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INTRODUCTION
It is generally accepted that the level of service (LOS) at intersections can significantly
affect the overall LOS of a road. It is also known that the LOS at an intersection can be
adversely affected by frequently allowing the left-turning vehicles to block the through traffic.
Crash rates also tend to be higher at intersections than on through sections of a road. The
separation of left-turning vehicles from through traffic can be an important condition for the safe
and effective operation of an intersection.
Existing guidelines use the volume of left-turn traffic, the volume of through traffic, and
the volume of traffic opposing the left-turn maneuver to determine whether a left-turn lane is
needed at an intersection. The most frequently applied guidelines for determining the need for a
left-turn lane were developed from analytical equations for specific left-turn percentages for
unsignalized intersections and for specific g/C ratios for signalized intersections. The
unsignalized guidelines are available as tables in the American Association of State Highway
and Transportation Officials’ (AASHTO) A Policy on the Geometric Design of Highways and
Streets, commonly called the Green Book (2001). These guidelines are not available for all
percentages of left-turning vehicles and do not go below 5% left turns. In the development of
these guidelines, the researchers also assumed a fixed value for the critical gap, which represents
a simplification of reality. In contrast to unsignalized intersections, there are no widely accepted
guidelines for installing left-turn lanes at signalized intersections. For the guidelines that do
exist, the researchers did not develop separate guidelines for different percentages of left-turning
vehicles.
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PURPOSE AND SCOPE

The purpose of this study was to develop left-turn lane guidelines for signalized and
unsignalized intersections. Specific objectives were as follows:
• Critically evaluate existing guidelines for providing left-turn lanes at unsignalized and
signalized intersections through a literature review.
• Determine how engineers at the Virginia Department of Transportation (VDOT) use
existing guidelines.
• Develop, calibrate, and validate event-based simulation models to simulate traffic at
unsignalized and simple two-phase signalized intersections.
• Develop new guidelines for left-turn lanes using the event-based simulation models
that are applicable for all turning percentages.
• Create a prioritization system for ranking the need for left-turn lane improvements
among a number of candidate intersections.

This study was limited to developing guidelines for installing left-turn lanes at signalized
and unsignalized intersections. Developing guidelines for left-turn signal phasing was not within
the scope of this study. Although each event-based simulation program used for developing
guidelines in this project could theoretically simulate any condition seen in the field, the scope of
each program was limited to ensure that program coding could be completed in a timely manner.
Thus, the scope of the program was defined as follows:
• A maximum of four approaches can be simulated using this program.
• The program can handle all kinds of lane sharing and also accommodate up to four
lanes