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provides the best value as it requires the least amount of widening, while still
meeting the local highway agency's operational standards. Alternative 2, the
addition of a shared through plus right-turn ATL, will be carried forward into
the preliminary horizontal geometric design process.
PRELIMINARY HORIZONTAL GEOMETRIC DESIGN
The following section describes a step-by-step process to develop a
preliminary horizontal design for the preferred alternative, the addition of a
shared ATL. Refer to the exhibit at the end of this chapter for a detailed
illustration of the preliminary horizontal design of the shared ATL in comparison
to the existing eastbound approach configuration. The theory behind the design
process is described in detail in Chapter 5.
Design Input Data
· Lane width: W = 11 feet;
· Approach design speed: S = 35 mph;
· Intersection width: IW = 110 feet.
Step 1: Calculate the Length of the Design Elements
To gain an idea of the overall picture of the design needs for the eastbound
approach and potential property, slope, drainage, and infrastructure impacts, the
first step is to calculate the length of each of the four sections of the ATL. Steps
1a-1d identify the procedures for calculating the length of each of the ATL design
elements:
· Passive taper
· Upstream ATL length
· Downstream ATL length
· Active taper
Step 1a: Determine the Length of the Passive Taper
Chapter 5 indicates that the minimum passive taper rate should be 10:1.
Using an 11-foot lane width and applying the passive taper rate of 10:1, the
minimum length for the passive taper is 11x10 = 110 feet.
Step 1b: Determine the Upstream ATL Length
The upstream ATL length should be sufficient to accommodate the
maximum 95th percentile maximum vehicle queue on the approach along with
any additional distance that is desired for deceleration. As shown in Exhibit 6-6,
the maximum 95th percentile back of queue is 300 feet in the CTL. Given the 95th
percentile back of queue in the ATL is expected to be 200 feet and prevailing
speeds on the approach, a distance of 300 feet is deemed sufficient to
accommodate safe deceleration (which is assumed to begin in the taper) for a
vehicle that departs the CTL and reaches the back of queue in the ATL.
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Step 1c: Determine the Downstream ATL Length
The minimum length for the section departing the intersection is computed
using the computational engine and following the procedure described in
Chapter 5. Exhibit 6-7 summarizes the geometric parameters for this design. The
downstream ATL length calculations based on the two methodologies show the
following results:
DSL1 = 220 feet
DSL2 = 250 feet
The greater of the two distances should be used to determine the minimum
downstream ATL length, which for this example is 250 feet. Given the posted
speed (35 mph), lack of driveways, clear sight lines, and available right-of-way,
the downstream ATL length of 275 feet is selected for this design.
Upstream ATL Exhibit 6-7
Length Downstream ATL Length Summary of Geometric Parameters
Based on Storage Downstream ATL Based on CTL-Gap
Condition Q-Length (ft) Length (ft) Acceptance Distance (ft)
Add Shared ATL 300 220 250
Step 1d: Determine the Length of the Active Taper
The minimum length for the active taper is calculated in a manner consistent
with MUTCD (3) recommendations. In this example the width of the ATL is 11
feet and the speed of the minor arterial is 35 mph. The downstream merge
section length is calculated as:
Step 2: Assess the Viability of Installing the ATL
Once the design elements are defined, existing slopes, drainage areas, rights-
of-way, utilities, and other infrastructure should be evaluated out to a width of
approximately 15 to 25 feet from the existing curb or edge of pavement line to
gain an idea of the effects that will occur due to the addition of the ATL. In this
example, the cross section assumed for the ATL addition is an 11-foot wide ATL,
with a 6-inch vertical curb, a 6-foot attached sidewalk, and a 4:1 cut/fill slope to
tie into existing ground. In this example, there are no major conflicts within the
cross-sectional area.
Step 3: Design the Upstream Full-Width Lane Segment of the ATL
This step involves identifying the signing and pavement markings for the
upstream segment given the length of the passive taper and upstream ATL
length described in Step 1. The following treatments are recommended:
· Place a 10-foot skip stripe with 30-foot breaks along the entire length of
the upstream full-width lane.
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