Houston metro is nearly six million people. It wasn’t really until Wednesday when there was much certainty. Forty-eight to 72 hours is impossible for a metro area.
Here’s a story from Wednesday:
...Hurricane Harvey is heading toward the Gulf Coast and could become a major Category 3 hurricane by Friday.
The storm’s track, while still unpredictable, appears to be heading south of Houston but could dump 12 to 20 inches of rain or more across a broad area of the Texas coastline...
“Houston metro is nearly six million people. It wasn’t really until Wednesday when there was much certainty. Forty-eight to 72 hours is impossible for a metro area.”
I don’t buy that, they don’t need to evacuate the entire city just the low elevation areas which they know from past flooding will flood the most.
The rest of the city is then more easily managed using emergency services.
As for the parking lot theory, the beauty of doing only select neighborhood evacuations means you can limited the number of people using the roads to get out at any one time.
You can limit it further by assigning specific time slots to the evacuation for each section, and calling people up according addresses to insure that they leave during their slot.
A city can even uses the police according to traffic tested plan to close select roads and open both lane to force people to leave thou the most efferent routes(not allowing people to go back into the city) thus maximizing your thou-put.