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DESIGN ELEMENTS
This subsection addresses typical user questions and provides appropriate
guidance from a design point of view. When considering an ATL design, typical
questions include:
· What are the minimum and desired distances for the upstream ATL
length and downstream ATL length?
· What signs and pavement markings should be applied for the ATL, and
where should they be placed?
· What are the preferred taper rates for beginning (passive taper) and
ending (active taper) the ATL?
Key design features should be communicated to drivers as they travel
through an ATL at a signalized intersection. As illustrated in Exhibit 5-7, there
are four unique segments of the ATL that require driver actions that differ from
those required by the geometric ATL design sections. These driver interpretation
segments consist of Approaching ATL, Approaching Signal, Departing
Intersection, and Merge at End of ATL.
Exhibit 5-7
ATL Segments Requiring
Unique Driver Action
These segments overlap, and each has a unique purpose; requires a specific
set of driver actions; and provides segment-specific guidelines for geometric
parameters, signing, and pavement markings.
Signing is an important element of ATL design. Signing needs are influenced
by the characteristics of each individual ATL segment. In addition, sign type and
placement influence the operations of the ATL. A review of local highway
agency signing practice for ATLs found that most agencies call for sign spacing
standards less than the spacing guidance identified in the MUTCD as depicted in
Exhibit 5-6. Based on field observations, sites with sign spacing less than the
guidance identified in the MUTCD did not appear to experience adverse safety
or operational performance. In many cases the MUTCD sign spacing guidance
could not be achieved at the study sites due to constraints. For these reasons, the
signing guidelines presented in this chapter call for sign spacings less than
MUTCD guidance where constraints are present.
The visibility of pavement markings is typically influenced by weather
conditions (especially during the presence of snow), and practitioners should not
rely solely on striping guidance for channelization. Striping needs must be
evaluated in conjunction with the conceptual geometric design.
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Approaching ATL Segment
This segment informs the approaching driver of the start of an additional
through lane at the next intersection. Supplemental signage and pavement
markings should encourage drivers to use all intersection through lanes. Exhibit
5-8 illustrates the associated signing and pavement markings along this segment.
The following subsections provide guidance for determining the key design
elements associated with the Approaching ATL segment.
Exhibit 5-8
Signing and Pavement
Markings for Approaching
ATL Segment
Upstream ATL Length
The upstream ATL length should be long enough to accommodate the
design queue in the ATL and ensure that it is not blocked by the CTL during any
point in the cycle. It should also accommodate deceleration from the
approaching CTL to the back of the queue in the ATL. In addition, the start of the
ATL should be visible early enough for approaching motorists to make informed
decisions. The facility's approaching horizontal alignment, the presence of a
vertical crest curve, or a horizontal-vertical alignment combination may require
the ATL upstream portion to be lengthened to ensure the ATL introduction is
visible.
Exhibit 5-9 provides a step-by-step approach for determining the minimum
recommended upstream ATL length based on the anticipated back of queue in
the CTL and ATL. The practitioner may determine that a longer upstream
distance is needed based on prevailing traffic and geometric conditions. Note
that the approach shown in Exhibit 5-9 requires application of the operational
procedure described in Chapter 3. Appendix C contains a detailed description
for calculating the minimum upstream and downstream ATL lengths.
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Exhibit 5-9
Analysis Steps for Determining
Upstream ATL Length
Step 1 Gather Input Data
· Total approach through and right-turn flow rates.
· Cycle length and effective green time for the subject approach.
· Saturation flow rate for both through and right-turn movements.
Step 2 Estimate ATL flow rate based on the one-CTL
or two-CTL model in Chapter 3
Step 3 Calculate the ATL through flow rate using HCM 2010
· Assume equal lane volume-to-saturation flow rate (v/s) based on HCM 2010
shared or exclusive lane group volume distribution.
Step 4 Select the ATL volume as the lower ATL flow rate
from steps 2 and 3
Step 5 Calculate performance measures for ATL and CTL
· Includes lane volumes, capacity, control delay, and back of queue using HCM
2010 signalized intersection procedures.
· For shared ATLs, include the right-turn flow rate in the lane flow computations.
Step 6 Estimate the 95th percentile queues
· Calculate for both ATL and CTL using HCM 2010 procedures.
Step 7 Determine minimum upstream ATL length
· Should provide both storage and unimpeded access to the ATL.
· Determine based on the maximum of the 95th percentile queues in the ATL
and CTL, respectively.
· Calculate the queue storage distance based on an estimate of average
vehicle spacing in a stopped queue for a given vehicle fleet mix
(approximately 25 feet per vehicle).
Passive Taper
A passive taper rate of 10:1 or greater should be applied where the ATL is
introduced. During slow-speed congested conditions, a 10:1 passive taper is
adequate. A higher taper rate is appropriate for higher-speed locations. Local
agencies typically have design guidelines for the introduction of an additional
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through lane and/or turn lane, which are either for a reverse curve along a
specific length or a straight taper.
