Maryland highway workers applied the brakes last night to traffic on the Woodrow Wilson Bridge in an effort to preview the effect that a major bridge reconstruction project will have.

By mid-evening, two of the three eastbound lanes were blocked, and traffic from Virginia into Maryland slowed from its usual 55 to 60 miles an hour to a sedate 20. Initially, half-mile backups and delays of several minutes were reported on the approaches to the bridge that carries Capital Beltway traffic over the Potomac between Alexandria and Prince George's County, but by midnight traffic had thinned considerably, according to police.

Workers also planned to place barriers to restrict westbound traffic to a single lane.

A second practice session is scheduled for tonight, starting at 8 p.m. Starting on Friday, the 20-year-old span will be redecked -- a $24 million process scheduled to be completed in January 1984. Traffic will be restricted to one lane in each direction on almost every night.

Last night, project manager Dick Dooley said things seemed to be going well. He said traffic was unusually light and attributed that to an effort to inform motorists that delays were expected.

The American Automobile Association has said nighttime traffic backups of two hours will probably be common when work begins. Motorists may obtain a free map showing 13 alternate routes by sending a stamped, self-addressed envelope to the Public Affairs Office, AAA Potomac, 8111 Gatehouse Rd., Falls Church, Va. 22047. For information on bridge closings call AAA-6200.