  Recognition of road surface conditions should always be at the forefront of
intelligent transportation systems for the enhancement of transportation safety
and efficiency. When road surfaces are covered by ice or snow, accident rate
would increase due to the reduction of road surface roughness and also friction
between tire and road. High-resolution recognition of natural and manmade
surfaces has been proved to be achievable by employing radars operating at
low-terahertz frequencies. In this work, we present theoretical investigations
on surface condition recognition of an icy road by employing low-terahertz
frequencies. A theoretical model combining integral equation method (IEM),
radiative transfer equation (RTE) and Rayleigh scattering theory is developed.
Good agreement between the calculation results and measured data confirms the
applicability of low-terahertz frequencies for the evaluation of icy road
surface in winter. The influence of carrier frequency, ambient temperature,
impurities inside the ice layer and frozen soil surface conditions on the
efficiency of this method is presented and discussed.
