Do you see the triangle of gravel in the picture above? The 90 angle is caused by the sweeping action of south-bound and west-bound vehicles pushing the gravel to the inside. The arc-like base of the triangle is caused by right-turning vehicles pushing the gravel to the outside. There the gravel will stay until a street sweeper comes by on its annual (if ever) mission.
Imagine bike lanes again, that are seldom or never swept. What falls there stays there.
BTW: The cost of sweeping bike lanes? $1,000 a year per linear mile, plus or minus a few hundred dollars. See why they don't get swept? A 200 mile bike lane system could cost $500,000 a year just to sweep... in a City that can't even afford to replace stop signs when they get knocked down.