Development of Warrants for Left-Turn Lanes

Transcription

1 Transportation Kentucky Transportation Center Research Report University of Kentucky Year 1979 Development of Warrants for Left-Turn Lanes Kenneth R. Agent Kentucky Department of Transportation, This paper is posted at UKnoledge. researchreports/138

2 Research Report 526 DEVELOPMENT OF WARRANTS FOR LEFT-TURN LANES KYP-75-7; HPR-PL-1(15), Part Ill B by Kenneth R. Agent Research Engineer Principal Division of Research Bureau of Highays DEPARTMENT OF TRANSPORTATION Commonealth of Kentucky The contents of this report reflect the vies of the author ho is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official vies or policies of the Bureau of Highays. The report does not represent a standard, specification, or regulation. July 1979

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4 Technical Report Documentation Page 1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 4. Title and Subtitle 5. Report Dote Development of Warrants for Left-Turn Lanes July 6, Performing Organiation Code 7. Author! s) Kenneth R. Agent 8. Performing Organiation Report No Performing Orgoni:.:otion Nome and Address 1. Work Unit No. (TRAIS) Division of Research Kentucky Bureau of Highays 11. Controct or Grant No. 533 South Limestone Street KYP 75-7 Lexington, Kentucky Sponsoring Agency Nome and Address 13. Type of Report and Period Covered Interim 14. Sponsoring Agency Code 15. Supplementary Notes Study Title: Development of Warrants for Separate Left-Turn Lanes and Signal Phasing 16. Abstract Warrants for the installation of separate left-turn lanes ere developed. Literature as revieed, and policies and practices in other states ere surveyed. Accident analyses of locations ith and ithout separate left-turn lanes ere conducted. Computer simulation as used to determine the relationship beteen and among traffic delay and load factor and traffic volume, percent left-turns, cycle length, and cycle split. The relationship beteen left-tum accidents and conflicts as investigated. Warrants ere developed involving the folloing three general areas: (1) accident experience, (2) volumes (based on delay), and (3) traffic conflicts. 17. Key Words 18. Distribution Statement Left-Tum Lane Left-Turn Accident Computer Simulation Traffic Conflicts Delay Volumes Warrants 19. Security Clossif. (of this report) Form DOT F Security C!assif. {of this page) 21. No. of Pages 22. Price i i J Reproduction of completed page outhoried

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6 CALVIN G. SECRETARY GRAYSON COMMONWEALTH OF KENTUCKY DEPARTMENT OF TRANSPORTATION Division of Research 533 South Limestone Lexington, KY 458 JULIAN M. CARROLL GovERNOR July 6, 1979 MEMO TO: G. F. Kemper State Highay Engineer Chairman, Research Commi!tee SUBJECT: "Development of Warrants for Left-Turn Lanes"; Research Report 526; KYP-75-7; HPR-PL-1(15), Part III-B This report is one of a series concerning the left-turn problems at intersections. The preceeding reports ere: No. 446; K. R. Agent, "A Survey of the Use of Left-Turn-on-Red"; May No. 456; K. R. Agent, "Development of Warrants for Left-Turn Phasing"; August No.519; K. R. Agent, " An Evaluation of Permissive Left-Turn Phasing"; April l979. In this study, arrants ere developed for use as guidelines in determining hen the need for left-turn lanes becomes critical. The addition of left-turn lanes alays provides an improvement in the traffic flo; hoever, left-turn lanes cannot be built at all locations. The recommended arrants involved accident experience, traffic volumes, and traffic conflicts.,j (je tfully su mitt, gh Enclosure cc's: Research Committee.Havens Director of Research.

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8 INTRODUCTION A vehicle stopped in the traffic stream to turn left creates an accident potential and impedes the flo of through traffic. On divided highays ithout grade separation at crossings, a considerable reduction in accidents has been accomplished here the median as of sufficient idth or could be idened so that left-turn lanes could be built. In locations here a \eft-turn lane cannot be cut into or substituted for the median, some form of flush median, delineated on the roaday to separate opposing streams of traffic and to mark separate turning lanes, has been used. The addition of left-turn lanes alays provides an improvement in the trafflc flo; hoever, left-turn lanes c:mnot be built at all locations, and arrants have not been established for determining hen the need for left-turn lanes becomes critical. This study as part of a larger study of the lefttun) problem at intcrseclions. Warrants for the addition or left-turn phasing ere developed (1), and a i.l survey of the use of left-turn-on-red as made (2). In this study, arrants or guides ere developed for installing separate left-turn lanes. Computer simulation as used to determine the relationship beteen traffic delay and such variables as pcrcen tage left-turns, traffic volume, and cycle length. Accident data ere compared at locations ith and ithout left-turn lanes, and the average number of left-turn accidents for approaches ith no left-turn lane as determined. The relati1l$hip beteen left-turn accidents and conflicts as also investigated. Using these sources of input, criteria for determining needed left-turn lanes ere derived. REVIEW OF LITERATURE Traffic accidents, delay, benefit-cost ratios, and left-turn capacities have been used to justify adding the left-tum lane. TI 1 e number of left-turn and rear-end accidents in a certain time period as resolved as a arrant in one instance (3). Unsignalied intersections having a total of four or more left-turn plus rear-end accidents in 12 months (involving vehicles from intersection legs to be channelied) or six or more left-turn plus rear-end accidents in a 24-month period qu<jlified. Several arrants ere tested at locations here left-turn lanes had been added. The <Jrrant or criterion yielding the most cost effective results as selected. An Index of Haard as developed in another study (4). It as based on the difficulty of a vehicle making a left-turn due to the lack of gaps in the oncoming_ vehicles and the physical features of the intersection. The Index of Haard (I.H.) as stated mathematically as follos: I.H. = V L V (I+ F e + F e + in hich = 8 hour maximum volume of left-turning vehicles, through movement in opposition to the left-turn movem men! during the same 8-hour period, clearance idth factor (representing the increased haard to left-turning vehicles crossing more than one lane of opposing traffic), escape idth factor (measuring the usable shoulder area for an overtaking vehicle to bypass to the right of a left-turning vehicle), F sa sight distance ahead factor, F so sight distance overtaking facm tor, F s through vehicular speeds factor, and F111 = miscellaneous factors. The Oregon State.Highay Department used this Index of Haard to convert the original relative arrant to one independent of construction costs. The folloing formula as used: R.W. = [I. H. (I + A p )J /124, in hich R.W. = relative arrant and number of preventable accidents in a SMyear period. This arrant as used as a guide hen comparing several alternative construction locations. Computer simulation has been used to develop arrants for left-turn channeliation (5). Probability curves ere developed to determine the delay likely to occur for a given set of conditions. The variables included the approach and opposing traffic volumes, percentage of left-turns, and traffic signal timing. Delays ere given in terms of the percentage of all inside lane vehicles delayed more than one signal cycle. By selecting the level of delay hich ould be permitted, probability charts indicate if a left-turn lane should be provided. ''

