I have often envisioned something similar, only it would spray blood. Hopefully triggering the thought sequence "ohgodthere'sbloodohgodwhodidIhitohgodI'mintheshitohg..eh..oh sweet relief I didn't kill anyone, I never want to feel like that again, I'm giving those fucking cyclists a wide berth now"
Unfortunately I imagine the reality would be something like ‘the little fucker. Cyclists are all the same, ill close pass the next entitled 2-wheeled shit I see’
Remember that bicycle that painted stuff on the road as it went?
I've always thought about having a sensor that detects close passes and vehicle speeds and sprays a legible warning message onto the car as it passes.