One thing about flying from Washington to Los Angeles hasn’t changed: It takes essentially the same amount of time to go from Ronald Reagan National to Los Angeles International now as it did several decades ago. So why have schedules for the flights gotten noticeably longer?

The answer is that as the number of travelers has increased and airlines have added flights to accommodate them, airports have become more congested. As a result, planes spend more time waiting to take off and then, when they land, more time waiting for an open gate at the terminal.

That means passengers are spending more time confined in the cabin.

“Airlines pad their schedules as a defensive move to protect their overall reliability,” said Henry Harteveldt, founder of Atmosphere Research Group, a travel analysis firm in San Francisco.

Airlines for America, the industry trade association based in Washington, recently analyzed data compiled by the Department of Transportation. It found that from 1990 to 2018, taxi time increased 19 percent at the nation’s 30 largest hubs and 24 percent at 31 medium-size airports. At Reagan National, taxi time increased by six minutes, to 27 minutes, in that time. At Seattle-Tacoma International Airport, the taxi time rose by nine minutes, to 26.