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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. Page 62

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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. Page 63