This is certainly unprecedented flooding for houston and it looks to get worse over the next few days if the rain continues.
Houston flood regularly and honestly it depends a lot on where the rain dumps. From what i hear the mayor wanted to avoid a repeat of the issue we had when i think it was Ike hit a few years ago. Tons of people tried to evacuate and as a result the freeways backed up and people ran out of gas and gridlocked the freeways. I know a number of people died as a result of it and they felt it would be better to stay and shelter in your home.
Last i heard only 2 people have died here in houston, its not over and i am certain that number will rise but 2 people for all the flooding we have had isn't bad at this point.