This chapter investigates the quality of travel time measurements made by probe vehicles and illustrates the kinds of measurement errors that may be evident in the data. It discusses the properties of travel time data, draws implications for vehicle movement simulation logic and analyses the statistical properties of travel time estimates obtained on the basis of travel time data. At the information gathering stage, the quality of probe-based Advanced Traveller Information Systems (ATIS) service may suffer due to measurement errors as well as due to sparse observation of the network at least in the initial stages of deployment. Data on the sequence of start and end times of signal cycles and phases on a continuous clock are generally not available in an ATIS and Advanced Management Information Systems. The 'all-encompassing' property of probe travel time data on signalized arterials is the presence of dependence in the data.