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HomeMy WebLinkAboutRPT 2 LEFT-TURN STDY 01-18-88 REPORTS '..~-, !:~ ":'~ ~ ?.f t! ,'.;,' ~' DATE: JANUARY 12, 1988 ~ TO: FROM: WILLIAM HUSTON, CITY MANAGER PUBLIC WORKS DEPARTMENT/ENGINEERING DIVISION SUBJECT: REPORT ON LEFT-TURN PHASE STUDY FOR WALNUT AVENUE AT CHERRYWOOD LANE RECOMMENDATION Exclusive left-turn phasing for Walnut Lane at the intersection of Walnut Avenue and Cherrywood Lane is recommended. BACKGROUND Pursuant to your request, an engineering study was initiated for the subject intersection. The purpose of the study was to evaluate the need for exclusive left-turn phasing on Cherrywood Lane. The previous staff report, copy attached, provides additional background information for this report. EXISTING CONDITIONS Cherrywood Lane is a local residential street which is located between Walnut Avenue and Sycamore Avenue. At the signalized intersection of Walnut avenue and Cherrywood Lane there are two approach lanes on Cherrywo0d Lane for each direction of traffic flow. Similarly, Walnut Avenue has two approach lanes for the north and south directions but also has exclusive left-turn lanes for each direction. However, there are no left-turn signal indications for these lanes at the present time. Field data collection and research for the left-turn phase warrant study included manual turning movement counts for the morning, noon and afternoon peak hours of operation at the subject intersection. In addition, the traffic accident data for the period of January 1, 1984 through November 30, 1987 was reviewed. LEFT-TURN PHASE GUIDELINES Nationally accepted guidelines for protected left-turn phases, documented in the national Manual of Uniform Traffic Control Devices and the Caltrans Traffic Manual are used as the basis for recommendations relative to protected left-turn phases. These guidelines include accident experience, left-turn delay, vehicular volume, intersection capacity and impaired sight distance. EVALUATION A review of the accident history indicated that there were three reported left-turn accidents during 1984 and one during 1987. Each of these accidents are considered correctible by the installation of a protected left-turn phase. The accident experience guideline is satisfied if there are five or more left-turn related accidents for a particular left-turn movement during a recent 12-month period. LEFT-TURN PHASE STUDY, WALNUT AND CHERRYWOOD LANE January 12, 1988 Page 2 The- traffic volume guideline was met on Cherrywood Lane, eastbound direction,.with regard to the number of left-turn vehicles during the peak hours of 7:00 - 8:00 A.M., 8:00 - 9:00 A.M., 11:00 - 12:00 Noon, and 4:00 - 5:00 P.M., and on Walnut Avenue for the southbound direction during the hours of 4:00 - 5:00 P.M. and 5:00 - 6:00 P.M. The volume guideline is satisfied for actuated signals if there are 50 or more left-turning vehicles per hour in one direction and conflicting through traffic during the peak hour of 100,000 or more. The cross product of 100,000 was satisfied only for Walnut Avenue. To evaluate the left-turn delay guideline for the subject intersection, a field observation was made of Cherrywood Lane during the peak hour of 8:00 - 9:00 A.M. on a Tuesday morning and of Walnut Avenue during the evening peak hour. The results indicated that no more than two vehicles were observed being delayed in making their left turns from Cherrywood Lane onto Walnut Avenue. A delayed vehicle was defined as a vehicle still waiting to turn left after the signal cycle changed from the beginning of green time back from Cherrywood Lane to Walnut Avenue. However, vehicles on Walnut Avenue were able to complete their left turns only on the yellow signal indications. It should be noted that a mathematical analysis of the available gaps in the opposing traffic on Cherrywood Lane indicated that the left-turn demand is less than the available gaps in both directions during all three peak hours. Hence, there are ample opportunities to make a left- turn movement onto Walnut Avenue when the signal displays a green indication for Cherrywood Lane. However, there are no available gaps for the southbound left turns on Walnut Avenue because of the large opposing traffic volumes. Therefore, based upon the results of the study, exclusive left-turn phasing for the Walnut Avenue/Cherrywood Lane intersection is recommended for Walnut Avenue. The provision of exclusive left-turn phasing will improve safety and operation at this location. Director of Public Works/City Engineer 'BL/DDB/mv Dennis D. Barnes City Traffic Engineer CONSULTANTS, Consultants to' Governmental Agencies July 20, 1986 To: Bob Ledendecker, City Engineer From: Jerry Crabill, Traffic Engineer Subject: Left Turn Phasing Study - Walnut Avenue at Cherrywood