9 In another study, volume-based arrants ere determined for left-turn storage lanes at unsignalied, at-grade intersections (6). Charts developed from theoretical analyses and field studies included opposing and advancing volumes, percentage left-turns, number of lanes, and speed. Benefit-cost ratios have been used to develop guidelines for inclusion of left-tum lanes at rural highay intersections (7). Field data ere analyed by multiple regression to obtain equations for predicting stops and delays. Benefits to road users by reducing stops and delays to through and right-turning vehicles ere added to the potential reduction in accident costs. These road-user benefits ere then compared to the cost of providing a left-turn lane to detennine the cost effectiveness of the construction. Another study as based on benefits and costs as a method of establishing need and feasibility of constructing a median lane (8). Multiple linear regression as used to develop expressions for predicting the seconds of delay per hour caused by left-turning vehicles to through vehicles and the number of accidents per million vehicles caused by left-turning vehicles at approaches to intersections in both rural and suburban areas. The benefit-cost analysis indicated that construction of median lanes as arranted at almost every intersection on a divided highay having a median idth of 16 feet (4.9 m) or more and many intersections on other four- and to-lane highays. The goals of one study ere to assess the benefits of left-tum storage lanes in terms of accident reduction and to develop predictive equations for use in benefitcost arrants (9). It as found that left-tum lanes had no significant effect on rates for accidents involving left-turning velticles, but some significance as observed ith respect to total accident rates for gross classes of approaches. Models to predict the total number of accidents ere developed Elsehere (1), arrants ere applied to signalied intersections on four-lane, major arterial streets. It as found that left-tum lanes are arranted hen one of the folloing criteria is met: (I) more than to accidents per year are caused by left-turning vehicles, (2) hen there is at least one [eftturn per green interval for 75 percent or more of all green intervals in a peak hour, or (3) hen the left-turn lane ould provide the desired level of service. A procedure based on left-turn developed (11) to determine if a left-: arranted at a signalied intersection l on the street is permitted to move simul1 common green indication. The total opp green-time-to-cycle-length ratio, and nurr ing through lanes ere used to estimate t a left-turn movement here no left-tun tected signal phase is provided. If the lef exceeds 8 percent of the estimated ca turn lane is arranted. A level of s assumed. The method of calculating the cap: tum Jane as developed by Leisch (12) a for the procedure taught by the Traffi Northestern University (13). The desig the left-turn lane (the larger of values c to charts) is determined for the sit: separate left-turn lanes are provided b separate signal indication is provided. only the cycle length and the assump vehicles ill tum left on the amber a each cycle to determine the design c chart ould govern conditions ith he volumes hen most left-turns ould hav during the amber light. Another chart gil design capacity here the opposing thrm relatively small. This chart uses volume, ratio of green time to cycle ler percentage of trucks and buses to deterrr After choosing the level of service at hi lane is needed, the left-turn capacity plied by the appropriate factor to deterrr The level of service describes the quali flo on a particular approach to the inters The Highay Capacity Manual al procedure 'for determining the capacity turning lanes having no separate signal, In this procedure, the service volume o lane (of adequate length) is given (in passenger cars) as the difference bet vehicles and the total opposing traffic volu of passenger cars per hour of green, but to vehicles per signal cycle. This proce basis of the Leisch nomographs (ith tl that minimum vehicles per signal cycle : 1.6). 2

10 SURVEY OF OTHER STATES Only six of the 45 states responding to an inquiry. listed definite arrants. The arrants ere as follos: (I) When an intersection is designed, left-turn lanes are provided henever left-turning volume exceeds I vehicles during the peak hour. (2) When the individual movement is 25 vehicles or more per hour, a separate turning lane is arranted. (3) A. On multilane, divided highays, leftturn lanes are arranted: hen the design speed is 4 mph (a) (17.9 m/s) or higher. (b) if the access point serves an industrial, commercial, or a substantial trip-generating area or if the access point serves more than three residential units. (c) at all median openings. B. On to-lane highays, left-turn lanes are: (a) not normally provided here the 2- year projected annual average daily traffic (AADT) is under I,5 or the design hour volume (DHV) is under 4. (b) provided hen the access is to a public road, an industrial tract, or a commercial center. (c) provided hen there are more than five accidents per year involving leftturning vehicles. (d) provided hen the projected toay DHV exceeds 7. (4) Controlled median openings ith leftturn lanes are constructed: (a) for public roads and dedicated streets hich arc open and in use. (b) for drive-in theaters. (c) for shopping centers hich provide off-street parking for I cars. (d) for hospitals, schools, industrial complexes, and cemeteries. Openings arranted under b, c, and d ould not be spaced less than 33 feet from any other median opening. (5) Left-turn bays are provided on the main roaday here side-road volumes are in excess of an AADT of 1. (6) At unsignalied locations, the procedure outlined in "Volume Warrants for Left-Turn Storage Lanes at Unsignalied Grade Intersections'' is used (6). At signalied locations, nomographs produced by the Traffic Institute at Northestern University are used (13}. Many states expressed the optmon that, on divided highays here sufficient right of ay exists, left-turn lanes are arranted herever a left-tum can be made. Some respondents indicated that left-turn lanes are provided at all median openings on fourlane divided highays ith no control or semi-control of access as ell as at all intersections of major routes on partially controlled access routes. Another respondent said that left-turn bays are constructed at each city street intersection, here practical, on urban projects ith four or more lanes. Hoever, this type of construction is limited by availability of funds; so analyses may be conducted to determine the locations hich ill yield the greatest benefits. Although fe specific arrants ere listed, nearly all of the states gave guidelines (both general and specific) hich ere used to justify separate leftturn lanes. A list of the general guidelines (areas hich should be considered) follos: (I) (2) (3) (4) (5) (6) (7) (8) (9) (1) (I I) (I 2) accident experience, main-line volume, cross-traffic volume, left-turn volume, available right of ay, benefit-cost ratio, capacity analysis, sight distance, speed limit, geometries, left-turn, rear-end conflicts, delays, (13) gaps, (14) effect on surrounding intersections, (IS) opposing volume, (16) queue lengths, type of facility, number of opposing lanes to cross, and left-turn volume versus opposing through volume. Several states gave specific guidelines or methods hich they used. There as a ide range in the volumes necessary to justify a left-turn lane. A summary of guidelines used to justify a separate leftturn lane follos: (I) Opposing AADT of 85 or more and leftturn volume equal to at least 25 percent of the opposing volume, (2) Left-turn volume of at least 25 vehicles per hour (to-lane streets), (3) Left-turn volume of at least 25 vehicles per hour opposed by a volume of at least 6 (four-lane streets), (4) Left-turn AADT of 5 or more (divided highays here funds are not available to construct all the left-turn lanes), 3 (17) (18) (19)

