  The interest to use probe vehicles for traffic monitoring is growing. This
paper is focused on the estimation of flow rate from probe vehicle data and the
evaluation of sample size requirements. Three cases are considered depending on
the available information on the percentage of probes p and the flow rate
\lambda: i) p is known but \lambda is unknown ii) \lambda is known but p is
unknown, and iii) both parameters are not known. Estimation methods for all
three cases are presented along with the reliability of these estimates. For
the first two cases, count data provide sufficient information to estimate the
unknown parameters p and \lambda individually when the other one is known. For
these two cases, simple analytical expressions are derived to analyze the
accuracy and sample size requirements. However, when both parameters are
unknown then additional information beyond count data is required. The position
of probe vehicles in a queue at signalized intersections is used as additional
information in that case. The results show for how long data need to be
collected to estimate the parameters at acceptable confidence levels.

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