On a sunny day (Mon, 31 Dec 2018 11:01:04 +0100) it happened "R.Wieser"
 wrote in :

>mm0fmf,
>
>> you could put the camera in an upstairs window looking down at the road
>
>In that case I would rather put a cheap reflector on/in the road and aim a
>lightbeam at it.   As long as the beam comes back the spot is free.    Add a
>bit of leeway to account from strange objects (pedestrians?) passing thru
>the beam and it should work nicely.
>
>... apart from autumn and or winter debri (leaves, snow) ofcourse.   Than
>again, that would most likely also pose a problem for image recognision.
>
>> Also worth adding some more image recognition to the system so it can tell
>> you if there's some gammon faced bod who can't read and has had a triple
>> sense of humour bypass hanging about looking for the inflatable bollards.
>
>:-)
>
>Regards,
>Rudy Wieser

Maybe something like this:
  http://www.ebay.com/itm/263367772077

A HFS-DC06-5-8GHz-Microwave-Radar-Sensor-Module-DC-5V-ISM-Waveband-Sensing-12m
Only 5$ inclusive shipping

Range 12 meters, 360 degrees, maybe add some parabolic antenna to narrow the
angle.

ebay is full of things like that.

It is not a camera, so that cannot be an objection.
Anyways I have 5 security cams monitoring house and environment,
and those have already payed for themselves, bad guy caught in the act.

Doing image processing is possible, but indeed with sun position changes,
weather, frost,
reflections from pools of water, etc etc that maybe be more difficult.
I once wrote some soft to read a digital alarm clock via a webcam:
 http://panteltje.com/panteltje/7s_parser/index.html

Monitoring a fixed pattern (ground) for presence should not be so easy, weather
/ snow / day / night permitting.

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