11 (5) At signalied intersections herever possible unless the approach has very little left-tum traffic (AADT of under 5), (6) In urban or rural areas here a continuous median of sufficient idth (usually 16 feet (4.9 m) or greater) is available, one or to accidents ould justify the minor construction, (7) I left-tum vehicles during the peak hour (in urban areas), (8) In rural areas, left-tum volume of at least 3 vehicles during the peak hour plus a related accident experience, (9) A rural intersection accident rate higher than 12 per 1 million entering vehicles, (1) Left-tum volume equal to 1 percent of the total intersection volume, (11) Sum of left-tum and opposing volume equal to 8 vehicles during the peak hour, (12) Side-road volume of 5 or more per day on a ne to-lane highay ith a design speed of 5 mph (22 m/s) or greater, (13) Signalied intersections here left-tum signal phasing is required (non-divided roadays), (14) High percentage of left-turning vehicles (2 percent or greater), and (15) Left-tum volume of 2 vehicles per hour, (16) A DHV value of approximately 1 vehicles making a left tum. To states listed guidelines for the use of double left-turn lanes. They ere as follos: (I) When left-turn volumes exceed 3 vehicles in the peak hour. (2) When the left-tum volume exceeds about 1,5 ADT. ACCIDENT DATA PROCEDURE The data base used here consisted of several years of accident analyses of intersections in Lexington. These analyses, including collision diagrams, ere avail able for the years 1968 through Accident rates at locations ith and ithout left-tum lanes ere cal- culated. This as done using sections for a 12-hour period assumption as made that 8( occurred in this 12-hour peri then multiplied by 1.25 to obt Using the same data bas left-tum accidents for the app lanes as calculated. The ave1 as used to calculate a criti accidents. A computer summary valved a left-turning vehicle i as also obtained. Comparisc dents and conflicts as ell as TRAFFIC VOLUME Computer simulation v lationships beteen traffic ' traffic volume, percentage lei cycle split. The simulation UTCS-1 Netork Simulation Federal Highay AdministJ intersection as input into runs ere made assuming b signal control. When a signal as spec cycle split ere given. Durin the side street of a semi-acl heavy that a ftxed cycle Data ere simulated for a: lane and a to-lane street. for both main street apprc left turns ere varied on onc approach had 1 percent o Cycles of 6, 9, and 12 splits of 7/3 (7 percent main street), 6/4, and 5, speed of 45 mph (2 m/s) and the load factor ere approach. Load factor is d< total number of green-sign utilied by!raffle during t number of green intervals fc same period (14). Its maxim 4

12 Graphs ere dran relating the variables to critical delay and load factors. The critical delay as found to be 3 seconds. This as found using a procedure given in another report (16). In that study, engineers ere asked for their opinion of hat constituted maximum tolerable delay for a vehicle controlled by a traffic signal. A mean value of 73 seconds as found. A criterion that 85 percent of all the left-turn approach vehicles be delayed less than this maximum level of 73 seconds Was then used. Assuming the distribution of delays becomes approximately nonnal during peakflo conditions, the folloing formula can be used: 85th percentile= X u in hich 85th percentile value of delay of the 85th percentile of the normal distribution (73 seconds), X = mean value of delay, and u = standard deviation of the distribution. The assumption as made that the standard deviation as approximately equal to the mean. Substituting these values gave a value of 3 seconds for the mean delay. Thirty seconds as used as the minimum average delay necessary because this value constituted the loer bound of excessive delay. A critical load factor of.3 as used because it represents the upper bound of level of service C ( 14), the upper limit of stable flo. Level of service D represents a one of increas ing restriction approaching instability. An additional procedure as used for simulation of non-signalied intersections. One hundred percent of the volume on one approach turned left hile 1 percent of the volume on the opposing approa h ent straight through the intersection. Volume on the leftturn approach as held constant hile the opposing volume as changed. This permitted a plot of leftturn delay as a function cf the left-turn and opposing volumes. Data ere simulated for an intersection on a four-lane and a to-lane street. The UTCS-1 model has been tested 1md validated. One test dealt specifically ith the response of the model to variations in primary and opposing flo levels and left-turn percentages. The tests indicated that the model performed realistically ith regard to left-turns at intersections. The delay per vehicle includes deceleration and acceleration as opposed to the stopped-time delay only. CONFLICT DATA Conflict counts involving left-turn vehicles ere taken at several intersections and related to the number of left-turn accidents and traffic volumes. The conflicts ere classified into several categories (17, 18). Basically, there ere four types of left-turn related conflicts. The first occurred hen a left-turning vehicle crossed directly in front of or blocked the lane of an opposing through vehicle (opposing left-turn conflict). The second as caused by a vehicle aiting to turn left (rear-end type). A third as a eave resulting hen a vehicle, evading a left-turning vehicle ahead, veered into the path of another vehicle. The fourth involved running the red light. An attempt as made to classify the conflicts according to severity. Hoever, in the analysis, no distinction by severity is made because of inconsistency of data taken by different observers. RESULTS ACCIDENT WARRANT Accident Rates at Intersections ith and ithout a Left-Tum Lane -- Using the Lexington data base, accident rates (left-turn accidents per million leftturning vehicles) ere calculated for intersections ith and ithout!eft-turn lanes (Table 1). Left-turn-related accidents ere based on the folloing definitions: (1) hen a left-turning vehicle turned into the path of an oncoming vehicle, (2) hen a!eft-turning vehicle as struck in the rear hile aiting to turn, or (3) hen a vehicle eaving around a vehicle stopped to turn left as involved in an accident. TABLE 1. COMPARISON OF ACCIDENT NO SIGNAL NO RATES AT LOCATIONS WITH AND LANES LEFT-TURN LANE WITH LEFT-TURN LANE WITH. SIGNAL NO LEFT-TURN LANE WITH LEFT-TURN WITH LEFT-TURN AND PHAS I :-.JG WITHOUT LEFT-TURN LANE LANE ACCIDENT RATE!LEFT-TURN ACCIDENTS PER MILLION LEFT TURN VEHICLES)

