ICriteria forthe Geometric Design of Triple Left-lUrnlanes BY KENNETH W. ACKERET onc of the newest challenges to the transportation design engineer is the geometric design of triple left-turn lanes. The use of double left-turn lanes has gained acceptance over the years as a solution to increasing intersection design capacity where there is significant left-turning volume. As a general rule, dual left-turn lanes are considered in locations with left-turn demands of 300 vehicles per hour (vph) or morel Years of experience and research on the design and operations of dual left-turn lanes have provided guidelines to transportation engineers for the geometric design of these intersection facilities. As traffic congestion and travel demands continue to grow on our street systems, transportation engineers are continually challenged to develop alternatives to satisfy this demand. In recent years the use of triple left-turn lanes has been introduced as a concept for handling at-grade intersection locations with severe left-turn capacity and operational problems where land-use constraints and construction costs preclude a grade-separated interchange. To date transportation engineers considering triple leftturn lanes as a design alternative have had to rely on personal experience and engineering design practices applied to dual left-turn lanes for the design of these facilities. Presented in this article are general criteria for the geometric design of triple left-turn lanes based on the design and operational experiences obtained within the Las Vegas Metropolitan area of Clark County, gaining acceptance for design and con- Nevada (Figure 1). struction. These schemes are extensions of those commonlv found for Triple Left-turn dual left-turn lanes.z For-this article, Configurations the three general types of triple leftturn lane configurations are classified Three general types of triple left- below and in Figure 2: turn lane configurations have been Type A Exclusive triple left-turn Road and Valley View Blvd., Las Vegas, Nev. Completed for operation 1989. ITE JOURNAL* DECEMBER 1994 27
TYPE A TYPE B TYPE C Exclusive Triple Left Turn Lane Exclusive Triple Left Turn Lane Permissive Triple Left Turn Lane (All Lanes Shadowed) (Outside Trap Lane) (Outside Lane Optional) Figure 2. Types of triple left-turn configurations. lanes with all lanes shadowed T or Y intersection configura- Type B Exclusive triple left-turn tions lanes with outside trap lane No opposing or nonconcurrent Type C Permissive triple left-turn opposing left turns lane with outside third left optional w Concurrent opposing left turns with For each configuration there are either single, double or triple left-turn numerous variations related to site- lanes specific conditions. These design varia- Design vehicle tions may include: Travel lane widths NEW DENOMINATOR TRAFFIC COUNTERS AND TALLY BOARDS Iacauered masonite board Sewing those who count since 1914 THE DENOMINATOR CO., INC., WOODBURY, CT 06798 (203) 263.3210 Intersection angle Left turns from two-way to one way or one-way to two-way streets The geometric and delineation features discussed are defined in Figure 3. Design Concerns and Advantages The geometric design of triple leftturn lanes at signalized intersections within the Las Vegas metropolitan area has raised various issues and concerns. The most common concerns include such issues as: Lane widths for receiving three travel lanes from the triple left-turn movement. Intersection width to accommodate the design vehicles turning three abreast through the left-turn maneuver. w Driver confusion and acceptance of the simultaneous three left-turn maneuver and fears of side swipe in the middle lane (Lane 2). Clearance between opposing vehicles during concurrent left-turn movements. All weather and permanent pavement markings to accommodate design vehicle tracking and turning characteristics as well as vehicle drift during the turn maneuver. Placement of offset stop bars between through and left-turn lanes. w Existing and additional right-of-way requirements for the triple left-turn intersection geometry. Vehicle weave downstream of the triple left. 28 ITE JOURNAL* DECEMBER 1994
Increased pedestrian vehicle conflict potential. Roadway width and its resulting impacts upon pedestrian crossings and signal timings. Pedestrian-vehicle clearance times and intersection capacity. The design experience and operation of triple left-turn lanes offer several distinct advantages which have promoted their further use within the Las Vegas metropolitan area: The ability to increase intersection capacity to handle a large volume of left-turn maneuvers (600 vph or more) and reduce vehicle delays and intersection queues. Reduction in upstream driveway conflicts by reducing vehicle queue lengths and resulting vehicle storage lane lengths per left-turn lane. The ability to reduce the minimum green time given to the left-turn movement so that it may be assigned to other intersection movements. intersection of Valley View Boulevard and Flamingo Road, as shown in Figure 1, was designed using the following criteria. Between its opening in January 1990 and October 1993, there have been no reported side swipe accidents associated with the center triple left-turn lane movements The geometric design elements for the development of triple left-turn lanes follow the general standards of practice for dual left-turn lanes. l The major difference is that the number of turning vehicles being handled is generally larger and the space