National Academies Press: OpenBook

Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections (2011)

Chapter: Chapter 5 - Geometric and Traffic Design

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Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
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Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
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Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
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Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
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Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
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Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
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Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 42
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Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 43
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Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 44
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Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 45
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Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 46
Page 47
Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 47
Page 48
Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 48
Page 49
Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 49
Page 50
Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 50
Page 51
Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 51
Page 52
Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 52
Page 53
Suggested Citation:"Chapter 5 - Geometric and Traffic Design." National Academies of Sciences, Engineering, and Medicine. 2011. Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections. Washington, DC: The National Academies Press. doi: 10.17226/14617.
×
Page 53

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Page 37 5. GEOMETRIC AND TRAFFIC DESIGN This chapter describes the typical design approach for an ATL and provides guidance for determining the upstream and downstrea m 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 reso urces on intersection design highlighted in Chapter 1 , including the AASHTO Green B ook ( 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 relationship s 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 tra ffic 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: • U pstrea m ATL l ength . 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 . S ign ing and pavement marking s that encourage use of the ATL as a through lane are likely to r esult in an increase in its use. Similarly, signing and pavement marking s 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 affec t safety by causing drivers to “tune out” and ignore the messages.

Page 38 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. As shown in Exhibit 5 - 2 , access to the ATL is blocked in the constrained scenario , which increas e s 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 i s perceived by drivers as being too short, many will avoid the ATL and continue to use the CTL. For these reasons , it is likely a n 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 assess ing 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 t o accommodate merge Exhibit 5-1 Unconstrained Site Exhibit 5-2 Direction of Flow Constrained Upstream ATL Direction of FLow Unconstrained Upstream ATL Constrained Site

Page 39 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 capacit y 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 presenc e 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 ap plications 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. • A n existing intersection on an existing facility . Traffic growth along a facility may trigger the need to add capacity at an existing intersect ion. Adding an ATL could provide operational relief for an intersection that is not meeting the desired operational performance. These guidelines focus on an intersection confi guration that includes an upstream lane add and a downstream right - hand merge as illustrated in Exhibit 5 - 3 . Exhibit 5-3 Downstream ATL Length Upstream ATL Length (passive taper) merge (active taper) Right-hand lane add Right-hand lane Intersection Direction of Flow ATL Configuration

Page 40 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 th at 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 show s t he following range of options that can be considered for improving a one - CTL facility with out an exclusive right - turn lane : • Add a right - turn lan e • Add an ATL with a shared through/right lane • Add an ATL with an exclusive right - turn lane One-CTL Shared Through/Right Add Exclusive Right Add ATL with Shared Through/Right Convert Right to ATL with Shared Through/Right Add ATL with Exclusive Right Add ATL with Exclusive Right ExclusiveRight Two-CTL Existing Intersection Configuration Potential Improvements Exhibit 5-4 Design Approach Flowchart

Page 41 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. Exhibit 5-5 Potential Improvements for One-CTL Configuration

Page 42 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 . 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 t he “Lane Ends” W4 - 2 sign or “Lane Ends Merge Left” W9 - 2 sign sh ould 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 trav el 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. Exhibit 5-6 Current MUTCD Signing and Striping Guidance

Page 43 DESIGN ELEMENTS This subsection addresses typical user questio ns 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 si gnalized intersection. As illustrated in Exhibit 5 - 7 , there are four unique segments of the ATL that require driver action s that differ from those req uired by the geometric ATL design sections . These driver interpretation segments consist of Approaching ATL, Approaching Signal, Departing Intersection, and Merge at End of ATL. 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 observation s, sites with sign spacing less than the guidance identified in the MUTCD did not appear to experience adverse safety or operational performance. I n 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 presen ce 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. Exhibit 5-7 ATL Segments Requiring Unique Driver Action

Page 44 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 i llustrates 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. 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 motorist s 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 vi sible. Exhibit 5 - 9 provides a step - by - step approach for determining the minimum recommended upstream ATL l ength 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 t h e 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. Exhibit 5-8 Signing and Pavement Markings for Approaching ATL Segment Upstream ATL Length

Page 45 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 pa ssive taper is adequate . A higher taper rate is appropriate for higher - speed locations. Local agencies typical ly have design guidelines for the introduction of an additional Exhibit 5-9 Analysis Steps for Determining Upstream ATL Length Passive Taper Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Gather Input Data Select the ATL volume as the lower ATL flow rate from steps 2 and 3 Calculate performance measures for ATL and CTL Estimate the 95th percentile queues Determine minimum upstream ATL length • • • 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. • • 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. • Calculate for both ATL and CTL using HCM 2010 procedures. • • • 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). Estimate ATL flow rate based on the one-CTL or two-CTL model in Chapter 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.