13 The left-tum accident rate dropped significantly for intersections ith left-turn lanes. For unsignalied intersections the left turn accident rate as 77 percent loer. The rate as 54 percent loer at signalied intersections. At signalied intersections, the rate dropped even loer hen left-tum phasing as added. Critical Left-Tum Accident Number -- Using the Lexington data, the average number of left-turn accidents for the approaches ith no left-turn lanes as calculated. Separate averages \1-.rere calculated for intersections ith and ithout signals. Using the average number of left tum accidents, the critical number of accidents as also detennined. For unsignalied intersections, the average number of accidents as found to be.8 left-tum accidents per approach per year. This corresponded to an average of 1.2 at signalied intersections. The difference as probably due to higher volumes at signalied intersections. The formula used to determine the critical number of accidents as derived from a formula for the average, critical accident rate (1): = in hich critical number of accidents, average number of accidents, and K constant related to level of statistical significance selected (for P =.95, K = 1.645; for P =.995, K = 2.576). For P =.995, the critical number of left-turn accidents in I year for an approach as found to be four at an unsignalied intersection and five at a signalied intersection (Table 2). TABLE 2. NUMBER OF LEFT-TUR ACCIDE TS NECESSARY TO BE A CRITICAL INTEKSECTION lllne APPROACH} NUMBER OF LEFT-TURN ACCIDENTS lone YEAR} NO SIGNAL!TH SIGNAL 4 5 VOLUME WARRANT Excessive Delay at a Signalied Intersection - The computer simulation as used to d termine the delay on an approach as a function of the opposing volume, percentage left turns on the subject approach, cycle length, cycle split, and number of opposing lanes. While all other variables ere held constant, the percentage left turns as increased, resulting in relation ships shon in Figure I. The delay per vehicle on the left-turn approach increased as the percentage of left turns increased. The critical delay as found previously to be 3 seconds. As shon in Figure I, this critical delay as reached at various percentage left turns as a function of the opposing volume. For this example, the critical delay as reached at three percent left turns for an opposing, peak-hour volume of I,2 vehicles. This compared to the critical delay at about 2 percent left turns hen the opposing peak hour volume as 8 vehicles. The points at hich delay became excessive ere taken from data such as shon in Figure I and plotted as best-fit lines. One of the relationships found is in Figure 2. Given the cycle length and split and the total peak-hour, main-street volume (peak hour, both directions), the percentage left turns on an approach necessary to create excessive delay could be found. In Figure 2, for a n ain-street volume of 1,6 vehicles and a 6/4 cycle split, 19 percent left turns ould be the point at hich delay becomes excessive. Plots, such as Figure 2, ere dran for 6-, 9-, and 12-second cycle lengths for to- and four-lane highays. These figures are given in APPENDIX A. 6

14 9 «" " 8 ".! en 1 " -' (.) > GO "... ;: du :I: 5 >n. a:" <( 4 n.z ".... ao,a,.a: <{::> _,I-.!- 3 au. --' 2 :: 1-- oo PERCENT LEFT TURNS ON THE LEFT APPROACH 2 25 Figure I. Relationship of Approach Delay to Opposing Volume and Percentage Left Turns (Four-Lane Highay, 9-Second Cycle, 6/4 Cycle Split). 1 CYCLE SPLIT EQUATION,, 9 7/3 y =942e' 21X BX 6/4 p731 e /5 y«ll66e -.3'2X.99 a: ::> "- --' (.) a: 4 n \ TOTAL MAI N STREET VOLUME!PEAK HOUR) Figure 2. Percentage Left-Turns When Delay Becomes Excessive (Four-Lane Highay, 9-Second Cycle). 7

