I came across the following NYC article from on the internets whilst researching the situation for ATX:
The original law recognized the injustice of installing a new parking sign while cars were parked in spaces regulated by the old sign, and issuing parking tickets based upon a violation of a parking sign that didn’t exist when people parked their chariots. Horrible but true.
As per the above, in NYC, it appears that there are currently explicit laws on the books that prevent installing the sign and issuing the parking tickets on the same day/week that the sign gets installed, and/or providing an affirmative defence for issued tickets.
What about other jurisdictions, specifically, Austin in Travis County, in Texas? Wouldn't an appropriate notice and grace period be required for the cars parked under the old signs (or lack thereof) provided that the new signs are respected within XX hours? (All other local laws in ATx did seem to have similar provisions, e.g., it was widely reported that at first only the warning was to be given for violating the hands-free ordinance.)