ed
controller settings. In the case of actuated signals, the guidelines were not developed in the form
of tables and/or graphs. This is due to the complexity involved in the actuated signal control.
Average cycle length and green times are to be estimated through multiple simulation runs and
are to be applied to pretimed signalized guidelines.
Length of Left-turn Lane
In case the proposed left-turn lane guidelines recommend the installation of a left-turn
lane, the length of the lane needs to be determined. Given that the purpose of installing a left11
turn lane is to prevent left-turn overflows, the probability of left-turn lane overflows for varying
left-turn bay lengths was investigated using the event-based simulation program.
Recommended left-turn lane length is provided in the form of a graph at a given traffic
volume, geometry, and intersection control type. In other words, the probability of left-turn bay
overflow is plotted against left-turn bay length. At each left-turn bay length, 100 simulation runs
were made to obtain an average left-turn bay overflow probability. Left-turn bay length was
evaluated from 0 to 1,200 feet in every 50 feet for signalized intersections and was varied from 0
to 500 feet in every 50 feet for unsignalized intersections.
Prioritization Tool
Traffic engineers are sometimes faced with a problem of allocating limited funds to a set
of candidate intersections that are considered for left-turn lane installation. Under these
circumstances, a ranking methodology that prioritizes the candidate intersections on the basis of
both safety and mobility measures is needed. A unique prioritization tool that considers both
operational and safety aspects of installing left-turn lanes was developed in this project. This
tool can be used to provide rankings of multiple candidate intersections.
The prioritization methodology tool is applicable for both signalized and unsignalized
intersections. The ranking score of individual site is calculated using the following equation.
i o ii s i RS =W × NO +W × NS [2]
where
RSi = ranking score of i-th intersection
Wo = weight factor for mobility measure (default value = 0.5)
NOi = normalized operational measure score of i-th intersection
Ws = weight factor for safety surrogate measure (default value = 0.5)
NSi = normalized safety surrogate measure score of i-th intersection
Wo + Ws = 1.
The normalized operational measure score at intersection i is calculated as follows.
( ) O
NO Oi
i max
= [3]
where
Oi = operational measure at intersection i
max(O) = maximum operational measure from all candidate intersections.
As noted earlier, unsignalized and signalized intersections are using different operational
measures. The percentage of left-turn vehicles blocking through vehicles is used for
unsignalized intersection, and the v/c ratio and left-turn delay of left-turn vehicles are used for
signalized intersections.
12
The normalized safety surrogate score, NSi, is based on two conflict opportunities: leftturn
and rear-end conflict opportunites. The reduction in confict opportunities with added leftturn
lane is calculated, and then it is normalized by dividing maximum value among candidate
intersections.
( CO)
NS CO i Δ
= Δ
max
[4]
where
ΔCOi = reduction in conflict opportunities with added left-turn lane at inters
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