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Suggested Citation:"Chapter 2 - ATL Characteristics." 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 2 - ATL Characteristics." 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 2 - ATL Characteristics." 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 2 - ATL Characteristics." 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 2 - ATL Characteristics." 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 10
Page 11
Suggested Citation:"Chapter 2 - ATL Characteristics." 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 11
Page 12
Suggested Citation:"Chapter 2 - ATL Characteristics." 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 12
Page 13
Suggested Citation:"Chapter 2 - ATL Characteristics." 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 13

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Page 7 2. ATL CHARACTERISTICS In many ways , ATLs at signalized intersections are similar to CTLs . Both have the same physical foot print at the intersection and both carry through traffic . As a result, many aspects of the analysis and design process are the same for an ATL and a CTL . However, there are some unique characteristics of ATLs that require special attention . These relate to the lane - change maneuver required to enter the ATL upstream of the signalized intersection and merge back into the CTL downstream of the intersection. These lane - change maneuvers influence traffic operations and safety performance as well as design elem ents such as lane length, signing , and pavement markings. The purpose of this chapter is to (1) identify and define key terms used to describe ATLs , and (2) draw attention to characteristics and user needs that are unique for ATLs compared to CTL s . These guidelines do not attempt to duplicate fundamental guidance related to the traffic operations, safety, and design characteristics of through lanes at signalized intersections as described in the resource documents identified in Chapter 1 . Rather, these gu idelines focus on practices and procedures that are unique to the analysis and design of ATLs. TERMINOLOGY The following list includes key terms and definitions used throughout these guidelines to describe ATLs. Auxiliary Through Lane (ATL) : A limited-length through lane added upstream and downstream of an intersection . Shared ATL : An ATL that accommodates right - turning movements in addition to through movements. Exclusive ATL : An ATL that does not include turn movements. Continuous T hrough L ane (CTL) : A n a pproach through lane that is adjacent to the ATL and continuous at least one - half mile upstream and downstream of the intersection. Upstream ATL L ength : T he available queue storage on the approach measured between the end of taper and the stop bar at t he intersection. The upstream ATL length should be sufficient to ensure the ATL is accessible throughout the cycle. Downstream ATL Length : T he downstream length of the ATL measured from the stop bar for the opposing direction and the beginning of taper. T he downstream length of the ATL should be sufficient to ensure that vehicles in the ATL are able to merge adequate ly at the desired prevailing speed. Prevailing Speed: The majority of drivers feel comfortable traveling at this speed on a given roa d section, regardless of the posted speed.

Page 8 Passive Taper : The ATL taper upstream of the intersection that allows vehicles to enter the ATL. Active Taper : The ATL taper downstream of the intersection that requires vehicles to merge. X T : The demand - to - capacity ratio for the through movement assuming the ATL is not in place . APPLICATION Similar to CTL s, ATLs are implemented to increase the stop - bar capacity on approaches at signalized intersections that represent a “choke point” along an arterial st reet. They can be applied in urban, suburban, or rural environments on either the major - street or minor - street approach. ATLs are often applied in lieu of a CTL when: • Construction of a CTL is not feasible; • The capacity added by the ATL adequately accommod ates current or projected traffic demand through the intersection bottleneck ; and • Sufficient length is available to accommodate upstream storage and downstream merge activity. CONFIGURATION TYPES These guidelines address four types of ATL configurations a s shown in Exhibit 2 - 1 : • One CTL with a shared ATL • One CTL, one ATL, and an exclusive right - turn lane • Two CTLs with a shared ATL • Two CTLs, one ATL, and an exclusive right - turn lane

Page 9 Each of the configurations shown in Exhibit 2 - 1 consists of a right - hand lane addition upstream of the intersec tion and a right - hand merge downstream of the intersection. Results from a web survey conducted as part of this research effort found that 85 percent of ATL applications had both a right - hand lane addition upstream and right - hand merge downstream. The rema ining sites included right - turn drop lanes downstream that generally ended at a commercial driveway entrance or left - hand merges. The guidelines presented in this document do not address ATLs with right - turn drop lanes an d left - hand merges given the relati vely few known applications of that configuration . Exhibit 2-1 One CTL with Shared ATL Two CTLs with Shared ATL One CTL, One ATL, and Exclusive Right-Turn Lane Two CTLs, One ATL, and Exclusive Right-Turn Lane ATL Configuration Types

