> So you probly don't want a gizmo that needs turf dug up or
> dangles loops or is fiddly to align

IMHonestO, a laser is the least problematic solution. It won't
get dirty, is cheap, reliable, and not really that much of a chore
to set up. The beam is quite plainly visible on the target so it's
easily aligned. The detection process is very simple. Although
people will be sensed, their passage across the beam is of much
shorter duration than a vehicle and can be ignored / filtered out

If the beam can also be modulated, that makes it (almost) totally
immune to ambient perturbations, especially with a specific colour
filter over the target sensor

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