Has any city or nation adopted anything remotely along the lines of what you've recommended, with beneficial results?
What if a city did require police to hesitate before shooting at civilians, and more cops were killed as a direct result of that policy change? Would you decry that outcome?
How long were the police supposed to wait before concluding the gun wasn't real, or Tamir wasn't really going to shoot them?
Does it occur to you at all that you're looking at this with hindsight, and you know more than the cops could possibly know at the time of the incident?