Lane RECOMMENDATION Exclusive left turn phasing for left turns at the Walnut Avenue/Cherrywood Lane intersection is not recommended. BACKGROUND Pursuant to your request, we have investigated the potential of installing exclusive left turn phasing for the Walnut Avenue intersection with Cherrywood Lane. Generall-y, the number of separate left turn phases provided by a traffic signal should be held to a minimum. When a separate signal phase is added to improve protection for left turn movements, the amount of green time available for other phases will be reduced. Additionally, overall vehicle delay generally increases after left turn phasing is installed because longer signal cycles are necessary to accommodate the separate left turn phase. Delay for left turns also increases during many off-peak traffic hours such as at night when opposing traffic volumes are -light. Also, left turn phasing should not be installed ~f alternate routes are available to accomplish the same purpose. LEFT TURN WARRANT CRITERIA The State Traffic Manual published by the California Depart- ment of Transportation (Caltrans) includes guide].ines that describe the conditions whereby separate left turn phasing sh-ould be considered. The Manual states that exclusive phasing should be considered when there have been 5 or more left turn accidents during a recent 12 month period or, when there is left turn delay to one or more vehicles remaining in the left turn storage lane after each cycle for one hour. 1415 East 17th Street · Sa nra A ne Ca l~forr,~a 92701 · [714) 558-1952 A Ber,,,man & Stephe~n I~du-~:"es Company Mr. Ledendecker July 20, 1986 Page Two LEFT TURN DELAY ANALYSIS To evaluate the potential of existing left turn delays at the intersection of walnut Avenue and Cherrywood Lane, the results of special intersection counts, conducted in May of 1986, were used for this evaluation. The traffic volumes collected and tabulated at that time incorporated the peak morning travel period (7:00 to 9:00 A.M.) and the peak afternoon travel period (3:30 to 6:30 P.M.). The delay analysis centered around a tabulation of all vehicles waiting to make a left turn during the hours described above. As previously indicated, left turn delay by warrant definition, occurs when all of the vehicles waiting in the left turn storage lane at the beginning of the green indication are unable to turn left during that portion of the signal cycle (either on the green or on the yellow) and must therefore wait for the next green indication. For this intersection, the survey indicated that a total of 81 vehicles made southbound left turns during the morning and afternoon peak hours, while 196 vehicles made eastbound left turns and 37 vehicles ~.ade westbound left turns during the same time period. Of these, there were no incidences of left turn delay. This means that for every signal cycle observed (174 cycles), all waiting vehicles were able to safely turn left on the green during the same cycle in which they entered the left turn lane. The northbound left turn displayed a low incidence of left turn delay The survey revealed that a total of six cars were delayed in making their left turns during the morning peak ho~rs, while a total of 445 vehicles had no delay during the morning or afternoon peak hours. It is significant to note that in many cases, several more left turns were 'made at this location other than those waiting for the beginning of the cycle. For example, if three cars were waiting to turn left when the light turned green, there was enough time for those three vehicles to turn as well as enough time for other vehicles to turn left who had not yet reached the intersection prior to the green signal indication. Therefore, based on this information, delay is not a factor at this location and exclusive left turn phasing is not required because of delay. Mr. Ledendecker July 20, 1986 Page Three ACCIDENT DATA In order to review historical left turn accident data at the subject intersection locations, information presented in the State-wide Integrated Traffic Records System (SWITRS) was evaluated..This information is presented below. Left Turn Traffic Accident Data Walnut Avenue at Cherrywood Lane Year 1983 1984 1985 NuKber of Accidents 3 3 1 State criteria stipulates that if five or more left turning accidents have occurred during a recent 12 month period left turn phasing should be considered. As shown in the chart, the left turn accident history falls well below the limiting criteria. On that basis, exclusive left turn phasing is not warranted. CONCLUSION Based on a review of left turn delay and accident histories at the subject intersection locations, we do not recon%mend the installation of exclusive left turn phasing at the Walnut Avenue intersections with Cherrywood Lane.