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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. Page 37
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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 Page 38
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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 Page 39
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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 Page 40
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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. Page 41
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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. Page 42