Cruising on the San Diego Freeway in west Los Angeles, two California highway patrolmen saw a car moving erratically and tried to make a routine arrest. When they cornered the driver on an off-ramp, the man started firing an M-1 semi-automatic rifle, spraying their patrol car with bullets and wounding one of the officers.

In remote Shasta County, in the mountains of northern California, another highway patrolman in July arrested what appeared to be a harmless drunk driver and put him in the back seat without bothering to handcuff him. Five miles down the road the man pulled out a revolver and murdered the lawman.

In Chula Vista, south of San Diego near the Mexican border a "freeway sniper," firing at motorists from his car, murdered three persons last month.

In Sacramento, nearly 500 miles to the north, a young motorist cut off a pickup truck and the man in the other vehicle retaliated by shooting him dead.

In San Jose, a young man was critically wounded when his passenger accused another motorist of being a reckless driver.

These incidents point to a new trend toward violence on California's 98,000 miles of state highways, which include some of the world's busiest freeways. There were 49 assaults on highway patrol officers alone in the first half of this year.

In 1974, according to statistics kept by Sgt. Jim Edison of the Highway Patrol Academy in Sacramento, there were nine shooting incidents on the state's highways: last year there were 40. So far in 1977. Edison reported, gunfire has rung out at least 57 times on state roadways.

"We just started collecting these statistics because they kept coming out at us all the time," said Deputy Highway Patrol Commissioner A. S. Cooper, a 29-year veteran of the 5,000-man force. "It's something that sort of creeped up on us. We've all of a sudden found that we have one hell of a problem now".

Three weeks ago Cooper and his wife got an up-close look at the tension on the highways. Driving in their private car, they were cut off by an angry motorist, who screamed at Cooper and blared his horn. The man tried for a half hour to force Cooper into an accident, for no apparent reason.

"It's just a matter of looking at terribly frustrated people who feel they have a right to do anything they want against anyone," Cooper said. "We have always had fights on the freeways, but now people don't just fight, they come out with weapons."

In the San Jose incident, a 22-year-old was driving his father's moving van when his passenger yelled that a passing '65 blue Chevy was being driven recklessly. In response, the man in the Chevy rolled down his window and shot the driver three times.

Today the young man, after two weeks in a coma, is recovering in a San Jose hospital. But according to police, he has permanent and severe brain damage.

The driver of the Chevy was never caught."We had no license plate or anything. We just couldn't get him," said San Jose police officer Dan Gianni. "The problem is that there really is no motive. It was your old act of passion. These guys didn't even know each other, so it's much more difficult."

Often freeway violence erupts unexpectedly, and from people with little or no criminal past.

In March, highway patrol officers Frank Edwards and Lob Magill saw a yellow, late model Capri weaving erratically through heavy traffic on the San Diego freeway.

They took off, lights flashing, after the Capri. When it stopped, "We thought it would be just another arrest," Magill recalled. "He just sat there in the driver's seat. Then he started firing two clips. We were moving out of our seats pretty darn fast. Frank got it on the hand. When he was shooting it seemed like he was shooting forever."

The driver riddled the car inside and out before Magill grabbed a shotgun and blasted him through the neck. Seven months later, Magill and Edwards, whose wounded hand has now healed, still have trouble explaining why the driver, a man with no criminal record, would start firing at them.

"Maybe it's just that people are uptight in L.A.," said Edwards. "These people are commuting 45 minutes a day in heavy traffic and you come along and it's the proverbial straw. They don't want to see you and they get very hostile. Sometimes you come along at the wrong time and that's all they need to break down."

"You can't walk around with your gun out of your holster when you're writing a ticket or helpinge a disabled vehicle." said Edison. "but you have to always be careful-you never know if the guy you're helping is the one who'll get you."

Officer Bob Nath, who works out of the central L.A. station, a one-story building nestled among the rumbling ramps of a half-dozen incoming freeways, said his greatest problem is with "professional types." When he arrested a woman doctor in her 50s who tried to knock another car off the freeway, he said, the case was thrown out of court because the judge "couldn't believe this woman could act so crazy."

Officer George Caravas said another doctor he arrested threatened him when he was writing a speeding ticket. "This guy told me that I'd better not get on an operating table before him." Caravas recalled.

As the freeway violence toll mounts, many highway patrol officers believe all they can do is stay vigilant and keep their weapons ready.

"Let me put it this way," said instructor Edison. "The only time I carry my off-duty weapon is when I'm on the freeway. I'd go anywhere else without it except on the freeway. It's getting so there that I'd feel naked without it."