“Athletes are sleepy, hungry and need to pee. Could we get to the Olympic Village please,” Mr. Clement wrote as the driver, struggling to understand the route given by the bus’s GPS device, finally abandoned repeated forays up dead-end streets and pulled out a map. “Um, so we’ve been lost on the road for 4hrs. Not a good first impression London,” he added.

The misrouted buses episode came as thousands of competitors and team officials began arriving at Heathrow on a day that officials said was the busiest in the airport’s 80-year history. There were none of the three-hour backups that have plagued the airport in recent days, raising fears of gridlock with the Games less than two weeks away. Instead, travelers passed smoothly through passport control, many in just minutes.

The main M4 highway into London was open again after the completion of emergency work on an unstable flyover that had closed six miles of the route for two weeks, and traffic moved smoothly along the miles of “Zil lanes” — dedicated parts of the main roads across London that have been set aside for the exclusive use of Olympic officials and V.I.P.’s, named after the Russian-made limousine and the Soviet custom of reserving road space in Moscow for top Communist Party officials.

Many of the 100 Olympic venues were officially opened, and while there were glaring examples of no-shows among the private security details, troops and police officers drafted to replace them began to rebuild confidence that the Games would be properly protected. Last week, acknowledging that the private security contractor G4S was going to be several thousand guards short of its commitment to provide 10,400 for the Games, the government announced it would add 3,500 troops to the 13,500 already preparing for Olympic duty.

And then there was Mr. Clement, the American hurdler, twice a world champion and the silver medalist at the 2008 Olympic Games in Beijing. Before boarding his trans-Atlantic flight, he had brimmed with optimism. “It’s always exciting seeing other athletes on the flight to London,” he posted on Twitter. “We are all after one thing. GOLD.”