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xi EXECUTIVE SUMMARY Left-turn movements at intersections, including drivewaysâespecially movements that are made from lanes that are shared with through trafficâcause delays and adversely impact safety. The objectives for this National Cooperative Highway Research Program (NCHRP) research were to develop an objective and clear process for the installation of left-turn lanes at unsignalized intersections and provide guidance on the design of these lanes. A review of the literature was performed using many sources, including research reports, state and federal design manuals, and handbooks. Although many procedures are currently in use by various organizations to determine the need for left-turn lanes, several are either very similar or identical. Most statesâ criteria are based on the American Association of State Highway and Transportation Officials (AASHTO) A Policy on Geometric Design of Highways and Streets (commonly known as the Green Book) values or NCHRP Report 279 valuesâboth of which are based on work done in the 1960s by M. Harmelink. Interviews were conducted of representatives from state departments of transportation, county governments, city governments, and consultants. The 25 questions in the interview were structured into planning, design, legal/policy/finance, and potential future applications. All interview participants indicated that left-turn treatments are provided at unsignalized intersections. In addition to left-turn lanes, many use two-way, left-turn lanes and bypass or shoulder widening (typically used when options are limited). One respondent indicated roundabouts are another treatment considered for dealing with left-turn movements. A legal review conducted as part of this research addressed the following question: When a government seeks to fulfill a broad public objective such as safety and, in this project, left-turn accommodation, who should bear the costsâthe developer who would be adding traffic to the roadway network or the general public? Unless the facts of the case are clear cut, the outcome is unpredictable even within specific jurisdictions. Results will be more predictable if agencies have the documented authority to manage access. This authority could be achieved in a number of different ways (state or municipal codes for access management, administrative rules, etc.). Otherwise, there are too many hazy cases and multiple factors so that predicting the outcome is impossible. This projectâs driver behavior study used videotaped recordings of vehicle and pedestrian operations collected at 30 sites located in College Station/Bryan, Texas; Houston, Texas; Staten Island, New York; and Phoenix, Arizona. The behaviors for a total of 2945 vehicle drivers that started from a stopped position were used in the evaluation. The most influential variables on the amount of time used to clear the intersection are crossing width and posted speed limit. The relationship between clearance time and the accepted lag or gap time was in the direction expected although it did not have as large an influence as initially thought. Gap is defined as the time interval between two opposing vehicles that is necessary for a left-turning vehicle to safely complete a left-turn maneuver. The relationship between gap and posted speed limits from the field studies was similar to the finding from a simulator study reported in the literatureâsmaller gaps are accepted at higher posted speed sites. Gap acceptance values increase as the crossing
xii width increases, although only by a small amount (less than 1 sec between the one-lane group and the two-lane or very-wide-one-lane group). Because of several concerns with the Harmelink procedure, including the lack of a clear relationship between the assumptions in the model and delay or safety on a highway, the research team recommends that the results from the benefit-cost ratio be used as the basis for left-turn lane warrants. Economic analysis can provide a useful method for combining traffic operations and safety benefits of left-turn lanes to identify situations in which left-turn lanes are and are not justified economically. A benefit-cost approach was used in this project to determine when a left-turn lane would be justified. The steps included simulation to determine delay savings from installing a left-turn lane; crash costs, crash reduction savings determined from safety performance functions and accident modification factors available in the AASHTO Highway Safety Manual; and construction costs. For rural conditions, different safety performance functions are provided for two- and four-lane highways and for three- and four-leg intersections. For urban and suburban arterials, prediction equations are provided for three- and four-leg intersections. Separate urban and suburban prediction equations are not provided based on the number of lanes on the major- road approach. The prediction equations are not a function of speed limit; therefore, the developed warrants are also not a function of speed limit. A range of values was used in the benefit-cost evaluation to identify volume conditions when the installation of a left-turn lane at unsignalized intersections and major driveways would be cost- effective. Plots and tables were developed that indicate combinations of major-road traffic and left-turn lane volume where a left-turn lane would be recommended. Warrants were developed using the following: ⢠A range of values for the economic value of a statistical life, ⢠Crash costs based on values in the Highway Safety Manual, ⢠A range of construction costs, and ⢠A benefit-cost ratio of 1.0 and 2.0. The research team suggests a benefit-cost ratio of 1.0 along with the mid-range economic value of a statistical life and moderate construction cost to identify the warrants for a left-turn treatment. For urban and suburban areas, that is a left-turn lane. For rural two-lane highways, that is a bypass lane. A benefit-cost ratio of 2.0 has been argued as being a more practical value to use to offset the potential variability in other assumptions. The warrants based on a benefit- cost ratio of 2.0 were selected for a left-turn lane on rural two-lane highways. These values were similar to the warrants that resulted when the lower crash costs based on older Highway Safety Manual values were used. Left-turn lanes can reduce the potential for collisions and improve capacity by removing stopped vehicles from the main travel lane. Left-turn lane warrants were developed as part of NCHRP Project 3-91 using an economic analysis procedure for rural two-lane highways, rural four-lane highways, and urban and suburban roadways. NCHRP Project 3-91 also developed a Design Guide on Left-Turn Accommodations at Unsignalized Intersections that discusses left-turn lane designs, traffic control treatments, and case study examples.
