Are speculators sitting on vacant lots in downtown Phoenix because property taxes are low so it behooves them to wait to potentially hit the jackpot with future land value jumps? Why not create an overlay district that is revenue neutral in which property taxes on vacant land are raised and property taxes on developed land are reduced by a comparable amount? This would encourage speculators to either do something productive with their land or sell it to someone else who will do something productive with it. Investors should make their money off of generating productive value, rather than sitting speculatively on assets (which drags down the value of surrounding property and hinders density, walkability and mass-transit economics).