>
> Optical methods are also used (beam across the road, etc) but are subject
> to being tripped by humans, animals, rain.
>

I was thinking about this also.  I thought of two ways to make the system
more reliable, but would not necessarily work in all situations. First, use
a software timer to detect how long the beam is broken. Short breaks would
indicate humans or animals. Second, another beam placed ten - fifteen feet
further down the drive than the first could be used to tell if someone was
standing in front of the first beam. In other words, first beam is broken,
set a timer, if timer expires and beam is still broken, it's probably a
vehicle but it could just be obstructed. Now watch the second beam to see
if it gets broken ...

Eric