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Millimeter-wave radar-based sensors are being considered for a number of automotive applications including obstacle detection and collision warning, true-speed, and road-surface recognition. In this paper, the interaction of electromagnetic waves with asphalt road surfaces, possibly covered with ice or water, at millimeter-wave frequencies is studied. First, an experimental procedure for determining the effective dielectric constant of bituminous mixtures used in road-surface constructions is developed. In this procedure, the effective dielectric constant is derived using a simple inverse-scattering algorithm to the measured radar cross sections of cylindrical specimen of a standard asphalt mixture. Then the vector radiative transfer equation is used to formulate the scattering from a multilayer medium representing an ice-or water-covered asphalt surface. The University of Michigan polarimetric 94-GHz radar system was deployed for characterizing the polarimetric backscatter responses of asphalt surfaces under many physical conditions near grazing incidence angles (70-88). The measured backscatter coefficients and parameters of copolarized phase difference statistics of a dry asphalt surface with smooth interface at one incidence angle were used to derive the phase and extinction matrices of the asphalt medium. The experimentally determined phase and extinction matrices are substituted in the radiative transfer formulation to predict the scattering from asphalt surfaces under all conditions. Excellent agreement between theoretical predictions and measured quantities is obtained. Index Terms-Electromagnetic scattering, millimeter-wave measurements. I. INTRODUCTION A CCORDING to the National Highway Traffic Safety Administration, traffic accidents are among the leading causes of death in industrial countries. A study has shown that 90% of rear-end and 60% of head-on collisions can be prevented, provided that the drivers had been given one extra second of warning [1]. Advanced sensors that alert drivers to the potential hazards such as objects on the road or slippery surfaces can drastically reduce traffic accidents. Radars, when compared with optical and infrared sensors, offer two major advantages: 1) their operation is not hampered with inclement weather and 2) they provide the range and speed of targets directly. With advances in millimeter-wave (MMW) technology, millimeter-wave radar-based sensors for automotive applications have become economically viable.

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