Helping Motorists Out of a Jam : Transit: A computer system displays maps on a screen in the car and assists the driver in avoiding congestion. It will be tested on Southland freeways.

You’re behind the wheel of a car on the Santa Monica Freeway during the morning rush hour. You’re new around here, so you’re not sure where you are, or exactly where it is that you want to go. And the traffic looks like it might be building up ahead.

According to Caltrans, the Federal Highway Administration and General Motors, it’s time for “Pathfinder.”

You push some buttons, and a map appears on the bright little television screen mounted on your dashboard. An arrow shows you where you are and which direction you’re headed. A series of triangles pinpoints the traffic congestion ahead.

You push some more buttons, and a voice tells you more about the traffic blocking the freeway. Or, if you prefer, the same information is printed on the screen.

You take an off-ramp, pull to the curb and punch in the address where you’re headed. Within seconds, your destination is marked on the screen with a star.

Using alternate routes marked on your map, you avoid the congestion and keep going until your arrow and the star merge.

You’re there.

In the meantime, data on the location and movement of your car has been fed into a centralized computer system to help monitor the overall flow of traffic and detour drivers around bottlenecks.

Of course, Caltrans already has sensors in the pavement of the Santa Monica, San Diego and Harbor freeways that do a lot of this sort of monitoring. There are broadcasts over dozens of radio stations that provide much of this information. And all of that is free to the individual driver, as opposed to the $9,000 to $10,000 it currently costs to equip a car with the Pathfinder system.

But the sensors and the radio can’t tell you where you are or show you exactly where you want to go. They can’t provide you with constantly updated, variable-scale maps that display all this, plus traffic congestion. And engineers say that once in general use, the per-vehicle cost would drop dramatically.

“We’re not going to expand our highway system significantly, so we’ve got to learn to use it better,” Jerry Baxter, Caltrans regional director here, told a news conference at the agency’s downtown offices Monday. “We are confident that if drivers have complete information, they will make wise commute decisions.”

For now, the Pathfinder system is confined to 25 cars, all donated by General Motors, that will ply the Santa Monica Freeway and environs for a field test over the next year. The overall project, to be funded by Caltrans, the federal Department of Transportation and GM, is pegged at about $1.65 million.

The special electronic hardware in each car is based on an ETAK Travelpilot--a computer-driven navigational system that displays road maps on a dashboard television screen. In addition to an advanced ETAK system, each car has a two-way radio that receives traffic data from Caltrans’ traffic operations center in downtown Los Angeles while providing the center with data on the car’s speed, direction and location.

The maps used for the test cover the whole Los Angeles Basin. Using the smallest scale, the entire region is displayed on the screen, with only freeways and a few other major streets delineated. Using the largest scale, areas covering only a few bocks appear, with each street clearly marked and named.

Engineers say that within months, a single disc drive for the on-board computer will contain mapping data to cover the entire United States.

To minimize the dangers of distracting the driver, some of the functions--such as typing in the address of an intended destination--can only be entered while the car is stationary.

Steve Leung, a Caltrans engineer, said the yearlong test of the system will include three phases:

* Caltrans employees will commute to and from work along the Santa Monica Freeway to evaluate the system’s usefulness in avoiding congestion.

* Drivers representing a cross-section of Southland residents will commute between a variety of origins and destinations.

* Commercial drivers will use the vehicles as part of their regular business trips.