Flying on a commercial jetliner has never been safer.

It will be four years Tuesday since the most recent fatal crash in the United States, a record unmatched since propeller planes gave way to the jet age more than half a century ago. Worldwide, last year was the safest since 1945, with 23 deadly accidents and 475 fatalities, according to the Aviation Safety Network, an accident researcher. That was fewer than half the 1,147 deaths, in 42 crashes, in 2000.

"The lessons of accidents used to be written in blood, where you had to have an accident, and you had to kill people to change procedures, or policy, or training," said Deborah Hersman, the chairwoman of the National Transportation Safety Board. "That's not the case anymore. We have a much more proactive approach to safety."

The last fatal accident involving a U.S. commercial flight was Colgan Air Flight 3407, which crashed near Buffalo, N.Y., killing 50 people, on Feb. 12, 2009. The pilot did the opposite of what he was supposed to do when ice formed on the wings.

Perhaps even more noteworthy, there has not been an accident involving a major domestic carrier since an American Airlines flight to the Dominican Republic crashed after takeoff in New York in November 2001, killing all 260 people on board.

But while flying is safer, it is still not risk-free.

Air traffic is set to grow in the next decade, and airports are more congested. Near-misses on runways and taxiways have risen. And with 2 million U.S. passengers boarding more than 30,000 flights every day, maintaining that safety record will be a challenge.

The Colgan accident also cast a troubling light on regional airlines, which hire young pilots, sometimes with little experience, at a fraction of the salaries paid by the bigger carriers.

Since the crash, the Federal Aviation Administration has mandated longer resting periods for pilots. But in the face of opposition from airlines, it is still working on new rules for more extensive co-pilot training.

"It's important not to define safety as the absence of accidents," said Chesley B. Sullenberger III, the US Airways pilot who became a hero when he landed an Airbus A320 in the Hudson River in January 2009 after both engines lost power. All 155 passengers and crew escaped.