>Microwave is immune to sun problems but might
>be interfered with in other ways.

Depending on the frequency you use, it might also get detected by traffic
radar detectors, which might not suit you (i.e. it tells the subject where
they were photographed, and depending on end use may result in equipment
vandalisation).

I like Jinx's idea of putting down a dose of scoria to get a low dust area,
and you could bury a coil of wire in the scoria for a traffic detector
similar to those used for traffic lights in cities, and automatic gates. Use
cable with a heavy duty sheath should do it, but you could always put it in
plastic water pipe for extra protection. Run a couple of lengths right
across the roadway each side of a gap in a fence line (so they cannot drive
around them if they suspect a sensor of some sort) and then if the wire is
discovered and ripped up it is easily replaced.

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