between turning queues of traffic must be carefully evaluated. The geometric design of triple left-turn lanes within the Las Vegas Metropolitan area includes the following elements as determined during the design and development of previous facilities. Selection of Design Vehicle The intersection being evaluated for the installation of triple left-turn lanes should be classified as to the types of vehicles anticipated to make the three abreast left-turn movement. The triple left-turn lane geometries should be designed for one of the following two types of traffic conditions: Traffic Condition l Passenger vehicles with single unit trucks andlor single unit buses. The roadway is not on a truck route and truck tractor semitrailer combinations are either restricted from the roadway or are not anticipated to be within the traffic flow. The design is governed by SU) and Bus design vehicles. Traftlc Condition 2-Passenger vehicles, single unit trucks and bus with the probability of semi-trailer WB-40 or WB-SO ( WB-15 or WB-17 ) Installation Criteria Triple left-turn lane facilities have been considered not to be appropriate for installation at a signalization intersection when: There is a potential for a high number of pedestrian-vehicle conflicts. Left-turning vehicles are not anticipated to queue evenly within the provided left-turn storage lanes due to downstream conditions (this may occur where a high potential for downstream weaving exists). Conditions exist that obscure or result in confusing pavement channelization markings within the intersection. Right-of-way restrictions prohibit adequate design vehicle turning maneuver space within the intersection. The installation is not economically justified when compared with other alternatives to improve intersection capacity. Geometric Design Criteria The following geometric design criteria have been applied to the design of triple left-turn facilities within the Las Vegas metropolitan area. The,11 L SET BACK MEDIAN NOSE \ 7 1 \ D I J\ w L STOP BAR (TYP) \ APPROXIMATE OUTSlOE VEHICLE PATH,, Ilttttl[,, LANE NUMBERING I I Y I-IF(P 4- -) t o - lntersecwhangle b. LEFT TURN BAY LfNCTb t. TAPER LENC1l. Figure 3. Definitions of geometric and delineation features. d = MIN DISTANCE EEIwfEN WPOSING LEFT TURN WHICLES ITE JOURNAL* DECEMBER 1994 29
design vehicles. The triple left-turn design is governed by the WB- 50 design vehicle. Design Vehicle Turning Paths The transportation engineer can determine the turning and offtracking characteristics of the design vehicle (Traffic Condition 1 or 2) by using turning vehicle templateso,s and/or computer program models. G The design vehicles should be placed three abreast and tracked through the leftturn movements. The lateral clearance between the running design vehicles should be maintained with a minimum clearance of 2 feet (ft) on each side of the design vehicle overhang limits within the turning maneuver. Following this lateral clearance criteria the center left-turn lane (Lane 2) increases in width to accommodate offtracking of the design vehicle turning characteristics. The wider center lane reduces the potential for vehicle sideswipe by passenger vehicles when turning through the intersection (see Figure 1). The additional lane width within Lane 2 also allows for variation in passenger car driver operation. Under conditions of concurrent opposing left-turns within the intersection, a recommended 10 ft of lateral vehicle body clearance between opposing vehicles was found to be an acceptable design criterion. This clearance is measured between the opposing turning paths (Traffic Condition 1 or 2) as they pass each other (Dimension d, Figure 3). Approach and Departure Lane W@lhs Left-turn approach lane widths to a triple left-turn lane intersection have been designed at least 11 ft wide with a desirable width of 12 ft. Similarly downstream departure lane widths have been designed with an absolute minimum of 11 ft wide with a desirable lane width of 12 ft. A key factor controlling the geometry of the downstream receiving throat width is the tracking path of the design vehicle as it transitions from a circular to a tangential motion. The tracking path approximates a spiral as the design vehicle completes the left-turn movement. Therefore, the width of the clear portion of the intersection may need to be widened based on the design vehicle turning characteristics. The turning geometry may be accommodated by setting the median island nose of the receiving cross street a significant distance back from the intersection. A 2-ft offset from the vehicle turning path in Lane 1 has been used in locating the median island nose. The receiving street width at the intersection may also be widened by increasing the curb return radius of the opposite street corner. These geometric adjustments have to be carefully evaluated for the intersection angle and street widths. Due to the high volumes of vehicle traffic, raised median islands of at least 2 ft wide (4 ft desirable) are used on the approach and departure legs of an intersection with two way traffic. When wider roadway median islands are available, they provide the intersection with large radius curves to improve the intersection s left-turning geometry. A raised median island has been found to WEIGH-IN-MOTION VEHICLE SPEED VEHICLE CLASS With state-of-the-art technology, ATI now offers complete traffic monitoring at affordable prices. Using low cost, above-the-surface sensors, ATI installs portable WIM systems for temporary applications on highways, local roads and truck routes. Type of Data Collected: 0 Individual Axle Weight o Arrival Time o Gross Vehicle Weight n Vehicle Class o Axle Spacing D Vehicle Length D Vehicle Speed n Vehicle by Vehicle Recording Formore informationon ourwim equipmentandvarioustypesof surveys offered: In the US call ATI at Tel.No.: (718) 447-5161 Fax: (718) 447-7995 In Canada call OTI at Tel/Fax: (9o5) 890-1430 30 ITE JOURNAL* DECEMBER 1994
