Fallout worry is not a thing with modern ICBMs and warheads (not the way you're imagining it). By the time the ICBM enters the terminal phase nothing can (currently) stop it. It's just a deployed MIRVed/MARVed warhead, ie. a set of a dozen or more hypersonic glides tipped with nukes and you have barely minutes to do anything even if you could. No, the way you stop a modern ICBM is you shoot it down with a nucler tipped anti-ICBM missile mid flight during its mid flight/coast phase, way before it had a chance to deploy. This basically happens in space and no fallout is going to reach the ground.