Flying on a commercial jetliner has never been safer.

It will be four years on Tuesday since the last fatal crash in the United States, a record unmatched since propeller planes gave way to the jet age more than half a century ago. Globally, 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 less than half the 1,147 deaths, in 42 crashes, in 2000.

In the last five years, the death risk for passengers in the United States has been one in 45 million flights, according to Arnold Barnett, a professor of statistics at M.I.T. In other words, flying has become so reliable that a traveler could fly every day for an average of 123,000 years before being in a fatal crash, he said.

There are many reasons for this remarkable development. Planes and engines have become more reliable. Advanced navigation and warning technology has sharply reduced once-common accidents like midair collisions or crashes into mountains in poor visibility.

Regulators, pilots and airlines now share much more extensive information about flying hazards, with the goal of preventing accidents rather than just reacting to them. And when crashes do occur, passengers are now more likely to survive.