15 The total main-street volume as used because the volumes on both the left-tum and opposing approaches ould be factors in determining here delay becomes excessive. Equal volumes ere input for both approaches. This as done since it ould have taken a prohibitive number of computer runs to consider all possible combinations of volumes. The data shon in Table 3 indicate that using equal volumes on both approaches gives a reasonable approximation of the dday hich ould result from different volume combinations. Therefore, given the necessary input, the figures given in APPENDIX A give a critical volume arrant for a left-tum lane at a signalied intersection based on excessive delay. TABLE 3. VARIANCE OF DELAY PER VEHICLE OPPOSING VOLUME BOO 13 ON AS THE LEFT-TURN APPROACH VOLUMES VARY* LEFT-TURN APPROACh VOLUME BOO 12 7 DELAY PER VEHICLE ON LEFT TURN APPROACH I SECONDS I OALL COMBINATIONS OF OPPOSING AND LEFT-TURN VOLUMES YIELD A TOTAL OF 2 VEHICLES IN A ONE-HOUR PERIOD. Excessive Load Factor -- The critical load factor used as.3. This value represents the upper bound of level of service C, hich is the upper limit of stable flo. The same procedure as used to relate the critical load factor to the variables under consideration as as used for excessive delay. Percentage left turns ere increased hile holding ali other variables constant, giving relationships such as plotted in Figure 3. For this example, the critical load factor as reached at 3.5 percent left turns for an opposing peakhour volume of 1,2 vehicles. This compared to the critical load factor at 22.5 percent left turns hen the opposing peak-hour volume as 8 vehicles. It should be noted that the volumes necessary to exceed a load factor of.3 ere slightly higher than those necessary to exceed the critical delay. Data such as plotted in Figure 3 ere plotted as best-fit lines to produce relationships as shon in Figure 4. The graphical procedure relating an excessive load factor to the variables considered as identical to that used hen excessive delay as considered. In Figure 4, for a main-street peak-hour volume of I,6 vehicles and a 6/4 cycle split, 23 percent left turns ould be the point at hich the load factor becomes excessive. Plots such as Figure 4 ere dran for 6-, 9-, and 12-second cycle lengths for to- and fourlane highays and are presented in APPENDIX B. These plots proyide a critical volume arrant for a left-turn lane based on an excessive load factor. Unsignalied Intersection -- Critical volume arrant curves based on excessive delays using a procedure similar to that for signalied intersections are given in Figures 5 and 6. The excessive delay criterion used for signalied intersections as 3 seconds. It ould be logical that a loer delay ould constitute excessive delay at an unsignalied intersection. Therefore, a curve representing a delay criterion of 2 seconds is included in Figures 5 and 6. Hoever, there as only a small difference in the to curves. Higher volumes are necessary to create a critical condition at an unsignalied site compared to one signalied. Another procedure as also used for simulating delays at a nonsignalied intersection. In this procedure, the computer input specified that 1 percent of the volume on the left-turn approach turned left hile 1 percent of the opposing volume ent straight through. Delay to the left-turn vehicles as determined as the left-turn volume as held constant hile increasing the opposing volume (Figures 7 and 8). The point at hich left-turn delay started to increase drastically represents the point at hich a left-turn lane should be considered. Sum of Left-Tum and Opposing Volumes -- The minim1 m sum of peak-hour left-turn and opposing volumes, hich resulted in a critical left-turn delay, as determined (Table 4). To obtain these results, figures contained in APPENDIX A ere used for signalied intersections, and Figures 5-8 ere used for nonsignalied intersections. This table represents a simpler volume arrant hich may be used to determine if further investigation is needed. The volumes there ould tend to be loer than those given in the previous figures; it represents the minimum volumes necessary to create a left-turn delay problem. Of course, a minimum number of left-turns, such as 5 left turns per hour, ould be necessary. 8

16 ir " ",. ".6 > : f- u.5 D ".4...J,o PERCENT LEFT TU RNS 2 25 Figure 3. Relationship cf Load Factor to Opposing Volume and Percentage Left Turns (Four-Lane Highay, 9-Second Cycle, 6/4 Cycle Split) CYCLE SPLIT EQUATION r -.196X 7/3 y=757 e X 6/4 y =727e -.34X 5/5 y=l377e => f- f- "-...J f- u : a TOTAL MAIN STREET VOLUME {PEAK HOUR) Figure 4. Percentage Left-Turns When Load Factor Becomes Excessive (Four-Lane Highay, 9-Second Cycle). 9

17 1 EXCESSIVE DELAY CRITERION EQUATION r2 9 3 SECONDS -.225X Y"1625e SECONDS Y"l893 ; -D244X.96 : => "- _J 1-- : a BOO TOTAL MAIN STREET VOLUME (PEAK HOUR) Figure 5. Percentage Left-Turns When Delay Becomes Excessive (Four-Lane Highay, No Signal). 1 EXCESSIVE DELAY CRITERION EQUATION r2 9 3 SECONDS -.55X y:75 38e SECONDS,.. s,9 tse oo4ss.96 ; : => "- _J 1-- : a TOTAL MAIN STREET VOLUME (PEAK HOUR} 2 1 Figure 6. Percentage Left-Turns When Delay Becomes Excessive (To-Lane Highay, No Signal).

18 9 8 -., 7 frl GO ' ul -' 9 5 :< g;! : 4 ::> f-,!. "- 3 -' 2 >-.. -' 1 GOO 7 BOO 9 1 OP POSING VOLUME (PEAK HOUR) Figure 7. Left-Tum Delay as a Function of Opposing and Left-Tum Volume (Nonsignalied Intersection, Four Lanes). 9 8 u; 7 (.) W GO -' 5 :< > 4 : ::> f- 3 "- -' 2 1i -' 1 5 GOO OPPOSING VOLUME {PEAK.HOUR) 1, Figure 8. Left-Tum Delay as a Function of Opposing and Left-Tum Volume (Nonsignalied Intersection, To Lanes). ll

19 TABLE 4. SUM OF LEFT-TURN AND OP POSIN G VOLUMES DURING THE PEAK HOUR NECESSARY TO CREATE A LEFT-TURN DELAY PROBLEM'; SIGNALIZED INTERSECTION!FOUR-LANE HIGHW AY) CYCLE SPLIT CYCLE LENGTH 7/3 6/4 5/ SIGNALIZED INTERSECTION!TWO-LANE HIGHWAY! CYCLE SPLIT CYCLE LENGTH 7/3 6/4 5/ NON-SIGNALIZED INTERSECTION DELAY CRITERION 3 SECONDS 2 SECONDS FOUR-LANE HIGHWAY 1 9 TWO-LANE HIGHWAY 9 BOO *ASSUMING A MINUMUM LEFT-TURN VOLUME SUCH AS 5 LEFT-TURNS IN THE PEAK HOUR. i 12