provide a driver in Lane 1 with a visual point of reference to guide a vehicle through the left-turn maneuver. A raised median island also provides delineation for the stop bar location on the receiving street. This is especially important when the left-turn lane stop bar is offset from the through movement to accommodate triple left-turn lane geometry. Determination of Storage Bay and Taper Lengths Until more information is gathered on the lane utilization and capacity characteristics of triple left-turn lanes, the storage bay length is being determined based on the same procedures now applied to dual left-turn lanesd. The storage bay length is designed based on the anticipated left-turn arrival rates, the signal cycle length and the need to provide sufficient leftturn storage bay length to prevent vehicles from queuing into the adjacent through lane. The approach taper length to the triple left-turn bay has been designed based on the roadway design speed and a local preference for reverse curves versus taper sections. The storage bay length design also considers other factors such as adjacent upstream intersections and street access driveways. Engineering Judgement One of the most important geometric design elements is the design engineer s engineering judgment. Each intersection being considered for design of triple left-turn lanes is a unique situation. The final intersection geometry is reviewed for the human element to make the intersection, through design and control devices, responsive to the characteristics and needs of drivers such that they can comprehend and interpret the facility with resulting comfort, efficiency and safety. 7 Roadway Delineation and Signage Even though intersection geometry may be adequate to accommodate three-abreast left-turn movements, roadway delineation and signage are equally important to the safe operation of the facility. Advance overhead signage of the triple left-turn lane configuration (Figure 2) is a critical element to inform motorists of the forthcoming intersection and lane options. Advance overhead signage is located at or in advance of the entrance to the left-turn lanes. The advance signage is of appropriate size, generally 30 in x 36 in to provide proper drive perception, reaction and execution time. At the Valley View/Flamingo Road intersection, which is approximately 2,800 ft west of Interstate 15, a north interstate shield was placed next to the advance overhead left-turn use control sign (R3-5) over Lane 1 while a south interstate shield was placed next to the left-turn only sign over Lane 3 (see Figure 1).8 The advance signage allows a driver completing a left turn in Lane 3 to then make a downstream right turn to access a southbound freeway on-ramp, while a driver in Lane 1 makes a subsequent left turn to access a northbound tieeway ramp. The advance signage allows approaching drivers an opportunity to separate themselves among the left-turn lanes to avoid or minimize downstream weaving. It is recommended that additional overhead signs for Type B facilities (Figure 2) are placed in advance of the intersection to provide adequate warning to drivers of a forthcoming trap lane which would force them to turn left. Overhead signage is supplemented with post-mounted triple-turn signs R3-8T modified for the appropriate configuration on the left curb side of one-way streets or on the median islands of two-way streets. A modified R3-8 triple-turn sign also is mounted on the overhead signal mast arm to further delineate the triple left-turn lane configuration. Each left-turn lane is provided with pavement markings. Left turn arrow markings separated by the word ONLY are installed upstream of the intersection at 80 ft on center spacings within each of the exclusive left-turn lanes. Turn and through lane use arrows are also provided on Type C installations to warn road users and to regulate the intersection. Raised pavement markings are used within the intersection for three-abreast left-turn movement to control the turning paths and safely guide vehicles through the intersection. An important concern when delineating triple left-turn channelization NEVER-FAIL LOOP SYSTEMS S-++ i +1 do. YEAR * +.=.+; WARRANTY L o THE LEADER 0 K TO & b TECHNOLOGY =], [p ===----: BUILT 1 LONG QUADRAPOLE FEA T URES ASPHALT-RUBBER FILLED POLYPROPYLENE FLEX/BLE/FOLDABLE CONTRACTION CONSTRUCTION. EXPANSION JO/NTS, LOOPS IN EXCESS OF 100 FT. EAS/LY HANDLED. ERMETIC ASSEMBLIES - 4 LAYERS OF MOISTURE PROTECTION. ESISTANCE TO GROUND-TYPICALLY OFF SCALE. U-STABILIZED QUALITY OF LOOP DOES NOT FALL OFF WITH TIME. + USER REFERENCES AVAILABLE NEVE&FAIL LOOP SYSTEMS 6021 S.W. 48TH AVE. PORTLAND OR 9 7221 (503) 244-6345 ITE JOURNAL* DECEMBER 1994 31
