In Houston, shortly after Katrina occurred a hurricane called Rita put the city in it's sites. Because of the obvious incompetence displayed by the govt. in Louisiana for Katrina, officials overreacted with an abundance of caution - and Galveston Island was evacuated. People did not go far because it was unclear where it was actually going to land (storm was already turning North and it did end up way East of Houston), so Houston became very crowded.
In the last 48 hours, the storm came close enough to cause real concern and a mandatory evac was called for areas South of Houston (again abundance of caution). This is a good description from Wikipedia. As you can see, only up to 4 million people tried to leave within a 36 hour stretch. In a city where 5-lane freeways are common place, an obvious disaster unfolded, because you see, they run our of gas or don't have a stable vehicle to flee in:
Just three weeks after Hurricane Katrina devastated the northern Gulf Coast, the threat of yet another major hurricane prompted mass evacuations in coastal Texas. An estimated 2.5 – 3.7 million people fled prior to Rita's landfall, making it the largest evacuation in United States' history.
Officials in Galveston County (which includes the city of Galveston), which was devastated by the Galveston Hurricane of 1900, ordered mandatory evacuations, effective September 21 at 6 p.m., in a staggered sequence. Officials designated geographical zones in the area to facilitate an orderly evacuation. People were scheduled to leave at different times over a 24-hour period depending on the zone in which the people were located. The scheduled times were set well in advance of the storm's possible landfall later in the week, but not soon enough to ensure that all residents could evacuate safely in advance of the storm. Nonetheless, many residents remained in the county because they were either unaware of the danger of the storm or believed that it was more important to protect their belongings, particularly in the wake of looting following Hurricane Katrina. The evacuation included transfer of all inpatients from the University of Texas Medical Branch hospital to other regional hospitals. 400 patients were prisoners under the ward of the Texas Department of Corrections. These patients were systematically transferred to the University of Texas Health Center at Tyler.
Officials of Harris County hoped that the designation of zones A, B, and C would help prevent bottlenecks in traffic leaving the area similar to those seen at New Orleans prior to Katrina and Hurricane Dennis earlier that year. Also, people in certain zones were to be forced to go to certain cities in Texas and were not allowed to exit their designated routes except for food and gas — another feature of the evacuation plan which officials hoped would keep traffic flow orderly.
The evacuation-destination cities included Austin, College Station, San Antonio, Dallas, Huntsville, and Lufkin, Texas. Evacuees were asked to try hotels in the Midland/Odessa area when hotels began to sell out in other areas.
On Wednesday, Houston mayor Bill White urged residents to evacuate the city, telling residents, "Don't wait; the time for waiting is over," reminding residents of the disaster in New Orleans. After heavy traffic snarled roads leading out of town and gas shortages left numerous vehicles stranded, Mayor White backed off his earlier statement with, "If you're not in the evacuation zone, follow the news," advising people to use common sense. However by 3:00 p.m. that afternoon, the freeway system in Houston was at a stand-still.
To the east of Houston, officials had set up evacuation routes in response to the slow evacuation of residents prior to Hurricane Lili. During the Rita evacuation, these preparations and their execution were overwhelmed by the enormous and unprecedented number of people fleeing from the Houston area prior to the departure of local residents. By the time Jefferson County began their mandatory evacuation, local roads were already full of Houstonians. Traffic on designated evacuation routes was forced to go far slower than the speeds experienced with any previous hurricane.
By late Thursday (22nd) morning, the contraflow lanes had been ordered opened after officials determined that the state's highway system had become gridlocked. The Texas Department of Transportation was unprepared to execute such a large-scale evacuation. Coordination and implementation of the contraflow plan took 8 to 10 hours as inbound traffic was forced to exit. Police were stationed to assist with traffic flow. Evacuees fought traffic Wednesday afternoon through mid-day Friday, moving only a fraction of the normal distance expected. Average travel times to Dallas were 24–36 hours, travel times to Austin were 12–18 hours and travel times to San Antonio were 10–16 hours, depending on the point of departure in Houston. Many motorists ran out of gas or experienced breakdowns in temperatures that neared 100 °F (38 °C). Gas stations reportedly ran out of gasoline, forcing some evacuees to fill up with diesel, which is incompatible with gasoline engines, enabling them to leave the area but left them with expensive car repair bills afterwards. Traffic volumes did not ease for nearly 48 hours as more than three million residents evacuated the area in advance of the storm.
And Houston is a huge/spread out place. I shudder to think of the same event occurring in LA, with 15+ million people. Ultimately, the only way to get out of the city would be on foot.