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LEFT-TURN LANE DESIGN AND OPERATION
Principal
Investigator:
Dr. Lei Yu
Sponsoring Agency:
Texas Department of Transportation, TxDOT 0-5290
Period:
September 1, 2005 ~ August 31, 2007
Research
Abstract:
Left-turn lanes
are used to provide space for the deceleration
and storage of turning vehicles. They may be
used to improve safety and/or operations at
intersections. Multiple left-turn lanes may be
used to accommodate high peak hour left-turn
volumes. A left-turn lane includes both
deceleration and storage. Storage is based on
consideration of the queues projected for both
the turn lane and the adjacent through lane.
The through lane queue considerations are needed
because of the possibility of blocking the
entrance to the turn lane. The required storage
length is calculated based on the anticipated
queue. This length may be obtained using an
acceptable traffic model; however, if a traffic
model has not yet been employed then the Roadway
Design Manual states that the queue may be
estimated by the following equation:
L=(volume/number
of cycles/hr)(2)(queue storage length of
vehicles).
This equation
needs to be revisited to determine if it is the
best technique for estimating the left-turn lane
storage length. Questions that have been asked
include the following:
·
Which volume should be used in the equation –
existing or proposed?
·
Is
the assumed storage length of vehicles
reflective of the current vehicle fleet and do
the values shown adequately account for the
presence of trucks?
·
Is
there a need to review the taper rate, pavement
markings (e.g., use of dashed lines), and other
elements for left-turn lanes?
·
Should there be any difference for a left-turn
lane at a signalized versus unsignalized
intersection?
These questions
were discussed during a meeting of the project
advisory committee overseeing the development of
the Urban Intersection Design Guide. Additional
information and guidance was sought on the
design of left-turn lanes.
A recent FHWA
study found that the addition of left-turn lanes
can result in impressive reductions in crashes.
The study found that crashes were reduced
between 7 and 19 percent at signalized
intersections and between 27 and 47 percent at
stop-controlled intersections. (Values varied
based on the number of approaches and the number
lanes on the approach.) The FHWA study noted
that no research was found that quantifies the
safety effectiveness of extending the length of
existing left-turn lanes to eliminate traffic
overflow into through travel lanes and to allow
a greater proportion of vehicle deceleration to
occur in the turn lane rather than in the
through travel lanes.
In addition to
examining the components of the left-turn
storage length equation, research is needed to
examine the safety benefit of increased storage
length.
Research
Information
For further
information about the research, please contact
Dr. Lei Yu by telephone at
(713) 313-7282 or by e-mail at
yu_lx@tsu.edu.
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