From tomorrow, parts of downtown Zurich will be pedestrianised
(German PDF) - see the diagram on page 2 of the PDF (I got a copy from the nice police lady by the Münz Bar who tells me she doesn't know if my bike parking spot will still exist!
This means that no traffic will be allowed in there without permission (available to local residents, businesses, etc). I believe progress within the zone will also only be permitted at walking pace.
This might also be the reason behind people on bikes in the area being nicked for riding where it says "No Riding"