  Kaplan-Meier estimate, commonly known as product limit method (PLM), and
maximum likelihood estimate (MLE) methods in general are often cited as means
of stochastic highway capacity estimation. This article discusses their
unsuitability for such application as properties of traffic flow do not meet
the assumptions for use of the methods. They assume the observed subject has a
history which it went through and did not fail. However, due to its nature,
each traffic flow measurement behaves as a separate subject which did not go
through all the lower levels of intensity (did not "age"). An alternative
method is proposed. It fits the resulting cumulative frequency of breakdowns
with respect to the traffic flow intensity leading to the breakdown instead of
directly estimating the underlying probability distribution of capacity.
Analyses of accuracy and sensitivity to data quantity and censoring rate of the
new method are provided along with comparison to the PLM. The results prove
unsuitability of the PLM and MLE methods in general. The new method is then
used in a case study which compares capacity of a work-zone with and without a
traffic flow speed harmonisation system installed. The results confirm positive
effect of harmonisation on capacity.
