The National Academies

NCHRP 03-91 [Final]

Left-Turn Accommodations at Unsignalized Intersections

  Project Data
Funds: $350,000
Research Agency: Texas A&M Research Foundation
Principal Investigator: Kay Fitzpatrick
Effective Date: 4/24/2008
Completion Date: 10/30/2010

The objectives of this research were to develop an objective and clear process for the selection of left-turn accommodations at unsignalized intersections and to provide guidance on the design of these accommodations.  

The following reports are available:
Maximizing the potential capacity of existing roadways is a priority in light of growing traffic demands and the diminishing resources to develop more capacity. Left turns at unsignalized intersections, including driveways, cause delay and may reduce safety. The decision to install a left-turn accommodation is a complex one as state and local transportation agencies weigh the left-turn demand, the cost of the accommodation, and the anticipated operational and safety benefits. Clear and consistent application of left-turn accommodations is important for mitigating the impacts of left turns, both for reconstruction projects and for the permitting of new access points. 
Transportation and land-use agencies often require developers to mitigate their traffic impacts by implementing left-turn accommodations. Courts have begun applying the principles developed in the Supreme Court Case of Dolan v. City of Tigard. In that decision, a two-prong test was applied: “it must be determined whether an ‘essential nexus’ exists between a legitimate state interest and the permit condition…. If one does, then it must be decided whether the degree of the exactions demanded by the permit conditions bears the required relationship to the projected impact of the proposed development.”  These principles are now often referred to as “essential nexus” and “rough proportionality.” 
Three questions must be answered to provide clear and consistent application of left-turn accommodations: (1) When is a left-turn accommodation justified? (2) What type of accommodation is appropriate? (3) How should the accommodation be designed?

Task 1.  Conduct a review of the literature and ongoing research related to the selection, design, and impacts of left-turn accommodations at unsignalized intersections. Identify conventional and novel left-turn accommodations and summarize the knowledge on each, including safety and operational impacts. Conduct a legal review of cases on essential nexus and rough proportionality. Submit a summary of findings from this task.  

Task 2. 
Interview state and local politicians and transportation agency administrators, business owners, and developers to determine their understanding and concerns related to providing left-turn accommodations at unsignalized intersections. The interviewees should be geographically and jurisdictionally diverse. The list of interviewees and the interview guide must be approved by the NCHRP before conducting the interviews. 

Task 3.
Identify performance measures that may influence the decision to install a left-turn accommodation at an unsignalized intersection and that can be objectively measured. Develop a preliminary framework for the process that will be developed in Task 6.  

Task 4.
Identify the types and sources of data (existing, observed, and simulated) that will be used in Tasks 6 and 7. Describe the methods that will be used to analyze the data.  

Task 5.
Submit an interim report documenting Tasks 1 through 4 and including an updated work plan for subsequent tasks for review by the NCHRP. 

Task 6.
 Develop a process for determining whether a left-turn accommodation is justified at an unsignalized intersection and, if so, the types of accommodations that are appropriate. The process should be based on a probabilistic assessment of safety and operational efficiency but also consider costs to the agency, the public, and affected businesses and, if private funds are implementing the accommodation, application of the principles of essential nexus and rough proportionality. The process should be easy to understand and implement and be applicable to intersections across a broad range of traffic volumes and land uses. As far as practical, the inputs to the process should be directly measurable. The rationale behind the process should be defendable to a stakeholder and address the concerns raised in Task 2. The process should be applicable to all implementing agencies.  

Task 7.
Develop design guidance for left-turn accommodations that are outputs of the process developed in Task 6. The guidance should describe the likely benefits and impacts of implementation of the accommodation, as well as geometric design criteria. The guidance should describe design concessions that can be made if conditions do not allow a typical installation. It is expected that this work will be based largely on the literature and existing practices. Identify any recommended changes to the AASHTO Policy on Geometric Design of Highways and Streets. 

Task 8.
Submit a final report that focuses on the results of Tasks 6 and 7 and succinctly describes the work that was done in the project. 

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