Signing
Side-mounted signing at the start of the ATL should be considered to
effectively communicate to drivers that the ATL is intended to be used as a
through lane. Field observations and data analysis appear to show higher ATL
use at sites with advance lane-use signing. As described in Section 2B.22 of
MUTCD, Advance Intersection Lane Control (R3-8 series) signs may be used to
indicate the configuration of all lanes ahead.
Pavement Markings
Standard "Lane Line" pavement markings should be considered to clearly
define the added through lane. In addition, a supplemental "Standard Through-
Lane Arrow" should be considered at the start of the ATL to communicate the
purpose of the added lane.
Approaching Signal Segment
When approaching the intersection, drivers should be reminded that the lane
configuration on the approach to the intersection continues beyond the
intersection. Without this reinforcement through drivers may be discouraged
from using the ATL.
The design elements for this segment include the right-turn lane, curb radii,
signing (overhead and side-mounted), and pavement markings. Exhibit 5-10
illustrates the recommended signing and pavement markings along this
segment.
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Exhibit 5-10
Signing and Pavement Markings
for Approaching Signal Segment
Right-Turn Lane
As part of the ATL evaluation and design process, the practitioner should
assess whether a separate right-turn lane is needed in addition to the ATL in
accordance with local agency design practice. If a right-turn lane is provided, the
start of the ATL should be located a sufficient distance from the start of the right-
turn lane to separate decision points and minimize driver confusion, as
illustrated in Exhibit 5-11.
Exhibit 5-11
Appropriate Spacing between
ATL Start and Right-Turn Lane
Curb Radii
Practitioners should consider the appropriate design vehicle and potential
impact to other users when selecting a curb-return radius. Larger curb-return
radii or three-centered curves for right-turns result in less deceleration in the
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ATL and higher ATL use. However, larger radii result in longer crossing
distances for pedestrians and faster vehicular speeds at the crosswalk locations
compared to smaller radii.
Overhead Signing
Similar to advance side-mounted signing at the start of the ATL, overhead
signing for the mast arm or span wire should be considered to effectively
communicate to drivers that the ATL is intended to be used as a through lane.
Field observations and data analysis appear to show higher ATL use at sites with
overhead lane-use signing. Either the Mandatory Movement Lane Control signs
(R3-5 or R3-5a) or the Optional Movement Lane Control sign (R3-6) can be
applied. Both are regulatory (black-on-white lettered) signs.
Side-Mounted Signing
Depending on the length of the ATL and distance to an upstream side -
mounted lane configuration sign (if present), the sign should be located halfway
between the beginning of the ATL and the stop bar. This sign is an MUTCD
Advance Intersection Lane Control sign (R3-8 series).
Pavement Markings Guidelines
Standard "Lane Line" pavement markings should be considered to clearly
define the added through lane. In addition, a supplemental "Standard Through-
Lane Arrow" should be considered at the stop bar of the ATL to communicate
the purpose of the added lane.
Departing Intersection Segment
In this segment, which begins immediately downstream of the intersection,
drivers should be reassured that the through lanes are continuing for an
appropriate distance beyond the intersection before drivers in the ATL begin to
merge into the CTL. Exhibit 5-12 illustrates the associated signing and striping
along this segment.
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Exhibit 5-12
Signing and Pavement Markings for
Departing Intersection Segment
Downstream ATL Length
Limited guidance is provided in national resource documents for
determining the length of the downstream ATL. As part of this project, the
research team developed a theoretical model to assist practitioners in
determining this key design element. The method presented below is intended to
be used as a guide for estimating the minimum appropriate downstream ATL
length and not applied as a rigorous standard or requirement. It presents one
option for estimating downstream ATL length. Individual agencies may have
their own preferred method or guidelines.
Consideration should be given to the maximum downstream ATL length.
ATLs with long downstream lengths may be perceived as a CTL and drivers may
reach the end of the segment and realize unexpectedly that they are required to
merge. From a review of the ATL study sites and other literature, it appears that
a range of ¼ to ½ mile is an appropriate maximum value for downstream ATL
length depending on prevailing speed, sight lines, and driveway/side -street
activity.
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The method described in this subsection estimates the minimum
downstream ATL length such that:
(a) Drivers in the ATL are able to reach the desired prevailing speed
beginning from a stopped (queued) position and
(b) Adequate gaps are available in the CTL to enable a safe merge maneuver
for vehicles that approach the traffic signal during the green phase when
no queues are present.
In these guidelines, the distances described in (a) and (b) are referred to as
DSL1 and DSL2, respectively. The greater of the two distances should be used to
determine the minimum downstream ATL length. Additional distance may be
appropriate based on the prevailing traffic and geometric conditions of the ATL
approach. Appendix C provides a detailed description of the methodology used
to determine DSL1 and DSL2.
The calculation for DSL1, the minimum distance to accommodate
acceleration from a stopped position, involves two steps:
1. Estimate the average uniform, random, and oversaturation back of
queue (BOQ) for ATL and
2. Provide sufficient spacing between ATL vehicles at the prevailing
roadway speed through the intersection after queues have cleared.
