ORLANDO, Fla. (AP) _ ''Smart cars'' that give drivers the latest traffic information on a computer screen will soon take to the roads in Florida and California in an experiment aimed at heading off gridlock on overburdened highways.

Test programs in Los Angeles and Orlando over the next three years will feature cars equipped with computer screens using satellite-delivered traffic information from sensor-equipped ''smart highways.''

They are the start of a combined federal-state, private-and-public effort to avoid future shock on American roads: gridlock caused by ever-growing numbers of motorists on too few highways that are aging more quickly than they can be repaired, rebuilt or replaced.

The application of electronic and communications technology ''can add efficiency and capacity without adding pavement,'' contends the Highway Users Federation, one of the groups behind the project.

Experts say the technology is available, only the money and direction for its application are needed. The two test programs and an Orlando-area conference this week are intended to better define the problems and solutions.

The National Leadership Conference on Implementing Intelligent Vehicle- Highwa y Systems, starting Thursday, is expected to set forth a plan for action ''and maybe get some startup of a national organization to carry this forward,'' said Mark Norman of the Highway Users Federation, a lobbying group.

Transportation Secretary Samuel Skinner, who will speak at the conference, is calling for a transportation revolution because the nation's airports, highways, bridges, waterways and mass transit systems are simply wearing out.

Experts say that since no new systems are in the works, the existing infrastructure must be made safer and more efficient.

To get the new technologies on the road, Skinner wants a national research project supported by federal, state and local governments and by corporate and private groups.

The ''smart car'' concept involves the driver, the vehicle and a traffic management team - much as air travel involves the pilot, the aircraft and air traffic control.

In California's Pathfinder program, expected to get rolling in a month or two, 25 smart cars will operate on the Santa Monica Freeway and parallel arteries. They will receive traffic information on dashboard-mounte d electronic displays that are constantly updated.

The screens tell drivers of congestion and outline alternate routes. In return, the cars feed information about road conditions to a central computer.

The $2 million cost of the two-year test is being paid by the Federal Highway Administration, General Motors and the California Transportation Department.

In the larger and more sophisticated Orlando trial, called Travtek, 100 cars will be rented and leased to tourists and residents who drive frequently, said Jack Butler of the state Department of Transportation.

The computer screens will be able to suggest the shortest, least-congested routes and how to avoid construction areas and accidents.

The one-year evaluation is to begin in early 1992. The approximately $8 million cost will be borne by the federal and state transportation departments, General Motors and the American Automobile Association.

Surveillance cameras are being installed along a portion of Interstate 4 in the Orlando area to monitor traffic flow and problems like accidents. The data will be fed via satellite to the smart cars' dashboard computers, along with information from traffic spotters in helicopters.

Orlando, like other cities, already has the start of smart highways in the form of computerized traffic signals that monitor tieups at intersections. The computer changes traffic lights accordingly.

In Europe, an $800 million-plus project called Prometheus is aimed at improving highway and traffic safety and relieving congestion through smart cars, smart highways and computers.

In Japan, three similar programs are under way.

''People in this country feel that we are behind not because of technology, but in the institutional framework - establishing public and private partnerships,'' said Norman. ''That's what this conference is aiming at.''