Radar, basically sends out electromagnetic energy in the form of radio waves. These waves travel outward until they strike a surface that reflects or deflects them. The radar antenna then receives any returned energy. The design of the radar and the computer systems in it determine what can and is done with that signal. The most basic forms can basically tell you something is there. Measurements of the time of return can tell you a distance, while changes to that time as well as the Doppler effect will provide additional information as to velocity acceleration/deceleration, etc... To determine things like altitude you actually need an array of multiple antennas pushing signals.
Different forms of energy have different properties, for example, visible light may reflect off of something that a radio wave will pass through to a greater or lesser degree. A X-Ray may react differently to the same object. Depending on composition, paint can affect radar returns, but it is generally minimal. The shape of an object and how it is presented to the radar antenna can have a significant effect. Theoretically, a variation in the road surface that provides a flat face toward the radar could give it some level of return greater than the flat road that is parallel to the emissions. Materials have a significant effect, some reflect a great deal of energy others absorb it(actually allow it to pass through) or scatter it, preventing a direct return to the antenna. If you are getting an activation when, say passing from blacktop to cement, or more likely passing over metal expansion joints, it is probably an indication that the radar is pointed down further than it should be and you are getting too direct of a return off of the road. Shadows would have no effect. Blades of grass would, but it would be so small that a system like the one on a vehicle probably wouldn't register a return unless you are driving from a mowed area to a field of dense shoulder high grass and brush. Even then, it is questionable. Angle of approach has a small effect, in that the greater the angle, the less accurate a speed reading is. As the angle between the radar and the target increases it will always give a reading that is slower than the true speed. If the angle suddenly decreases, it can cause it to appear that the object is accelerating, when it is actually just giving a more true reading of speed.
As it is used in these systems, it is looking for a change in distance to an object that it senses ahead. If the time of return is decreasing, you are closing distance and it will activate warnings and/or slow you down. That means it should be a directional system that is pushing it out to the front, much like a police radar system.
