Sub-problem 1a - Page 4 of 8
Sub-problem 1a: Analysis of the
existing TWSC intersection
What default values should be used?
Driver behavior at a TWSC
intersection is described by two parameters, the
and the follow up
time. The HCM provides default values that represent average
values from a number of measurements made at sites throughout the United
States for critical gap and follow up time. However, conditions at the site
(unusual geometric features, high volumes causing more aggressive driver
behavior) that you are studying may produce values that are different than
these default values. It is always better to use values that are estimated
from the site that you are studying, if these values can be measured.
However, it should be pointed out that critical gaps cannot be
directly measured in the field. Rather, they are estimated using
statistical procedures based on the distribution of gaps that are accepted and
rejected by drivers in the field. An appropriate but somewhat complex
method for doing this was presented by
in 1992. A rough method to check the validity of
the critical gap would be to measure the follow-up time, which can be done
easily, and to estimate the critical gap using the approximate relationship, tf/0.6.
What time periods should be analyzed? The data
that have been collected for this site represent the volumes for
the afternoon peak period. If there are other peak times during the day, such as
the morning peak or sometimes a midday peak, these should also be included in an
What measure should be used to determine the
performance of the intersection? The level of service for a TWSC
intersection is based on control
delay. Control delay is the primary measure of effectiveness for TWSC intersections and will be used as the parameter to compare the
performances of the various alternatives that we consider in this problem.
In fact, control delay is also used as the primary measure of effectiveness
for signalized intersections. In addition, we will consider
two other performance measures, each describing another aspect of the
operation of the intersection. The v/c, or
is useful for showing how close the intersection is to capacity operation. The queue length provides
a way for the analyst to determine the adequacy of the geometric design of
the facility by examining the projected length of a queue compared with
the length of turning or storage lanes.
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