iws&m The definition of quality signs: Wells Signs & Manufacturing. sign (sin) n. [< L. signum] 4. publiclydisplayedboard, placard,etc.bearing information,usually manufacturedby Wells Signs & Manufacturing because no other company makes Fiber Optic, Blankout or Illuminated signage that last and perform as well. F!!El Wells Signs & Manufacturing ONLY a within the Las Vegas area was the number of raised channelization markers that were required within the intersection area. The amount of delineation placed within the intersection for triple left-turn movements could result in driver confusion on other intersection maneuvers. To address this concern, raised white channelization markers have been placed along the projection of the centerline and lane lines of the cross street. This placement sequence results in a varied marker spacing of approximately 6 ft during the initial portion of the left-turning maneuver, with the marker spacing increasing to approximately 15 ft as the vehicle completes its turn. Experience gained during the construction of triple left turns shows that the placement of raised pavement markers must be established by the engineer with survey control to ensure proper field placement. The placement of pavement delineation for triple left-turn lanes through an intersection is critical to the safety of the facility. The design engineer generally conducts field inspections of the intersection delineation during construction and before opening the intersections triple left-turn movement. In addition, the design engineer may elect to use temporary pavement delineation markings within the intersection during an initial trial period and make further adjustments as necessary andlor appropriate before placing the final intersection markings. Signal Design The design of the traffic signal at triple left-turn lane intersection is believed to be as important to the success of the facility as the geometric and delineation improvements. The high vehicle volumes associated with a triple left-turn lane operation require that the left-turn movement be a fully protected signal phase. Due to the potential unfamiliarity of drivers encountering triple left lanes, and in order to minimize driver confusion, all three leftturn lanes are provided with a signal indication over each turning lane. For Type C triple left-turn facilities, split phasing of the signal operation is required to prevent opposing left-turn and through movement conflicts on two-way streets. To provide left-turn signal faces over each turning lane, special mast arm and signal pole equipment with cantilever mast arm lengths of 60 ft to 70 ft have been required. These designs have resulted in special mast arm, pole and foundation designs. As an alternative to a cantilevered signal mast arm design, a signal bridge spanning the entire intersection has been considered. However, due to aesthetic concerns of the community, they have not been accepted for installation. U&!!?Ev Phone(206)699-1258 Fax (206)699-1259 Summary The described criteria used for the geometric design of triple left-turn lanes within the Las Vegas area should not be considered an exhaustive list, but as a reference guide for the design of these new and unique facilities. As with all geometric designs, site-specific conditions must be reviewed and good engineering judgement applied to each design situation. As more facilities are constructed 32 ITEJOURNAL* DECEMBER 1994
and used by the motoring public, additional research should be conducted to include topics such as: Accident rate comparisons between dual and triple left-turn lane installations. Lane utilization. Left turn lane adjustment factors. Determination of saturation flow rates for triple left-turn lanes. A comparison of left-turn capacity among single, double and triple turn lanes. The effects of downstream weaving on the uniform loading of triple leftturn storage bays and intersection leftturn capacity. References 1. Transportation Research Board, Intersection Channelization Design Guide, Report 279 (November 1985): 57-60. 2. Technical Council Committee 4L-M, The Use and Effectiveness of Double Left Turn Maneuvers, Traffic Engineering Institute of Traffic Engineers, Washington D.C. (July 1975). 3. State of Nevada Department of Transportation Data Base for Traffic Accidents, Carson City, Nev. 4. American Association of State Highway and Transportation Officials, A Policy on Geometric Design of Highways and Streets, (1990). 5. DeLeuw Cather Canada Ltd., Turning Vehicle Templates, Ottawa, Canada: Roads and Transportation Association of Canada (August 1977). 6. Graham S. Garlick, Davis N. Kanga, and Gary Glenn Miller, Vehicle Off tracking: A Globally Stable Solution, ITE Journal, (March 1993): 17-21 7. Jack E. Leisch & Associates, Planning and Design Guide AT- GRADE INTERSECTIONS, A Design Reference Book and Text for Course of Instruction, Jack E. Leisch & Associates, Evanston, Ill. 60201 (April 1988): 1.10 8. U.S. Department of Transportation Federal Highway Administration, Manual on Uniform Traffic Control Devices for Streets and Highways, (1988). I Kenneth W. A ckeret, P. E., is an associate at the Las Vegas office of Kimley-Horn and Associates He earned his bachelor s degree in civil engineering from the University of the Pacific and his master s degree from the University of Nevada. He is an Associate Member of ITE. SignaLog Helps You to:.:. Better Manage your Signal Maintenance Operation. (easier too!).:. Quickly Prepare Convincing Reports for Budgets, Personnel Changes, Equipment Purchases, etc..:. Prepare for Litigation involving Signals..:. Analyze any Maintenance Activity, FAST!.:. Plan your Maintenance Activities. Keep Complete Trouble Call and Equipment Records with Reports. I SignaLog S!gnalTek, Inc. 4919 South Yorktown Ave. Tulsa, Ok. 74105 918-749-8884 DEMO DISK AVAILABLE I 1 ITE JOURNAL* DECEMBER 1994 33