Methods and systems of monitoring traffic flow

A method of monitoring traffic flow comprises reading registration plates of the vehicles of a multiplicity of series of vehicles passing respective locations on a road network, if necessary selecting samples of readings from the readings relating to the vehicles of the series, and determining whether readings of the samples are deemed to contain matching readings. The samples are used as probes or targets to measure average journey times which are compared with reference journey times. The selecting of the samples at the respective reading locations is performed according to an algorithm common to all of the locations.