In recent years there has been an increasing need to improve traffic flow within urban centres without increasing the required roadway infrastructure. As a result of this requirement, automated traffic control systems have been evolving at a steady pace. Central to the operation of any automated traffic control system is the need to reliably detect vehicular traffic flow. Currently, this detection function was primarily carried out through the use of buried inductive loop sensors.Buried inductive loops have been in service for many years. Inductive loops typically require stopping or re-routing traffic during installation and are prone to breakage as a result of weather-related effects or construction. A variety of sensors have been developed for the replacement of buried loops. Of the technologies being employed in these sensors, those normally associated with all-weather, day/night capability are radiating (active). Active sensors pose a certain degree of public health concern and are hence not a favoured solution.This paper describes the design and performance of a new, overhead vehicle sensor which relies on naturally occurring radio emissions. The sensor, mounted in an overhead or sidefire position, is intended to provide all-weather, daylnight vehicle detection without any potential for public health safety impacts. Prototype test results are presented from a number of trial sites in varying weather conditions. BACKGROUNDA variety of techniques can be potentially employed to carry out vehicle detection functions within modern traffic management systems, replacing the functionality of conventional buried loop sensors. A top level comparison of these is shown in table 1. Of the passive sensing techniques, radiometry provides a suitable combination of attributes for the vehicle detection function. RADIOMETRIC VEHICLE SENSOR DESIGNThe principal of operation of the radiometric traffic sensor is based on the fact that there is a detectable difference in the radiometric response (or "temperature") of a viewed segment of roadway with and without a metallic vehicle present. The fundamental equation governing this phenomenon is;Where;T,,,,, = apparent radiometric scene temperature E = ground (or roadway) emissivity T,herntr,l = ground (or roadway) physical temperature p = vehicle radio reflectivity T,k, = sky noise temperature When considering T,,,,,, , (1) is reduced to its first term when no reflective vehicle target is present. The resultant term is approximately 200 -300 K depending upon the road surface material and wetness. When a reflective target, such as a vehicle, is present and occupying a significant portion of the field-of-view (FOV), (1) is reduced to its second term. The second term defines the amount of sky noise which is reflected from the target vehicle into the sensor. Naturally occurring sky noise arriving from deep space within the propagation "windows" centred at 35 and 94 GHz is minimally affected by atmospheric attenuation, weather, and seasonal/diurnal variations. Typically at 35 GHz, when co...