Police patrol cars with only one officer in them are safer and more efficient, as well as much cheaper, than cars carrying two policemen, according to a new study by the Police Foundation.

The year-long study was conducted in San Diego, Calif., but the issue of how many policemen to put in a patrol car is a serious one in Washington too, where the size of the police force has been cut by about 20 percent over the past four years.

As a result of the manpower cut, D.C. police have sharply reduced the number of two-man patrols on the streets, although some two-man patrol cars are still scattered around the city all day.

In an introduction to the new report, patrick C. Murphy, the president of the Police Foundation, declared that the study shows at least in San Diego, that clearly and unequivocally it is more efficient, safer, and at least as effective for the police to staff patrol cars with one officer."

"Most police departments can expect to have the same results as San Diego," added Murphy, who served as Washington's public safety director in 1967 and 1968. The foundation he heads was created the by Ford Foundation in 1970 to encourage innovation in police work.

"Most of the patrol cars in Washington by far carry one man now," said D.C. police information officer Chuck Collins. "We agree that in most cases the one-man unit is more efficient because the wide majority of police work is in nonemergency type calls. But it is those emergency calls that we have to be concerned about, and on those we want to be able to send two men. We think it's best to have a mixture of (one-man and two-man) units."

The San Diego study, which lasted throughout 1975, included 22 patrol cars with one police officer and 22 with two officers that patrolled similar neighborhoods in both high-crime and low-crime areas of the city. The data gathering and analysis was done by a team of three social scientists, headed by John E. Boydstun, of System Development Corporation, of Santa Monica, Calif.

According to their report:

The one-man police cars and the two-man cars handled about the same number of calls in the average eight-hour shift.

The one-man units made more arrests than did the police working in pairs, but they also required help from an extra police car much more often.

Officers working in both arrangements were equally likely to be injured and assaulted, but those working alone encountered fewer suspects who resisted arrest.

The one-man units were the target of fewer citizen complaints.

The two-man units handed out substantially more traffic tickets.

The two-man units also required less time to handle an average call - 37.3 minutes versus 48.8 minutes.

However, the report said the cost of operating a two-man patrol car is about 80 per cent higher than the cost of a car with one one policeman in it. Thus, it said, a police force can deploy 18 one-man cars for the same expense as 10 two-man patrols and any savins that the two-man units make by dealing with calls quickly is more than offset by their higher cost.

A police spokesman in San Diego said the city had followed the foundation's advice and cut back the number of its two-man patrol cars. He said a few two-man cars were still deployed in high-crime areas and in outlying parts of the city where it would take a long time to send reinforcements in case a lone policeman encounters serious trouble.

In the Washington suburbs police departments operate one-man patrol cars almost entirely except for Montgomery County, where a spokesman said there are a few two-man patrols both in high-crime areas and in remote parts of the upper county.