Page 10 TRAFFIC OPERATIONS T he operational characteristics of an ATL are similar to that of a CTL , as described in the signalized intersection chapters of the HCM 2010 ( 2 ) , with one critically important exception: lane utilization . The HCM 2010 does not currently account for the lane utilization impacts associated with limited - length lanes. A n adjustment to the HCM procedures is needed to more accurately reflect the amount of traffic that is anticipated to use the ATL. These guidelines present two approaches for estimating the amount of traffic that will use an ATL: • Statistical model . Directly estimates the amount of through traffic that will use the ATL using a deterministi c approach . This model is used in conjunction with the HCM 2010 signalized intersection procedure (Chapter 3). • Microsimulation. Through the modification of lane choice parameters to more accurately reflect actual lane usage , microsimulation software can b e applied to predict the performance of an ATL (Appendix A). SAFETY Adding an ATL may decrease an intersection’s safety due to the potential for additional sideswipe crashes compared to an intersection without an ATL . However, because an ATL will r educ e congestion it may result in fewer rear - end and other congestion - related crashes . It is not clear whether the trade - off between increases in some crash types and decreases in other s will generally result in net positive or negative changes in crash frequen cy . I t is clearer, though, that the types of crashes that may increase with an ATL would be less severe than typical crashes at major signalized intersections, on average. Overall, the expectation of a net positive safety impact from an ATL is not unreason able. The analysis of crash data certainly did not highlight any unusual safety concerns at the ATL sites investigated. GEOMETRIC AND TRAFFIC DESIGN Many fundamental geometric and traffic design principles of CTL s apply to ATLs: • The geometric design of th e ATL should meet driver s ’ expectations ; • Signing and pavement markings should be applied to reinforce the messages conveyed by the geometric design of the ATL; • Adequate sight distance should be provided to adequately accommodate advance decision making and emergency stops; and • Driveways and other impedances should be located outside of the intersection influence area (which for ATLs includes the entire effective ATL length including upstream and downstream tapers). The unique geometric and traffic design e lements of ATLs relate to the determination of their length and the use of signs and pavement markings. The upstream ATL length should be sufficiently long to accommodate the maximum back of queue on the approach (could be in the CTL or ATL) to ensure that the ATL remains accessible throughout the cycle.

Page 11 The downstream ATL length should enable vehicles from a stopped position to reach the desired prevailing speed before reaching the beginning of taper. It should also ensure that adequate gaps exist in the adjacent CTL , particularly at high speeds, to enable safe merge maneuvers before vehicles reach the beginning of taper. Supplemental signs should be applied in advance of the intersection and on mast arms or span wires to indicate that the ATL is intended for use as a through lane and not inadvertent ly assumed to be a right - turn - only lane. Pavement marking arrows should be considered for application in advance of the downstream merge to provide additional notification to the driver. USER CONSIDERATIONS This section presents considerations for the four primary modes (pedestrians, bicyclists, transit , and auto ) and highlights their unique considerations related to the analysis and design of ATLs. Pedestrians Similar to when a CTL is added , when an ATL is added , the distance pedestrians must travel to cross one or more intersection legs is increased . This increased distance produces the following effects: • Increased pedestrian exposure to traffic. By increasing crossing distance, ATLs may place pedestrians a t higher risk. According to the AASHTO Highway Safety Manual , the greater number of lanes at a signalized intersection , the higher is the likelihood of a vehicle – pedestrian crash ( 4 ) . • Reduced p edestrian level of service on side - street approaches. ATLs incr ease the total crossing distance for pedestrians, resulting in reduced pedestrian comfort and a lower level of service ( LOS ) for crossing pedestrians, per the Signalized Intersections LOS methodology for pedestrians included in the HCM 2010 ( 2 ). • Increased minimum pedestrian crossing time . Assuming a typical 12 - foot lane and a walk time of 3.5 feet per second, the addition of an ATL will increase the minimum walk time by approximately 3.5 seconds. When pedestrian walk times govern the minimum time a green li ght is provided to a side - street approach, this effect will increase the proportion of green time provided to the side street approach and reduce the proportion of g reen time allocated to the main - street approach. To the extent that the increased minimum g reen contributes to overall pedestrian delay, this will negatively impact pedestrian LOS. Bicyclists Unless a bicycle lane is provided, cyclists should be assumed to use the ATL rather than other through lanes. M any bicyclists may feel uncomfortable in an ATL’s merge section due to the “struggle” for available space. If a bicycle lane is available, its location relative to the general traffic lanes may create additional conflict points within a signalized intersection that contains ATLs.

