Just think about how few minutes the warning would be if it ever happened—and then imagine moving anywhere in Manhattan if such a warning were given.
The LEOs would be stuck in traffic just like everyone else.
True. Also, most who happen to be on duty at the time would abandon their posts and head out to their families.
The LEOs here have proved time and time again that they're completely useless. From weather disasters to a simple noise complaint, don't count on them for any info or assistance. YMMV but don't chance it with your loved ones. You're on your own.