Pete, if these are you main concerns, then they are fairly contained events, more like evacuating from a wildfire, rather than a huge mass migration for an approaching hurricane.

I'm not sure how a chemical attack would likely play out, but running from a toxic industrial accident seems fairly hit and miss IMHO. Unless you're quite close to the source and would likely die whether you're inside or outside, sheltering in place is likely sufficient until the main threat has subsided. Since winds can shift quickly, getting stuck on the road and caught up in a concentrated plume could be far worse than hunkering down.

A bio attack would probably result in a locked down quarantine zone. If you're not inside the hot zone, it's probably safe enough to stay where you are, unless you're downwind of certain bio agents.

But unless you already live in a sensitive area with military assets, like chembio-specific detection equipment, it could take a while before anyone knows that a chembio attack is responsible for some situation. The "fog of war" is likely to prevail during any time window that would allow you to flee before authorities figure out what is happening and probably lock things down.

Anyway, just some thoughts.