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HomeMy WebLinkAboutRPT 2 LEFT TURN STDY 9-2-86DATE: AtI~UST 20, 1986 TO: FROM: SUBJECT: WILLIAI4 HUSTON, CITY HAtC~GER PUBLIC WORKS DEPARTHENT/ENGINEERING DIVISION LEFT TURN PHASING STUDY - WALNUT AVENUE AT CHERRYWOOD LANE RECOMMENDATION: For the City Council meeting of September 2, 1986. Receive and file. BACKGROUND: Several months past, staff was directed to investigate the need for exclusive left turn phasing from Walnut Avenue to Cherrywood Lane. The City's Traffic Engineer Consultant has completed the investigation for the subject intersection and has concluded that the exclusive left turn phasing is not warranted at this time. A copy of this investigation is attached for the City Council's review. Staff concurs with the consultant's findings and recommends against any change in the traffic signal operation at the intersection of Walnut Avenue and Cherrywood Lane at this time. Bob Ledendecker Director of Public Works/City Engineer BL:jm Attachment CONSULTANTS, INC. 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. Generally, 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 if 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 guidelines that describe the conditions whereby separate left turn phasing should 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. Santa Ana, California 92701 . (714) 558-1952 A Berryman & Stephenson Industries 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 made 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 hours, while a total of 445 vehicles had no delay during the morning or afternoon peak hours. It is significant to note that in man~ 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 Number 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 recommend the installation of exclusive left turn phasing at the Walnut Avenue intersections with Cherrywood Lane.