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1
SUMMARY
1h January 1993, Me University of Idaho, in cooperation
with Kittelson and Associates, Ruhr University, and
Queensland University of Technology, initiated work on
NCHRP 3~6 to develop new capacity and level of service
analysis procedures for unsignalized Intersections. This
report, the first of two volumes describing We work
completed for this project, documents the work completed
for two-way stop-controDed (TWSC) Intersections.
Volume two includes eight chapters covering the work
completed for as-way stop-controlled (AWSC)
Intersections.
A new data base descnb~ng traffic operations at TWSC
intersections was assembled as part ofthis study. A total
of 68 unique sites wer~videotapeddurLng 79 different time
periods. In most cases, four video cameras were used at
each site producing a total of over 300 individual
videotapes. A total of 128 hours of usable data was
acquired. The data base includes information from all
geographic sectors of We United States and covers a wide
range of traffic operational and geometric conditions.
Two types of models were used to analyze traffic
operations at TWSC intersections: gap acceptance models
and empirical models. The theoretical background of
venous capacity models, delay models, and cntical gap
estimation procedures were documented Candidate models
and procedures were selected based on certain criteria and
then tested against field data.
Critical gap and foDow-up time are the two major
parameters used by various gap acceptance models. The
maximum likelihood procedure was recommended for
estimating critical gaps. Follo~up time was estimated
directly from field data. A completely new set of cntical
gaps and foDow-up bmes were recommended, based on the
new data set collected for U.S. conditions.
Gap acceptance capacity models developed by Harders
(used In the 1985 HCM) and Siegloch (used In the 1994
HCM) were tested. Harders' model, whose results were
marginally better Han the Smooch model, is recommended
for future applications. A comprehensive calibration and
validation process showed Hat the basic computational
procedures used in the 1994 HCM are valid; these
procedures are recommended to evaluate TWSC
intersections. However, the following changes are
recommended as additions to He basic procedure:
.
.
new values for critical gaps and follow-up times,
a weighting factor of 2 to be applied to major
street left turn flows to calculate conflicting
volumes,
a procedure to account for the effect of upstream
signals,
a mode! to account for the effect of a two-stage
gap acceptance process, flared approach, and
pedestrians, and
delay calculations for the major sheet through
vehicles.
A t~me-dependent delay mode! developed by Troutbeck
and Akcelik is recommended. (This is He same mode!
used in the 1994 HCM). The mode! provides estimated
delay in queue at TWSC intersections.
Other topics included in this document are discussions of
simulation models, MUTED signal warrants, and LOS
cnteria. Testing ofthe available simulation models showed
that only KNOSIMO, developed by Briton, gave
satisfactory results. Selection of intersection control type
based on the average intersection has demonstrated
consistency with the peak hour signal warrants given by
MUTCD provided that burning movement proportions are
"average". Finally, while delay can be a good indicator to
define intersection LOS at TWSC intersections, it is
probably more reasonable to define LOS with
consideration given to both delay and queue length.
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Representative terms from entire chapter:
twsc intersections