20 TRAFFIC CONFLICTS WARRANT Traffic conflicts at 25 intersection approaches not having a separate left-turn lane ere observed for three peak hours at each approach. In most instances, the data collection periods consisted of one morning rush hour (7:3 to 8:3a.m.) and to afternoon rusli hours (3:3 to 5:3p.m.). The peak hours ere found from traffic volume counts and varied from location to location. Data ere recorded on forms developed for conflict studies ( 18). All of the conflict types ere recorded; hoever, only those relating to left-tum accidents ere considered in the analysis. Those conflicts included in the analysis ere as follos: (I) opposing left-tum, (2) eave (involving left-turning vehicle), (3) sloed-for-left-tum, (4) previous-left-tum, and (5) ran-red-light (turning left). Further descriptions of these conflict types are given in APPENDIX C. The sum of these five conflicts as refimed to as the total left-turn-related conflicts. The 25 intersection approaches ere divided into to groups based on hether they met the previously developed accident arrant. Seven approaches did. The number of accidents used as the highest!-year number of accidents at a particular approach. The average number of left-turn-related conflicts as determined for the to groups of locations. Six of the approaches ere at unsignalied intersections. These approaches ere not analyed separately because there ere very fe conflicts directly involving the traffic signal (ran-red-light conflict). Also, six of the approaches ere on to lane streets. These approaches ere not analyed separately since eave conflicts ere not a high proportion of the total. A summary of the number of conflicts found at locations hich did and did not meet the accident arrant is given in Table 5. For each conflict type, the averages of the number of conflicts found in the highest hour as ell as all three hours for each approach ere summaried. Also, the 95th-percentile confidence interval as calculated for each average value. The sloed-for-left-turn type of conflict occurred most often. It as folloed in frequency by the previous-left-tum and opposing-left-turn conflicts. There as a smaller number of eave conflicts and a very small number of ran-red-light conflicts. The nuncber of conflicts as substantially higher at locations hich met the accident arrant. Hoever, there as a very large range in the data, as shon by the confidence intervals. An interesting comparison can be made beteen the upper bound of the con fidence interval for the locations hich did not meet the accident arrant and the average value at locations hich did meet the accident arrant. With the exception of the ran-red-light conflict, the average value for locations meeting the arrant as above the upper bound of the confidence interval for locations not meeting the arrant. This indicates that using these average values as a guideline ould not identify locations ith a lo accident potential. Hoever, some potentially high-accident locations could be missed. A determination of hich types of conflicts to use in a traffic conflicts arrant must also be made. To benefit from all data available, it ould be logical to include the total of all related conflicts in any arrant or guideline. In addition, any one type of conflict found to relate more to the accident potential should be included. Most accidents involved a left-turning vehicle turning into the path of an opposing vehicle. Therefore, the opposing left-turn conflict could be used as a guide. To determine hich types of conflicts related most directly ith accidents, equations of the best-fit lines relating left-turn accidents and left-tum-related conflicts ere determined (Table 6). When each approach as treated as a separate point, very poor relationships ere found, as indicated by the coefficients of determination (r 2 ). The highest r 2 as. 29. The equation shoed that the total conflicts and opposing-left-turn conflicts related best to accidents. The locations ere also grouped by the number of accidents and related to conflicts. Five accident groupings ere used. There ere four locations having no accidents, four ith one, seven ith to, four locations ith from three through five accidents, and six ith six or more accidents. Much better relation ships ere found hen this procedure as used. Substituting the number of accidents necessary to arrant a sigual into the equations provided another procedure for determining critical traffic conflict numbers. Five accidents ere used as input into the equations. Almost identical results ere obtained for both groups of equations. A summary of several alternate methods of developing traffic conflict arrants or guidelines is given in Table 7. These methods give similar results. Using both total conflicts and opposing-left -turn conflicts as guidelines ould provide a suitable procedure. The total left-turn-related conflicts provides maximum input; on the other hand, opposing-left-turn conflicts are the most severe and are the most representative of the type of accidents hich have occurred. 13

21 TABLE 5. COMPARISON OF CONFLICTS AT. LOCATIONS WHICH DID AND DID NOT MEET THE ACCIDENT WARRANT ' i LOCATIONS MEETING LOCATIONS NOT MEET! NG ACCIDENT WARRANT ACCIDENT WARRANT CONFIDENCE CONFIDEN CE INTERVAL INTERVAL TYPE OF (95TH (95TH CONFLICT AVERAGE PERCENTILE I AVERAGE PERCENTILE I TOTAL a PEAK HOUR b 45 AVERAGEc 't OPPOSING LEFT TURN PEAK HOUR AVERAGE SLOWED FOR LEFT TURN PEAK HOUR AVERAGE PRE VIOUS LEFT TURN PEAK HOUR AVERAGE WEAVE d PEAK HOUR l - 3 AVERAGE ol RAN RED LIGHT d PEAK HOUR.57 - lo3 o.so AVERAGE o.s -. 7 TOTAL OF LEFT-TURN RELATED CONFLICTS b AVERAGE OF THE HIGHEST NUMBER OF CONFLICTS FOUND IN ONE OF THE THREE PEAK HOURS STUDIED FOR THE LOCATIDNS CAVERAGE OF THE NUMBER OF CONFLICTS FOR THE THREE HOURS FOR EACH LOCATION d JNVOLVING LEFT-TURNING VEHICLES PEAK 14

