(57) The invention relates to monitoring means particularly for traffic using a road or motorway, the object being to provide information relating to traffic make-up, densities, speeds and the recognition of alarm situations. This objective is met by monitoring means characterised by at least two radiation detection means (1, 2) set in longitudinal spaced relationship in alignment with an intended direction of movement of objects passing below the radiation detection means, each detector being adapted to produce a signal pulse that is a function of the length and speed of an object passing below the detectors, and the at least two detectors being connected to processor means (7) adapted to store and compare the signal pulses generated by each detector. By providing two detectors the pulse outputs from which can be recognised as being from the same vehicle, and are a function of the length and speed of a vehicle, all manner of information can be provided by the data processor such as the categorising of vehicles by length, the volume of traffic per unit time, the speed and spacing of successive vehicles and the progressive slowing down of traffic on a particular stretch of road or motorway, to provide reliable information for analytical purposes and to activate appropriate warning signals positioned alongside in advance of and beyond the position of the detectors, as may be appropriate.