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SWUTC Research Project Description

Use of Directional Median Openings on Urban Freeways

University: Texas Southern University

Principal Investigator:
Yi Qi
Texas Southern University
(713) 313-6809

Project Monitor:
Jun Yao
(610) 862-7709

Funding Source: State of Texas General Revenue Funds

Total Project Cost: $50,000

Project Number: 161342

Date Started: 12/1/12

Estimated Completion Date: 11/31/13

Project Summary

Project Abstract:
On urban divided roadways, a directional opening is normally used to restrict crossing and left-turn movements from minor streets where it is desirable to limit the number of conflicts. While it may provide a potential safety benefits, the benefits to be achieved depend largely on whether it is properly implemented as well as on a series of factors, such as geometry, traffic, control, and environmental conditions.

Project Objectives:
The objective of this study is to investigate the impacts of installing directional openings on median-divided urban roadways. The approaches will include field traffic studies, micro-simulation-based studies on traffic operations and safety, as well as a crash data analysis. The outcomes of this project will provide traffic engineers with necessary understanding of the critical issues associated with the use of directional median openings on urban roadways.

Task Descriptions:

Task 1: Literature Review
The purpose of this task is to conduct a thorough literature review. The areas to be reviewed include, but are not limited to

1)      Existing design standards and best practices for designing median openings

2)      Operational and Safety performances of various types of median openings

Task 2: Design and Conduct Field Study
The purpose of this task is to design and conduct a field study for a selected road segment.

The selected study site should have raised median installed with relative heavy traffic volumes from/to the minor streets using the median openings. The data to be observed include field traffic volumes, traffic signal timing, crash data records and so forth, which are necessary for conducting traffic operational and safety analyses.

Task 3: Analyze the Operational and Safety Impacts Based on Traffic Simulation Studies
In this task, micro-simulation models will be developed to replicate the traffic conditions at the study site selected in Task 2. Hypothesized scenarios with directional or full median openings will be developed, so that comparison can be made to evaluate the operational and safety effects of a directional median opening under various geometric and traffic conditions. The appropriate conditions for using a directional median opening will be investigated by comparing the outcomes of various scenarios.

Task 4: Analyze Safety Impacts through Historical Crash Data Analysis
The purpose of this task is to analyze the safety impacts of directional median openings as opposed to full median openings. Actual crash records from TxDOT Crash Records Information System (CRIS) will be analyzed. Crash reports from the Crash Records Information System will be carefully reviewed to identify the related safety issues at the study site. Beside the selected study site, 10 additional locations will be selected to enable a before-and-after or cross-sectional comparative study. The crash type, severity levels and the likely contributing factors will also be analyzed.

Task 5: Document Research Findings
The purpose of this task is to document all the research findings from Tasks 1 to Task 5. A final report will elaborate the work performed, methods used, and results achieved.


Implementation of Research Outcomes:
This research investigated the safety impacts of installing directional openings on median-divided urban roadways.  The results of this research indicate that, since converting a full median opening to a directional median opening will generate more U-turns at the downstream opening of directional opening, the selection of U-turn location is critical for the safety performance of directional openings. Diverted left-turn traffic should not be allowed to make U-turns at a closely spaced opening that already have significant U-turns or left-turn volumes.

Products developed by this research include:

Model Developed:  This study developed a Poisson regression model to investigate the influencing factors on the crashes occurred at the downstream U-turn locations of directional median openings.

Presentation:  Operational and Safety Impacts of Directional Median Opening on Urban Roadways: Case Study in Houston, Texas, Qi ,Y., X. Chen, and G. Liu, presented and published in the proceedings of the 92th Annual Meeting of Transportation Research Board , Washington, DC, Jan 12-16, 2014

Impacts/Benefits of Implementation:
The results of this project provide guidelines to transportation engineers in appropriately implementing of directional median openings. The developed methods, models and major findings can be incorporated into the state or Federal roadway design manuals.

The results of this project will reduce the crash rates related to the use of directional median openings. Thereby, it will reduce social and environmental cost associated with the traffic accidents.

Web Links:

Final Report