The equation for calculating DSL1, measured from the far-side stop bar (in
feet), is as follows:
Equation 5-1
where:
DSL1 = Downstream length from far-side stop bar (in feet)
V = Prevailing roadway speed through the intersection after
queues have cleared (in feet/second),
a = Acceleration rate from stop-line (in feet/second2),
L = Spacing between vehicles at stop (in feet, typically 2025
feet),
T = Driver reaction time (in seconds),
BOQ = Average back of queue upstream of intersection, and
INTW = Intersection width measured from the stop bar to the far
curb (in feet).
Exhibit 5-13 provides a graphical illustration of the method for calculating
DSL1.
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Exhibit 5-13
Illustration of ATL Downstream
Length (DSL1) Calculation
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The calculation of the minimum distance to provide adequate gaps for
merging (DSL2) is shown in the following equation (see Appendix C for more
details):
Equation 5-2
where:
NUM = The number of rejected gaps in the CTL. This could be
either the mean value of rejected or a pre-specified
percentile number of rejected gaps, as explained in
Appendix C, and
Gr = Expected or average size of a rejected headway in the CTL
(in seconds).
Based on initial testing and validation, the research team recommends
applying DSL2 assuming an 85th percentile for rejected gaps, as opposed to the
mean value. This is consistent with the use of the 85th percentile in determining
the design speed of a facility.
For one-CTL sites, DSL2 will generally exceed DSL1 for low-volume and low-
speed approaches. For two-CTL sites, DSL1 will most always govern the
minimum downstream ATL length because of the higher volume of traffic and
queue in the ATL.
Exhibit 5-14 shows potential ATL downstream lengths based on a set of
operational parameters for an anticipated congested level at one-CTL facilities,
while Exhibit 5-15 illustrates potential ATL downstream lengths at two-CTL
facilities. These exhibits can be used for planning purposes to determine the
minimum downstream ATL length for a given speed and congestion level as
represented by XT.
Exhibit 5-14
Planning Tool: ATL
Downstream Guidance for
One-CTL Approaches
ATL Min Downstream Length (ft)
Level of Through-Movement Congestion (xT)
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Exhibit 5-15
Planning Tool: ATL Downstream
Guidance for
Two-CTL Approaches
ATL Min Downstream Length (ft)
Level of Through-Movement Congestion (xT)
Practitioners should consider site-specific conditions when determining the
appropriate downstream ATL length for their application. In addition,
practitioners should consider the following for constrained conditions:
· The equations for DSL1 assume that drivers in the ATL will accelerate to
the prevailing roadway speed before attempting to merge; however, in
many cases drivers will merge prior to reaching the prevailing speed. In
addition, speeds are typically lower during the peak periods due to
congestion along the corridor
· Observations indicate that drivers use a portion or all of the downstream
taper for merging; thus, in some cases it may be appropriate for the
practitioner to consider a portion of the taper as part of the downstream
ATL length.
The practitioner must apply judgment in assessing the anticipated safety and
operational effects of constrained conditions. In some cases, a constrained ATL
site may not provide sufficient downstream ATL distance to accommodate
merge maneuvers and driver expectations. In other cases, while ATL use at a
constrained site is less than desired, it may provide the additional capacity
needed to achieve a desired operating condition.
Signing
Signing in this segment is needed to effectively communicate to drivers that
the ATL is ending a specific distance beyond the intersection. To accomplish this,
consideration should be given to providing a side-mounted "Right Lane Ends"
W9-1 sign approximately 50 to 100 feet minimum from the crosswalk (extension
of opposite stop bar) along with a W16-2P or W16-2aP plaque that indicates the
distance to the beginning of the taper.
Pavement Markings
Standard "Lane Line" pavement markings should be considered to clearly
define the added through lane.
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Merge at End of ATL Segment
Near the termination of the ATL and the taper segment, the driver needs to
know that the extra lane is ending and that appropriate merge behavior is
encouraged. Exhibit 5-16 illustrates the associated signing and striping for this
segment.
Exhibit 5-16
Signing and Pavement
Markings for Merge at End of
ATL Segment
It should be noted that none of the observed study sites provided the
advance placement of the warning signs as outlined in the 2009 MUTCD Table
2C-4. Some sites had the W4-2 signs located along the active taper.
Geometric Design Guidelines
The active taper should be consistent with MUTCD Figure 3B-14 based on
the roadway speed.
Signing Guidelines
Consider providing the side-mounted W4-2 sign approximately halfway
between the "Right Lane Ends" W9-1 sign located with the Departing
Intersection segment and the end of the ATL. Ensure a minimum distance of 100
feet between W4-2 sign and the upstream "Right Lane Ends" W9-1 sign.
Pavement Markings Guidelines
Consider extending the standard "Lane Line" pavement markings to the end
of the ATL to define the total length of the added through lane. In addition,
supplemental "Lane Reduction Arrows" should be considered for speeds of 45
mph and higher.
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