15 Minute Parking - A Reader Asks if this is Legal

A reader writes: 

For a couple of years I have used the 15 minute parking zone on the west side corner of Wabash and Harrison, which was convenient for a barbershop right around the corner. Recently Dunkin Donuts/Subway placed what looks to be a makeshift sign on the sidewalk that prohibits non-customer 15 minute waiting. Is this legal? Thanks.

Interesting.  We don't know, but figured we would see if anyone does.

(Hat tip: OI!)