While cycling during one of the unseasonably sunny October days, I passed lots of cars on the opposite side of the road, but none seemed to be overtaking me. I wondered: how many more I should expect on one side compared to the other?
Making some unrealistic assumptions: I ride my bicycle at a constant speed of 
The unslowing cars approach from behind at a merciless relative speed of 
A pictorial calculation: cars in the green block start ahead of me; those in the red and yellow have gone by after one hour.
For example, if I cycle at 10mph in traffic of 30mph, I should be expected to be overtaken by half as many cars as zoom by in the other direction. (Quick sanity check: the formula also works if I am cycling at the same speed or faster than the cars; or if we change the units.)
Of course, that was all too easy: another implicit assumption I made, that was reasonable, is that we’re all travelling at velocities nowhere near the speed of light. Otherwise, pointlessly taking special relativity into account, relative to a fixed observer standing on the side of the road, if I travel at 
As a check, suppose instead of cars overtaking, we think of light beams (ie. 
For instance, if 
