It is now widely reported that Harvey has destroyed up to 500,000 new and used cars. That is as many as Katrina and Sandy combined. Telling people not to leave town was a mistake. It simply exacerbated property damage by keeping hundreds of thousands of vehicles in the driveways of homes that were destroyed.
The Houston bus system saved all of its buses by parking them up on freeway flyover ramps.
https://www.nytimes.com/2017/08/28/opinion/harvey-flooding-mayor-evacuation.html?mcubz=0Have you ever been to Houston around 5pm on a Friday? It's complete gridlock in most of the city, and that's a regular Friday where plenty of people aren't going anywhere. Add in the panic of a hurricane, and make it everyone in the city instead of the regular working folks, and you now have a new disaster.
120 people died purely in the evacuation last time they did it for Rita in 2005, when the city had even less people. Currently we're at 60 dead for Harvey, half as many as just the evacuation. Additionally, by the time they know we need to evacuate it's too late. Yeah they could've told everyone a week before it happened to evacuate and it might've gone okay, but if they take that strategy they're going to be evacuating the whole city multiple times a year just in case, and most of the time nothing will happen (and people will start ignoring evacuation orders). 48 hours isn't nearly enough time to evacuate everyone, and they had no idea how bad this storm would be until well within that window. Sure there were some weather channels talking about the "possibilities" of the storm getting really bad before that, but that happens with every storm.
Evacuation is one of those things that sounds really simple, but in practice is basically not possible in the timeline you have to work with.