Probe vehicles have been used to collect travel time for a number of years. Because the number of probe vehicles in a traffic stream can have a substantial effect on the accuracy of a travel time estimate, many studies have been conducted to study the required sample size of probe vehicles required for reliable travel time measurement. This paper addresses determining the sample size of probe vehicles in different traffic conditions on motorways. Travel time data from individual vehicles collected on seven motorway links were used to analyse the statistical characteristics of travel times. The relationship between mean travel time and standard deviation together with sampling theory has been used to calculate required probe vehicle sample sizes. The results indicate that fewer probe vehicles are required in more congested traffic condition while more probe vehicles are needed in lighter traffic condition. The relationship between traffic flow and the required sample size of probe vehicles can be described...