Could also post electronic signs on freeways that warn motorists to stop and pull over immediately, particularly before hazardous areas like bridges, tunnels, viaducts, and onramps. IIRC most of the civilian casualties of the 1989 Loma Prieta & 1994 Northridge earthquakes were from viaduct or bridge collapses that pinned cars underneath. With a minute's warning, you can prevent any more cars from entering the bridge and get most of the existing traffic out.