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Page 1 1. INTRODUCTION An a uxiliary through lane (ATL) is a limited length through lane added upstream and downstream of an intersection, as shown in Exhibit 1 - 1 . ATLs are typically applied as an intermediate-cost treatment to reduce recurring bottlenecks at signalized intersections. They can be applied to either the major - street or minor - street approach . When an ATL is present, through traffic is allowed to disperse across an additional through lane at the signalized intersection , wh ich increas es the stop - bar capacity of the approach. This increase capacity reduces delay and queuing for through vehicles. A n ATL also reduces the time required for the green light phase to serve the through demand on the approach, meaning the extra time can then be allocated to other movements at the intersection. ATLs are typically applied at locations where additional through capacity is desired but construction of a continuous through lane (CTL) is not feasible. ATLs can also be applied as an interim i mprovement until a CTL improvement is made or can be justified. In summary, an ATL achieve s a portion of the capacity benefits of a CTL for a portion of its cost and right - of - way/environmental impact. F ield data and observations were collected at 22 ATL a pproaches across the United States to analyze ATL performance and the relationship between the traffic operations, safety, and design characteristics of ATLs. Results from this data collection effort indicate that during the peak period an average of 24 pe rcent of through traffic used the ATL on approaches with one continuous through lane. Despite the relatively low utilization, ATLs provide significant benefits in terms of reduc ed vehicle delay. A nalysis of the field data shows that the presence of the ATL reduced delay by an average of 100 seconds per vehicle compared to the case with no ATL in place during the observational periods ( assuming all other factors remain equal ) . This delay reduction is because many of those approaches would have operated in an oversaturated state without the ATL in place. Similar conclusions were reached for ATLs with two CTLs . Thus, while ATLs do not achieve the full operational benefit of a CTL , their operational benefits are nonetheless substantial. ATLs are most effective under congested conditions when the demand - to - capacity ratio for the through movement approaches 1.0 without the ATL in place. If congestion levels are too low there is limited incentive for drivers to use Exhibit 1-1 Typical Auxiliary Through Lane Configuration
Page 2 the ATL. As congestion increases, the risk that a driver will not clear the intersection within the current cycle increases , along with the delay savings achieved by using the ATL, making it more likely that a driver will choose to use the ATL. This research found a strong correlation between congestion l evel and use of the ATL. This document presents guidelines that are a culmination of information gathered via a literature review, a survey of transportation practitioners, and an analysis of field data regarding ATLs, along with the research teamâs exper ience in the operation and design of signalized intersections. These guidelines are intended to be applied by transportation practitioners as a decision support tool and are intended to supplement national guidance documents and local agency policies and p ractices on intersection design. SCOPE OF THE GUIDELINES These guidelines apply to auxiliary lanes for through movements that begin upstream of a signalized intersection and end downstream of the intersection. It focuses on ATLs that begin with a right - hand add lane upstream of the signal and end with a right - hand merge downstream of the signal . The operational models presented in these guidelines assume that both the continuous and auxiliary through lanes are free from impedances from left - turn m ovements and downstream activity. These guidelines provide practitioners with the tools and guidance needed to answer the following questions: ⢠What factors affect the use of ATLs? ⢠How much traffic is likely to use an ATL? ⢠What is the safety performance of ATLs? ⢠What tools are available to evaluate operational and safety performance of ATLs? ⢠What minimum length is needed for the upstream and downstream components of the ATL? ⢠What signs and pavement markings should be applied on ATLs? ⢠How can simulation be us ed to supplement a deterministic analysis of ATLs? LIMITATIONS OF THE GUIDELINES The ATL guidelines do not address the following conditions: ⢠Non - signalized intersections ⢠Intersections that serve as transitions from either four - lane to two - lane roadways or six - lane to four - lane roadways ⢠Left - or right - turn lanes with an upstream addition and downstream drop ⢠A ppr oaches that have more than two CTLs ⢠A pproaches that include shared left â through lanes or downstream facilities w here queues extend into the ATL
Page 3 ⢠Appr oaches that experience blockage due to downstream conditions ⢠Approaches that operate within a well - coordinated signal system such that the majority of vehicles arrive during the gre en phase of the traffic signal In addition, t h e guidelines do not provide statistical or analytical models to predict the number of crashes or conflicts on an ATL. Rather, a summary of crash data obtained for ATL approaches is provided. Lastly, these guidelines do not provide guidance for applying ATLs relative to other capacity - enhancing intersection treatments. ORGANIZATION OF GUIDELINES These guidelines are organized to follow a typical analysis and design process for ATLs as shown in Exhibit 1 - 2 . Also included is the corresponding chapter that documents the information and procedures needed to carry out the appropriate step in the process . Th e title and content for all chapter s and appendices are described below : ⢠Chapter 2: ATL Characteristics. Describes the operational, safety, and design characteristics of ATLs, as well as needs and considerations for potential ATL user types. ⢠Chapter 3: Operational Analysis. Presents a statistical model for predicting the amoun t of traffic that will use an ATL for approaches with one or two CTLs . ⢠Chapter 4: Safety. Documents the results from an evaluation of field crash data and discusses geometric and operational factors expected to impact the safety performance of an ATL. Chapter 5: Geometric and Traffic Design. Describes an approach for preparing a functional design plan for an ATL, provides a method for determining the minimum upstream and downstream ATL length, and presents guidance on signing and pavement markings for ATLs. Chapter 6: Sample Application. Demonstrates how to apply the operations, safety and design tools, methods, and guidelines to a practical example. ⢠⢠⢠Appendix A. Describes how analysts can use traffic simulation models to estimate the operational performance and, to a limited extent, th e safety performance of ATL designs. ⢠Appendix B. Describes the computational engine that carries out the deterministic operational analysis procedure described in Chapter 3. ⢠Appendix C. Describes the method and equations for calculating the minimum required upstream and downstream ATL lengths.
Page 4 Exhibit 1-2 Guidelines Organization Sample AT L F unctional Design P lan N OTE S ⢠⢠No additional dat a r equired beyond traditional intersection analysi s A pplicable to approaches with one or tw o c ontinuous through lanes and an e xclusiv e o r s hared r ight-turn lane Assess M ultimodal Need s ⢠I dentif y f acility needs fo r pedestrians, bicyclists, and transit r ider s ( Chapter 2) E valuate T raffic Operations ⢠⢠HCM analysi s u s ing statistica l m odel to predic t AT L u se ( Chapter 3 & A ppendix B) Microsimulation (Appendix A) Assess Safety Effect s ⢠⢠Q ualitativ e e v aluation ( Chapter 4) Conflic t p rediction (Appendix A) Calculate Design Elements ⢠⢠⢠⢠Upstream Passive T aper Upstream AT L Lengt h Downstream AT L Lengt h Downstream Activ e T aper ( Chapter 5) L ay Out Individual Segments ⢠⢠⢠⢠A pproaching AT L A pproaching S ignal Departing Intersection Merg e a t E nd of AT L ( Chapter 5)
Page 5 RECOMMENDED RESOURCE DOCUMENTS There are many other resources regarding the operations, design, and safety of intersecti on treatments that contain information relevant to the analysis and design of ATLs. The following resource documents should be used in the analysis and design of ATLs along with these g uidelines , and in addition to relevant local agency resources : ⢠A Polic y on Geometric Design of Highways and Streets (AASHTO Green Book) ( 1 ) ; ⢠Highway Capacity Manual 2010 (HCM) ( 2 ) ; ⢠Manual on Uniform Traffic Control Devices (MUTCD) ( 3 ) ; and ⢠Highway Safety Manual (HSM) ( 4 ) .
