It definitely won't go well if you don't plan for it. Which they apparently hadn't for Rita. And clearly still haven't done. You need to come up with a good plan. Then you need to practice it, by that I mean table top and other exercises for emergency management folks, first responders, etc. That helps you find out where the plan breaks. Then you need to modify your plan as appropriate. Then communicate that plan widely to residents and other stakeholders.

While getting the entire 6.5 million Houstonites out clearly isn't feasible, there are a number of things that could have (and should have) been done to get people out of the most hard hit areas.

1. Do a staged evacuation, so everyone isn't trying to leave at once. Evacuate the most vulnerable areas and populations first. That means nursing homes in low lying areas, then residents of low lying areas. Then the next most vulnerable areas, etc. These days with GIS, hydrologic data, etc, it is pretty straightforward to identify the areas most at risk.

2. Identify evacuation routes, and find ways to make them work better. Identify which roads are most likely to flood first. Plan to reverse traffic flow on inbound lanes. In other words, make all lanes on key roads and freeways outbound.

3. Communicate this planned sequence to residents and stakeholders. In today's world, hurricanes are not surprise events. They had several days to communicate which areas would evacuate first.

4. Identify safe staging areas for law enforcement and emergency personnel, so that they can remain in flooded areas to discourage looting, and rescue those who can't get out in time. Practice staging them there.

4. Make evacuation orders mandatory. That doesn't mean police drag people from homes. But studies have shown that calling an evacuation "mandatory" leads to significantly better compliance for people.

5. Have a consistent message from officials. Don't have the Texas governor saying "Evacuate!" and the Houston emergency manager saying "Shelter in Place!".

All this does require planning and practice. It also does cost a little money to create these plans and rehearse them. And recognize that no plan will cover every possibility, but it is much easier to modify an existing plan to meet unforeseen circumstances than to create a plan on the fly. Gen. Dwight Eisenhower, who successfully ran the Normandy Invasion (probably the most complex military operation of all time) put it so well:
“Plans Are Useless, But Planning Is Essential”