Bike frontal cross section v speed perception
Perhaps some of the experts can help here. Down here, one of the main causes of bike/car accidents is where motorists will stop to turn into or cross a lane, causing an approaching bike to collide with the car. One of the 'excuses' for their action is that; they did see the bike; but did not realise the bike was travelling so fast.
I am both a biker and cager. So, I started paying special attention to this 'excuse'. And it's true; at least by my observation. It appears that one can better judge the approach speed of the wider car, than the approach speed of the narrower bike.
More, I have noticed that; where bikes have additional riding lights; arranged in a form of a triangle with the riding lights on the extremeties of the bike; judging their approach speed seems to be better and more accurate.