Page 46 through lane and/or turn lane, which are either for a reverse curve along a specif ic length or a straight taper. 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. F ield observations and data analysis appear to show highe r ATL use at sites with advance lane - use signing. As described in Section 2B .22 of MUTCD, A dvance I ntersection L ane C ontrol (R3 - 8 series) sign s may be used to indicate the configuration of all lanes ahead . Standard “Lane Line” pavement markings should be considered to clearly define the added through lane. In addition, a s upplemental “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 . Signing Pavement Markings

Page 47 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 su fficient distance from the start of the right - turn lane to separate decision points and minimize driver confusion, as illustrated in Exhibit 5 - 11 . P ractitioners should consider the appropriate design vehicle and potential impact to other users when selecting a curb - return radius . L arger curb - return radii or three - centered curves for right - turns result in less deceleration in the Exhibit 5-10 Signing and Pavement Markings for Approaching Signal Segment Exhibit 5-11 Appropriate Spacing between ATL Start and Right-Turn Lane Right-Turn Lane Curb Radii

Page 48 ATL and higher ATL use. However, larger radii result in longer crossing distances for pedestri ans and faster vehicula r speeds at the crosswalk locations compared to smaller radii. 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 tha t 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 C ontrol sign (R3 - 6) can be applied. Both are regulatory (black - on - white lettered ) signs. Depending on the length of the ATL and distance to an upstream side - mounted lane configuration sign (if present), the sign should be located halfw ay between the beginning of the ATL and the stop bar. This sign is an MUTCD A dvance I ntersection L ane C ontrol sign (R3 - 8 series ) . Standard “Lane Line” pavement markings should be considered to clearly define the added through l ane. 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 i mmediately downstream of the intersec tion , driver s 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 associate d signing and striping along this segment . Overhead Signing Side-Mounted Signing Pavement Markings Guidelines

Page 49 Exhibit 5-12 Signing and Pavement Markings for Departing Intersection Segment 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. Downstream ATL Length

Page 50 Equation 5-1 (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: 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 20–25 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. The method described in this subsection estimates the minimum downstream ATL length such that:

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

Page 52 The calculation of the minimum distance to provide adequate gaps for merging (DSL 2 ) is shown in the following equation (see Appendix C for more details) : w here: NUM = T he 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 G r = E xpected or average size of a rejected headway in the CTL (in seconds). Based on initial testing and validation, the research team recommends applying DSL 2 assuming an 8 5 th percentile for rejected gaps, as opposed to the mean value. This is consistent with the use of the 85 th percentile in determining the design speed of a facility. F or one - CTL sites , DSL 2 will gene rally exceed DSL 1 for low - volume and low - speed approaches . F or two - CTL sites, DSL 1 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 potentia l 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 X T . Equation 5-2 Exhibit 5-14 Planning Tool: ATL Downstream Guidance for One-CTL Approaches A TL M in D ow ns tre am L en gt h (ft ) Level of Through-Movement Congestion (xT)

Page 53 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 DSL 1 assume that drivers in the ATL will accelerate to the prevailing roadway speed before attempting to merge ; h owever, 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 mergi ng; 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. T he practitioner must apply judgment in assessing the anticipated safety and operational effects of constrained con ditions . 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 in this segment is needed to effectively communicate to drivers that the ATL is ending a specific distance beyond the intersection. To accomplish this, c onsider ation shoul d 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. Standard “Lane Line” pavement markings should be considered to clearly define the added through lane. Exhibit 5-15 Planning Tool: ATL Downstream Guidance for Two-CTL Approaches A TL M in D ow ns tre am L en gt h (ft ) Level of Through-Movement Congestion (xT) Pavement Markings Signing

Page 54 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 endin g and that appropriate merge behavior is encouraged. Exhibit 5 - 16 ill ustrates the associated signing and striping for this segment . It should be noted that n one 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. The active taper should be consistent with MUTCD Figure 3B - 14 based on the roadway speed . Consider providing the side - mounted W4 - 2 sig n 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. 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. Exhibit 5-16 Signing and Pavement Markings for Merge at End of ATL Segment Geometric Design Guidelines Signing Guidelines Pavement Markings Guidelines

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Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 707: Guidelines on the Use of Auxiliary Through Lanes at Signalized Intersections provides guidelines to help in the justification, design, and analysis of auxiliary through lanes (ATLs) at signalized intersections.

ATLs are lanes for through movements that begin upstream of a signalized intersection and end downstream of the intersection. ATLs are potentially a moderate-cost approach to increase intersection and overall corridor capacity.

A report that describes the research related to the development of NCHRP Report 707 has been released as NCHRP Web-Only Document 178: Assessment of Auxiliary Through Lanes at Signalized Intersections.

A spreadsheet-based computational engine is also available online.

Spreadsheet Disclaimer - This software is offered as is, without warranty or promise of support of any kind either expressed or implied. Under no circumstance will the National Academy of Sciences or the Transportation Research Board (collectively “TRB’) be liable for any loss or damage caused by the installation or operation of this product. TRB makes no representation or warranty of any kind, expressed or implied, in fact or in law, including without limitation, the warranty of merchantability or the warranty of fitness for a particular purpose, and shall not in any case be liable for any consequential or special damages.

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