ABUS CARRYING 45 people, mostly elderly nursing home patients, burst into flames Friday, apparently when passengers' oxygen tanks exploded. Families with small children were stranded for more than 24 hours on highways that turned into parking lots. The city of Houston, incredibly, appeared to run out of fuel, and the Texas National Guard was delivering gas directly to stranded motorists. In other words, we were wrong Friday when we spoke of an evacuation proceeding like clockwork: What looked, on Thursday morning, like a carefully planned, well-thought-out evacuation from the Texas coast had deteriorated, by Thursday night, into a nightmare for many people who had obeyed orders and evacuated when they were asked to do so. Asked what went wrong, many of the planners who were so eloquent in listing the mistakes made in New Orleans in the days after Hurricane Katrina were left with explanations along the lines of "there were too many cars on the road."

Mistakes were made, of course. As those who plan these things more frequently in Florida attested, Houston's mayor ought to have been much clearer about who should leave the city -- namely anyone whose house was in danger of flooding -- and who should, for the sake of those in more dangerous areas, stay home. The highways should have been made into one-way roads much earlier and more consistently. The gas shortages should have been anticipated sooner, and extra supplies should have been brought in.

Having said that, the double whammy of hurricanes Katrina and Rita may bear another lesson. Officials did succeed, with the public's cooperation, in getting an extraordinary number of people out of what was predicted to be Rita's way. In Galveston, Port Arthur and other coastal cities, they made a huge effort to move the immobile and the indigent. But there are limits to what even the best planning can achieve. In New Orleans, nursing home operators are being assailed for leaving their residents in the path of the storm; the bus fire in Texas sadly illustrates the risks of not staying put. A hurricane is more predictable than most calamities, but 72 hours before Rita made landfall -- even 24 hours before landfall -- forecasters were still not able to say where, exactly, the storm would hit, who would be most affected, who really needed to move and who should stay put. We should expect officials at every level to learn from each of these experiences, but there may be no perfect way to move 2 million people in two or three days.