WASHINGTON  The Federal Aviation Administration will add a runway version of traffic signals at 20 busy airports in the next three and a half years, the agency said Monday. The signals are part of a program to keep taxiing airplanes or vehicles from intruding on runways where other planes are taking off and landing.

Reducing runway incursions has been a difficult problem for the F.A.A. as the tempo of airport operations has increased.

The system will use a computer to determine when a runway is in use, and then turn on red lights embedded in the pavement at each intersection. Thus it would help to counter errors by pilots and controllers at the 20 airports, which include Kennedy International and Newark Liberty International, but not La Guardia.

The system could have prevented two close calls at airports in New York. In July 2005 at Kennedy, a fully loaded Boeing 767 moved onto an active runway as a cargo plane, a DC-8, was taking off; the cargo plane climbed steeply to avoid the passenger plane’s tail. In July 2007 at La Guardia, a controller mistakenly cleared a commuter plane to taxi across a runway as another jet touched down; the controller told the commuter plane to hurry up.