[throwway120385]

Could you use a different spectrum of EM radiation to detect water? There are parts of the microwave band that attenuate the signal by absorption and I wonder if you could use that. The only clue a human driver has in that situation is in the visible spectrum. The lines of the road disappear from view, which can be challenging to see at night.

[esseph]

In theory you could send different frequencies, but then you run afoul of all kinds of potential interference with other systems and other local regulations.