Today’s invention is based on a compelling visual illusion known as ‘Mirage‘ which consists of a couple of parabolic reflectors face to face, the upper one of which has a hole in it.
I imagine this being adapted as an extra road safety measure for those people who have to work in open manholes, protected from passing traffic only by a few flimsy poles and signs.
The workers would stand on a portable, shallow paraboloid reflector and temporarily fit another to the underside lip of the manhole. This would give the illusion that their hard hats were actually moving about above the road surface and cause drivers to give them a much wider berth.
This might be supplemented by a conical insert fitted in the manhole from above and which would be bonded to a cowcatcher device which would deflect any vehicles about to drive over the hole. This would be made in a transparent plastic (to enable the illusion to be seen). Any impacts on the sides of the catcher would transmit the load to the cylindrical rear face of the manhole ironwork.