I often don't need as much as 3 feet, and when I need more, I just negotiate for it by moving closer to the center of the road. So it would be pretty silly for me to wear a shirt requesting a static number that's almost never the clearance I need.

When I'm riding too close to the edge of the road, overtaking drivers interpret my position as an offer to share my lane with them. When I'm using enough of my lane, overtaking traffic moves into the other lane to pass.

As an added bonus (along with more passing room), riding farther from the edge improves my conspicuity and sight lines, and reduces right hooks, left crosses, and pullouts. Those three attacks from the front are much bigger problems for cyclists than same-direction traffic - which is all that's going to see the back of my shirt.