Mesh/Decentralized FM/AM radio
Legal FM transmitters are so low-powered in the US that it doesn't make sense to even try. Their effective range is something like 300 feet max. We'd need A LOT of these to make it work.
AM radio seems like a better option (though the chance of someone randomly scanning their car radio and finding the channel is almost nil on AM). The range is something like 1/4 to 1/2 mile for a good legal transmitter. To completely cover the entire area of Oakland we'd maybe need somewhere between 600 and 1500 transmitters. If we could get a decent transmitter setup for maybe $200 per unit, then that's $120,000 to $300,000. Which puts it not entirely outside of the realm of possibility.
The rangemaster is apparently a good legal AM transmitter. It's about $750 for the cheapest model though. Here's some info on running a decentralized AM transmitter network.
If we used e.g. a beagle bone black with a GPS for timing then it would be theoretically possible to generate a clock or control a PLL with the raspberry PI based on the GPS timing, thus ensuring that the carrier signal phases of all transmitters are synced.