22 TABLE 6. RELATIONSHIPS BETWEEN LEFT- TURN LEFT-TURN RELATED CONFLICTS ACCIDENTS AND EAC H LOCATION LOCATIONS GROUPED BY TREATED SEPARATELY NUMBERS OF AC CI DENTS TYPE OF CONFLICT EQUATION R 2 EQUATION R 2 TOT Al b PEAK HDUR y = X.18 y = X.85 AVERAGE d y = X Oo21 y = X o.8o OPPOSING LEFT TURN PEAK rlour y = lob.91x.26 y = lo6 o.eax.59 AVERAGE y =.64.66X.29 y =.6.64X o. 71 SLOED FOR LEFT TURN PEAK HOUR y = X. 16 y = 1o.o + 2. X.87 AVERAGE y = s.o l. 1 X.15 y = X o. 71 PREVIOUS LEFT TURN PEAK HOUR y = 5.6 = lol X. 19 y X.62 AVERAGE y = X.15 y = 3.4 o.1ax.51 WEAVE PEAK HOUR y = X Ool8 y =.95 + o.sox o.a2 AVERAGE y =.75 o.nx.13 y =.45 O. 25X.86 RUN RED LIGHT" PEAK HOUR y = = o.5a, 2X o.oos y.55 O.OlX o.oo3 AVERAGE y = X.2 y =.31 - O.OlX.2 ax = NUMBER OF ACCIDENTS y = NUMBER OF CONFLICTS b TOTAL OF LEFT-TURN RELATED HIGHEST NUMBER OF CONFLICTS CONFLICTS IN THE THREE PEAK HOURS STUDIED d AVERAGE OF THE THREE PEAK HOURS STUDIED rnvolving LEFT-TURNING VEHICLE 15

23 TABLE 7. METHODS OF DEVELOPING TRAFFIC CONFLICT WARRANTS OF GUIDELINES CRITICAL TRA FFIC CONFLICT LEVEL FOR GIVEN METHOD UPPER LEVEL AVERAGE VALUE OF CONFIDENCE INTERVAL AT SUBSTITUTING FIVE ACCIDENTS AT LOCATIONS LOCATIONS NOT INTO EQUATION TYPE OF MEETING ACCIDENT MEETING ACCIDENT RELATING CONFLICTS CON1'LICT WARRANT WARRANT AND ACCIDENTS TOTAL a PEAK HOURb AVERAGE' 3 2b 2b OPPOSING LEFT TURN PEAK HOUR b.o AVERAGE SLOWED FOR LEF T TURN PEAK HOUR AVERAGE PREVIOUS LEFT TURN PEAK HOUR AVERAGE WEAVE d PEAK HOUR AVERAGE 2.2 lob 1.7 a TOTAL OF LEFT-TURN RELATED CONfLICTS b THE HIGHES T ONE-HOUR NUMBER OF CONFLICTS c AVERAGE NUMBER OF CONFLICTS IN THE THREE PEAK HOURS d INVOLVING LEFT-TURNING VEHICLE 16

24 Based on these sources of input, the folloing arrant as developed: add a left-turn lane hen a conflict study shos an hourly average of 3 or more total left-turn-related conflicts or 6 or more opposing-left-turn conflicts in a 3-hour study period during peakmvolume conditions. Also, consider adding a lane if 45 or more total left-turn-related conflicts or 9 or more opposing-left-turn conflicts occur in any! hour period. SUMMARY AND CONCLUSIONS 1. Fe states use numerical arrants for the installation of left-turn lanes; hoever, most use some type of guideline. The guidelines ere usually based on either accidents, volume, or delay. 2. Left-turn accident rates ere found to be significantly loer at intersections having left-turn lanes compared to those ithout left-turn lanes. This flnding applied to both signalied and unsignalied intersections. 3. The critical number of left-turn accidents in one year necessary to arrant installation of a leftturn lane as found to be four at an unsignalied intersection and five at a signalied intersection. 4. Critical volume arrant curves for a leftturn lane at a signalied intersection ere developed on the basis of excessive delay. Using a critical delay of 3 seconds per vehicle, plots ere developed giving percentage left-turns necessary to create excessive delay as a function of total main-street volume. Plots ere dran for various cycle lengths and cycle splits for to-lane and four-lane highays (APPENDIX A). 5. Figures similar to those cited above ere developed to give a critical volume arrant for a leftturn lane based on an excessive load factor (APPENDIX B). A critical load factor of.3 as used. 6. The volumes necessary to arrant installa lion of a left-turn lane ere slightly higher hen based on an excessive load factor than hen based on excessive delay. 7. Critical volume arrant based on excessive delays ere developed for unsignalied intersections (Figures 5 and 6). 8. An alternate type of volume arrant as based on the minimum sum of peak-hour left-tum and opposing volumes necessary to create a critical left turn delay (Table 4). These volumes represent the loer bounds of the volumes necessary to create a left-turn delay problem and may be used to decide if further investigation is needed. 9. Traffic conflict studies ere conducted at intersection approaches hich did not have a separate left-turn lane. The data shoed that the average number of left-turn-related conflicts as higher at locations hich had a higher number of left-turnrelated accidents. Hoever, there as a very large range in the data, as shon by the confidence intervals hich ere found. 1. Equations of the best-fit lines relating leftturn accidents and left-tum conflicts ere determined (Table 6). When each approach as treated as a separate oint, very poor correlations ere found. The highest r as.29 hen only the opposing-left-turn conflict as considered. Hoever, much better correlations ere found hen the locations ere grouped by number of accidents. 11. A arrant based on conflicts as developed. The arrant states that a separate left-turn lane should be considered hen a conflict study shos an hourly average of 3 or more total left-turn-related conflicts or 6/ or more opposing left-turn conflicts in a 3-hour study period during peak-volume conditions. Also, consideration should be given to adding a lane if 45 or more total left-turn-related conflicts or 9 or more opposing-left-turn conflicts occur in any!-hour period. RECOMMENDATIONS The addition of left-turn lanes alays provides an improvement in the traffic flo; hoever, left-turn lanes cannot be built at all locations. It is recommended that the folloing arrants be used as guidelines to aid in determining hen the need for lefttum lanes becomes critical: 1. Accident Experience Install a separate left-turn Jane if the critical number of left-tum-related accidents (as deflned in the text) has occurred. For one approach in 1 year, four left-turn accidents at an unsignalied intersection and five at a signalied intersection are critical. 2. Volume -- Install a separate left-turn lane hen volumes meet the criteria given in the critical volume arrant graphs in APPENDIX A for signalied intersections. For signalied intersections, the number of Janes, cycle length, cycle split, total main-street volume (peak hour), and percentage left-turns must be knon. For unsignalied intersections, the number of lanes, total main-street volume (peak hour), and percentage left-turns must be knon. It is recommended that the curve representing a critical delay of 2 seconds be used for unsignalied intersections (Figures 5 and 6). Also, the volumes given in Table 4 representing minimum sums of peak-hour left-turn and opposing volumes giving critical left-turn delays may be used as a guideline to determine if further investigation is needed. 17

