Why not utilize the ad-hoc network that most citizens, tourists and vehicles already have instead? Set up virtual geo-fences in important locations, and have applications alert you when you're around something important.

This can be a passive or active system, depending on the nature of the application. You can track how many people are within a given area (city gathering information) OR, as a citizen or tourist, I can gain knowledge about places that I am near or will be near (using predictive / learned route information).

I don't want to have to carry around a bunch of different RFID chips to activate things, and it's cost prohibitive for cities to install readers all around town.