Scientists at the University of Hawaii are developing an eight telescope system, to be based in that state, to provide early warning of dangerous asteroids approaching Earth. The system is called ATLAS.
ATLAS, which will also search for deep sky objects from dwarf planets to supernovvae, will be able to give a one-week warning a 50-meter rock is on the way, and a three-week warning for a 150-meter body. That's not much, but it might allow the evacuation of a city.
It's a start. ATLAS should be operational by 2016.