I consider mass evacuations to be hazardous - there are many sharp hooves flying when the herd is in panicked flight. Hence I would try to stay put if possible.

That said, sometimes staying is riskier. If there's a major wildfire I'd have the car packed & ready to go (gas, etc), check which routes were safe, and go within minutes when told. A train wreck with chemical spill would be similar.

If I thought I could beat the crowd out of town I'd run even if I thought it unnecessary, just to avoid dealing with fuel & other disruptions. But this is almost certainly wishful thinking: I won't learn anything before everyone else. Beating the crowd just isn't realistic unless you're already past the suburbs.

Another consideration is that I have 30 days lifeboat food & water with a 2x safety margin (half in one side of the house, half in the other). Civil Defense recommendations almost certainly assume this is not true. There are cases where I'd stay in the face of recommendations to go if the assumptions behind the "go" announcement didn't apply.

Finally consider the inconvenience in an evac. I have a good friend 15 miles out of town on my main evac route, my parents are 30 miles north of town near a secondary evac route, and my first stop is with friends 180 miles away. It's not a big deal if I run unnecessarily assuming traffic conditions permit.