on a given approach.
• Unsignalized intersections on undivided highways are considered for only left-turn
lane guidelines. VDOTs Road Design Manual (2002) dictates that left-turn lanes are
to be provided for traffic on non-access controlled divided highways.
• The signalized intersection program can simulate actuated intersections but assumes
simple two-phase signals.
• The model is applicable only for left-turn lane analysis and cannot be used for rightturn
lane analysis.
• At signalized intersections, left-turns on the subject link must be permitted.
• The effect of available sight distance and grades on the approaches is not examined
explicitly.
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• The signalized program is a decision making tool for installing a left-turn lane and
should not be used to determine left-turn phasing.
methodology The research approach in this study involved (1) a literature review, (2) a survey on the
usage of existing left-turn lane guidelines, (3) field data collection and reduction, (4)
development of event-based simulation model, (5) validation of the simulation model, and (6)
development of new guidelines.
Literature Review
The current literature and previous research in this area were reviewed as the first task in
this study. The Virginia Transportation Research Council (VTRC) library and the University of
Virginia libraries were used for this purpose.
Survey on Usage of Existing Left-turn Lane Guidelines
In order to understand how VDOT engineers use current left-turn lane guidelines, an
online survey of VDOT resident engineers and district traffic engineers was conducted. A
complete questionnaire of the survey is provided in Appendix A.
Field Data Collection and Reduction
Data Collection
Site Selection
Field data had to be collected at existing intersections to gather information that could be
used to calibrate and validate the event-based simulation programs. The district traffic engineers
were asked to provide sites that were suitable for the study. All sites had to have a shared lane
(left and through), and signalized sites had to have permitted phasing for the left-turning
vehicles. Sites that had large opposing volumes and large delays for left-turning traffic were
preferred. Potential sites had to meet the following criteria:
Unsignalized site requirements:
• shared lane (left + through) on the subject link
• frequent instances of left-turn vehicles blocking through vehicles
• adequate shoulder width on the approaches so that the data collection crew could
work safely
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Signalized site requirements:
• shared lane (left + through) on the subject link
• frequent instances of left-turn vehicles blocking through vehicles
• adequate shoulder width on the approach to safely park the Smart Travel Van (STV)
(Other safety conditions, such as no power cables within 10 feet of the mast, are also
required for the safe operation of the STV.)
• permitted left turns on the subject link and the opposing link.
The research team visited 54 sites (11 signalized intersections and 40 unsignalized
intersections) suggested by the district traffic engineers and also obtained average annual daily
traffic (AADT) counts on available sites. Based on traffic volumes and site characteristics, these
sites were clas
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