Interesting discussion, and welcome insight from JimG.
I remember reading of Vascar in 'Motor' & 'Autocar' back in the 70s and thinking how beautifully simple clever it was. Try it for yourself... even on a dull day, you can see the flash of shadow, with split second accuracy, as a car way ahead in the distance passes under a bridge.
It also resulted in my one and only speeding fine, circa 1990, when i didn't bother slowing down for a temporary 50mph limit on a 70mph motorway. Until a few years earlier, temporary motorway limits had nearly always been just advisory, so they were often ignored.
I saw the police car, quite a long way off but gaining on me rapidly and so I eased off and slowed, but I was far too late. The officer conceded that my powers of observation were reasonable as he could see when I slowed that I spotted him at some distance, but he'd already clocked me at 67 and so a ticket was issued. Couldn't really argue. It was all quite an amicable exchange in fact.