Should there have been a mandatory evacuation of Houston?

Where?

Nationwide?

Over 100 people died during the evacuation of Rita. I'm sure that was on the mind of those who issue evacuation orders.

Charles LSU hit the nail on the head. Mayor Turner did the right thing.

They should have evacuated areas where flooding would have been likely for any storm and elderly or medically related for sure.

Most people would have stayed with where it went inland.

Nah. Except for those poor old people at the nursing home. The owners of that place should be put in jail.

Sure. Kill another 107 people just like Rita.

How in the hell do you evacuate 2.5 million people?

The mayor was technically correct but his message resonated as "stay at home, everything will be ok" which isn't right either.

It still would have been technically better to drop the population of the city to as low as feasible with the 2 days they had, so maybe a "voluntary evactuation" would have been better than the shelter in place at t-48 hours. Knowing of course the vast majority of people will stay in place during a voluntary evac.

The problem is if you get much outside of Houston the freeways will drop to 2 lanes each direction. If you don't contraflow the capacity then to evacuate Houston to the East and North (West to be assumed blocked off) is only 71,000 cars/day to the East and 71,000 cars/day to the North. Averaging 2 people per vehicle that is only ~560k people you can get out of town. Maybe double that to account for back roads (e.g. 190 to the East), but you'd struggle to move more than 20% of the population in 2 days with the road to San Antonio blocked off and the Houston - Austin stretch not actually having an interstate between the two.

Source

Maybe the message should have been something as follows

"We are likely to have heavy flooding in Houston, peoples lives will be impacted but with some precautions you are likely to be safe if you stay here, if you are of able body it is more dangerous to be in a vehicle than your house during a major flooding event.

But, if you have a place to stay in Louisiana, North Texas, or somewhere to the East or North of us, and the roads are clear you should pack up and go within the next day or so. Reducing the number of people in this city will help reduce the strain on the infrastructure. You can check Google maps or Waze for traffic. We do not advise leaving your home if a road looks to be high traffic on Google maps due to what happened in Rita"