I had a meeting the other day with a parking sign manufacturer and they said that my signs are so far the most exhaustively researched. That was a big surprise to me. Apparently, although road sign typefaces are tested in terms of readability at certain speeds and distances, they are not tested for usability.
Rather than having the P symbol followed by the time maybe you could use something like this NSW style by having ‘2P’ for 2 hours parking:
Also how will you show free / meter / ticket?
Finally, parking signs are not just an issue in LA, here is one from my home town Sydney: http://www.2gb.com/sites/default/files/field/image/20140124/confusing_signs.jpg
There is definitely still room for improvement. I’ve been thinking about how to visually differentiate free parking from paid parking while being mindful of costs (I learned that adding a color will easily double the price of a sign).
A question for you: What does NSW mean? Is this notation used in Sydney?