I had this idea about 15-years ago but chickened out.

I came back to the idea because AZ has a 3-foot passing law, I had a close passing incident captured on video, and yet could not really prove that the pass was inside 3-feet for enforcement. The flag solves that problem; it extends 35 3/4" inches beyond my handlebar. If you hit the flag, you're inside 3 feet.

I thought that I might get some guff for the flag but the only comments I have had are very supportive. One lady who commented on it yesterday really seemed to appreciate my use of it and claimed that it was very helpful to her as a driver.

It looks HUGE in small places and on small roads, but not so much on the highway as far as I can tell. The main benefit seems to be in places where drivers are tempted to "shoot the gap" in tight places.

It is, at least, an interesting experiment in driver psychology and a way to measure the minimum clearance.

PS. My commute is mostly highway with a wide shoulder and rumble strip. I ride right of the rumble strip except in two short sections where the the guard rail is too narrow to pass outside the rumble strip.