[bsder]

You have to weigh the death toll of the evacuation against the death toll of the disaster.

We see the flooding issue right now--an evacuation would have killed far more people.

A dirty bomb? Once the bomb goes off you will have plenty of time to evacuate where you need to. Keeping radioactivity contained would be more problematic than removing people.

A biological agent? You want to quarantine rather than evacuate.

A fire? San Diego showed how to deal with that--phased evacuations ahead of the actual fire path.

Earthquake? Well, once it's done you have plenty of time to evacuate.

I can't really think of anything that would affect something the size of Houston simultaneously.