Page 12 For bicyclists on side - street approaches, ATLs increase total crossing distance. This increase will negative ly affect bicycle LOS for side - street approaches , according to the Signalized I ntersection LOS methodology included in the HCM 2010 ( 2 ) . Exhibit 2 - 2 shows potential bicycle treatments with and without exclusive right - turn lane s at signalized intersections. Transit Bus stops could be located within an ATL on either the near side or the far side of the intersection , depending on transit agency policy, local land uses, and signal timing. Depending on the roadway classification and/or traffic volume, bus stops are sometimes also located within the near - side right - turn lane or in a bus pull out bay on the far side of the intersection. Exhibit 2 - 3 illustrates potential bus stop location s based on the final configuration of an intersection. Exhibit 2-2 Bicycle Treatment Examples

Page 13 Mergi ng with general traffic can often be difficult for bus operators. In some jurisdictions, it is the law to yield to buses. The Transit Capacity and Quality of Service Manual (TCQSM) provides a method for estimating delay as a result of buses merging into tr affic ( 5 ). Depending on the guidelines and preferences of the transit agency, and the agency owning the roadway, a right - turn lane may be used as a bus pullout, or a bus pullout could be provided beyond the intersection. Far - side stops often allow buses to take better advantage of the signal progression provided along the roadway, but other considerations such as facilitating transfers to bus routes on cross streets and proximity to transit passenger generators may dictate the use of a near - side stop. If no pullout area is provided, then buses would stop in the outside lane (e.g., the ATL). Motorists typically avoid the outside/auxiliary lane when buses are present, so a high frequency of buses along the street would tend to discourage ATL use. Other observa tions from the research indicate: • For relatively short cycle lengths, near - side bus stops had a limited impact on the intersection operations. • Far - side stops within the downstream ATL caused motorist upstream to reposition themselves. • Where buses stopped i n right - turn lanes (upstream) or bus pullout areas (downstream), they were able to find acceptable gaps to merge back into the traffic stream. Auto M otorists typically seek to minimize delay while traveling through intersections , which means they are likely to consider using the ATL when there is risk of not clearing the intersection in the green phase from the CTL and/or when they can wait in a shorter queue. Exhibit 2-3 Potential Bus Stop Locations

Page 14 Certain messages need to be conveyed throughout the ATL to make motorists aware of the ATL and to encourage its use: • Prior to the intersection , the driver needs to be made aware of the ATL lane being added. • At the intersection , the driver needs to know that the ATL serves through traffic and not just turn ing movements. • After departing the inters ection , the driver needs to be made aware of the impending mandatory merge condition. A noted concern from many motorists and highway agency staff members is the us e , or misuse , of ATLs by aggressive drivers. Aggressive drivers may choose to use the less - utilized ATL to by - pass vehicles in the CTL. Certain agencies, such as the Maryland State Highway Administration and Connecticut Department of Transportation ( 6 ) , ha ve experimented with signs indicating an alternating merge area to both encourage use of the ATL by removing the priority of the CTL over the ATL and to encourage courteous behavior. Exhibit 2 - 4 illustrate s some lane - merging signs. Exhibit 2-4 Alternate Merge Sign Examples

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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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