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600451-00044

SWUTC Research Project Description

Left-Turn Lanes at Unsignalized Median Openings

University:  Texas Southern University

Principal Investigator:
Yi Qi
Center for Transportation Training and Research
(713) 313-6809

Project Monitor:
Daniel Lynch
Traffic Engineers, Inc
Houston, TX

Funding Source:  USDOT

Total Project Cost: $60,000

Project Number: 600451-00044

Date Started: 10/1/12

Estimated Completion Date: 9/30/13

Project Summary

Project Abstract:
As a representative access-management technique, raised medians are widely used on urban arterials, and a median left-turn lane is a favorable design element for median openings. The lanes provide space for deceleration and then offer refuge for vehicles awaiting an opportunity to turn left, and thereby keep the highway travel lanes clear for through traffic. The placement of median openings determines the maximum available length that can be used for installing median left-turn lanes. Due to the frequent presence of median openings in urban arterial settings, the requirements for the deceleration and storage of turning vehicles (e.g. AASHTO Green Book) often exceed the available length between two adjacent openings, which leaves traffic engineers having to decide whether left-turn lanes shorter than the standards can be used or not.

The objective of this study is to investigate the operational and safety impacts of left-turn lanes with substandard lengths at unsignalized median openings. The potential approaches will include field conflict study, analytical modeling, and micro-simulation based operational and safety analysis. The outcomes of this project will provide traffic engineers with necessary understanding of the critical issues associated with left-turn lane design at median openings.

Project Objectives:
The goal of this research is to investigate the minimum required length for the left-turn lanes at the unsignalized median openings, and study the safety and operational impacts of such left-turn lanes with substandard lengths. The potential approaches will include field conflict study, analytical modeling, micro-simulation based analysis. To achieve this goal, the research will:

  • Synthesize existing, related research,
  • Develop models for storage lengths at unsignalized median openings
  • Develop models for estimating the delays caused by substandard deceleration lengths and the resulting excessive deceleration on main travel lanes
  • Analyze safety impacts of substandard median left-turn lanes.

Task Descriptions:
Task 1: Review Literature
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 left-turn lanes
  2. Safety performance of left-turn lanes at unsignalized median openings
  3. Operational performance of left-turn lanes at unsignalized median openings

Task 2: Design and Conduct Field Study
The purpose of this task is to design and conduct a field study at a selected road segment, on which median left-turn lanes of substandard lengths have been installed. The field study aims at providing field traffic data for the operational and safety analysis in Tasks 3 and 5.

Subtask 2.1 – Select study site
The selected study site should have raised median and median left-turn lanes of substandard lengths installed (as opposed to AASHTO or TxDOT standards). The presence of heavy through and left-turn demands is preferred.

Subtask 2.2 – Conduct field observation
Field studies will be performed at the selected study sites. The data to be observed include field traffic volumes, signal timing, crash records and so forth, which are necessary for conducting traffic operational and safety analyses.

Task 3: Analyze Operational Impacts of Substandard Deceleration Lengths
In this task, micro-simulation models will be developed to reproduce the traffic conditions at the study sites selected in Task 2. Hypothesized scenarios with extended lengths of left-turn lanes will be also developed, so that comparison can be made to quantify the effects of substandard lengths of turn lanes. Analytical models will be formulated and validated for estimating the operational impacts of substandard deceleration lengths, specifically for estimating the traffic delays caused by through vehicles being blocked by left-turn vehicles decelerating on main travel lanes.

Task 4: Estimate Storage Lengths at Unsignalized Median Openings
In this task, the developed simulation models will be used to estimate the minimum required storage lengths at unsignalized median openings. Conventionally, a “double-the-average-arrivals” method is used as a rule-of-thumb for estimating the storage lengths. There is a consensus that this method overestimates the required storage lengths especially for unsignalized median openings. In this Task, a new method/model will be developed for estimate the minimum required storage lengths at unsignalized median openings.

Task 5: Analyze Safety Impacts of Substandard Median Left-Turn Lanes
The purpose of this task is to analyze the safety impacts of median left-turn lanes with substandard lengths. Actual crash records from TxDOT Crash Records Information System will be analyzed. Crash reports from the Crash Records Information System will be carefully reviewed to identify the safety issues relating to the short left-turn lanes. Besides the study location in Task 2, the crash studies will also include more study locations with left-turn lanes of substandard lengths at median openings. In addition, based on the micro-simulation models, simulated traffic conflicts based on surrogate measures will be compared between scenarios with the short and extended lengths of turn lanes.

Task 6: 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:
Due to the frequent presence of median openings in urban arterial settings, the requirements for the deceleration and storage of turning vehicles (e.g. AASHTO Green Book) often exceed the available length between two adjacent openings which leaves traffic engineers having to decide whether left-turn lanes, shorter than the standards, can be used or not. The goal of this research was to investigate the minimum required length for the left-turn lanes at the unsignalized median openings, and study the safety and operational impacts of such left-turn lanes with substandard lengths.

Products developed by this research include:

Journal Article:  Estimation of Design Lengths of Left-turn Lanes, Yi Qi, L. Guo, L. Yu and H. Teng, ASCE’s Journal of Transportation Engineering, Volume 138, Issue 3, pp 274-283, 2012.

Presentation:  Operational and Safety Effects of Using Left-Turn Lanes with Substandard Lengths at Unsignalized Median Openings:  A Case Study in Houston, Texas, Xiaoming Chen, Yi Qi, Guanqi Liu and Kianing Wu presented at the 92nd Annual Meeting of the Transportation Research Board, January 2013.

Journal Article:  Safety Impacts of Using Short Left-Turn Lanes at Unsignalized Median Openings, accepted for publication in Transportation Research Record:  Journal of Transportation Research Board, 2014.

Model Developed:  This effort produced a new model for estimating the length of left-turn lanes at unsignalized median openings.  This is a unique model that can take account of the impacts of u-turn traffic at the median openings. This is an important feature because with the increasing use of directional median openings and indirect left-turns on urban roadways, u-turn traffic will become more prevalent.

Crash Modification Factors Developed:  Crash Modification Factors (CMFs) were developed for future applications in projecting the change in crash frequency, given a specific change in left-turn lane length.

Impacts/Benefits of Implementation:
The results of this project provide guidelines to transportation engineers in implementing of appropriate left-turn lanes at unsignalized median openings. The developed methods, tables, and models can be incorporated into the state or Federal roadway design manuals.

The results of this project will also reduce the crash rates at unsignalized median openings. Thereby, reducing the societal and environmental cost associated with the traffic accidents.

Web Links:
Final Technical Report