Same here. I procured an ACL problem skiing. Since then, I have started riding fixed and it went away, to the point that when I played some serious beach volleyball yesterday for the first time in 15 years, my knees and legs felt younger than when I stopped playing all these years ago.

Caveat - hard city riding might be different to what I do (10 mi one way commute, hills, no frequent stops on the way, 50x18)