the ATL either the "Lane Ends" W4-2 sign or "Lane
Ends Merge Left" W9-2 sign should be installed at the advance warning
sign distance indicated in MUTCD Table 2C-4.
· Prior to the required signage, the "Right Lane Ends" W9-1 sign may be
considered to emphasize that the travel lane is ending.
Exhibit 5-6 also illustrates the pavement markings (MUTCD Figure 3B-14)
associated with an ATL:
· The "Lane Line" pavement marking stops three-quarters of the advance
warning sign distance (MUTCD Table 2C-4) before the actual ATL end.
· Supplemental "Lane Reduction Arrows" to emphasize the ATL is
ending and motorists should merge.
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