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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. Page 43

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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. Page 44

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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 Page 45

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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. Page 46

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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 Page 47

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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. Page 48

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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 to mile is an appropriate maximum value for downstream ATL length depending on prevailing speed, sight lines, and driveway/side -street activity. Page 49

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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. Page 50

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Exhibit 5-13 Illustration of ATL Downstream Length (DSL1) Calculation Page 51

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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) Page 52

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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. Page 53

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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. Page 54