25 3. Traffic Conflicts - Consider adding a sepa rate left-turn lane hen a conflict study shos an hourly average of 3 or more total left-turn-related conflicts (as defined in APPENDIX C) or 6 or more opposing-left-turn conflicts (as defined in APPENDIX C) in a 3-hour study period during peak-volume conditions. Also, consider adding a lane if 45 or more total left-turn-related conflicts or 9 or more opposing left-turn conflicts occur in any!-hour period. REFERENCES I. Agent, K. R.; Development of Warran ts fo r Left Turn Phasing, Report 456, Division of Research, Kentucky Department of Transporation, August Agent, K. R. ; A Survey of Use of Left- Tum-On Red, Report 446, Division of Research, Kentucky Department of Transportation, May Hammer, C. G.; Evaluation of Minor Improve ments, Record No. 286, Highays Research Board, Failmeger, R. W.; Relative Warrants fo r Left Tum Refuge Construction, Traffic Engineering, April l Dart,. K.; Development of Factual Warrants for Left-tum Channeliation, Ph.D. Dissertation, Texas A & M University, January Harmelink, M. D.; Vo lume Warrants fo r Left Turn Storage Lanes at Unsignalied Grade Inter sections, Department of Highays, Ontario, Canada, January Riny, S. L.; and Carstens, R. L.; Guidelines fo r the Inclusion of Left- Tum Lanes at Rural Highay Intersections, Ioa State Highay Commission, September Sha, R. B.; and Michael, H. L.; Evaluation of Delays and Accidents at Intersections to Warrant Construction of a Median Lane, Record No. 257, Highay Research Board, Foody, T. J.; and Richardson, W. C.; Evaluation of Left- Tum Lanes as a Traffic Co ntrol Device, Ohio Department of Transportation, November Metasic, R. J.; Warrants for the Provision of Left Turn Bays, Master of Science Thesis, TI1e University of Ariona, II. Messer, C. J.; and Fambro, D. B.; A Guide fo r Designing and Operating Signalied In tersections in Texas, Texas Transportation Institute, August Leisch, J.; Capacity Analysis Techniques fo r Design of Signalied Intersections, Public Roads, Vol 34, No. 9 and 1, August and October Capacity Analysis Procedures for Signalied Intersections, Traffic Institute, Northestern University. Highay. Capacity Manual, Special Report 87, Highay Research Board, Netork Flo Simulation for Urban Traffic Control System. Phase II, Volumes 1-5, Federal Highay Administration, March Traffic Signal Warrants, KLD Associates, KLD TR No. 17, prepared for the National Cooper alive Highay Research Program, November Perkins, S. R.; and Harris, S. J., Traffic Co nflict 14. Characteristics Accident Potential at Inter sections, General Motors Corporation, December Zegeer, C. V.; Development of a Traffic Conflicts Procedure for Kentucky, Report 49, Division of Research, Kentucky Department of Transportation, January

26 APPENDIX A FIGURES GIVING PERCENTAGE LEFT-TURNS WHEN DELAY BECOMES EXCESSIVE (SIGNALIZED INTERSECTION) 19

27

28 1 CYCLE SPLIT EQUATION '' 9 7/3 Ygse -:.OOI9X.96 6/4 y =I X /5 y =IISOe -.26BX,99 : ::> _, 1- " : a TOTAL MAIN STREET VOLUME (PEAK HOUR) Figure AI. Percentage Left-Turns When Delay Becomes Excessive (Four-Lane Highay, 6-Second Cycle). I DO CYCLE SPLIT EQUATION,, 9 7/3 y -"942e -.2 1X.97 6/4 y "731e X /5 y =1166e -.32X.9 9 ; : ::> 1- I-... _, I- " : a TOTAL MAI N STREET VOLUME {PEAK HOUR) Figure A2. Percentage Left-Turns When Delay Becomes Excessive (Four-Lane Highay, 9.Second Cycle). 21

29 1 CYCLE SPLIT EQUATION,, 9 7/3 6/ X y I67e X Y" BO 5/ X y = 916e.99 : " f- f- LL.J f- u : L TOTAL MAI N STREET VOLUME (PEAK HOUR) Figure A3. Percentage Left-Turns When Delay Becomes Excessive (Four-Lane Highay, 12.Second Cycle). 1 CYCLE SPLIT EQUATION ' 9 7/3 -.ooex y;;522995e.9 I BO -:9X 6/4 y=2252 e 1. 5o; so y =59782e -.9X.9 B : " f- f- LL.J f- u : L 1 'l. "" "" 6 " " s I GOO TOTAL MAIN STREET VOL UME (PEAK HOUR) 22 Figure A4. Percentage Left-Turns When Delay Becomes Excessive (To-Lane Highay, 6.Second Cycle).

30 1 CYCLE SPIT EQUATION,, -.7X 9 7/3 y=4 225e.9 9 6/4 Y= 272Be -:7X ,OOBX 5/5 Y = 6592e.95 : ::> 1-1- u....j 1- u : a \ TOTAL MAIN STREET VOL UME! PEAK HOU R) Figure AS. Percentage Left-Tnrns When Delay Becomes Excessive (To-Lane Highay, 9.Second Cycle). 1 CYCLE SPLIT EQUATION,, 9 7/3 y =35435e -.oos x 1 6/4 y =7672e -.OOGX.99 : ::> 'l 'l 'l :;; s 5/5 -.7X y = l952e u....j 1- u : a "" BOO TOTAL MAIN STREET VO LUME( PEAK HOU R) Figure A6. Percentage Left-Turns When Delay Becomes Excessive (To-Lane Highay, 12.Second Cycle). 23