Marvin S. Cohen, chairman of the Civil Aeronautics Board, yesterday sought to assuage fears about the safety of commuter airlines, the small carriers beginning to serve more and more of the smaller towns and cities that larger airlines are seeking to drop.

In a speech to the American Bar Association meeting in Dallas, Cohen specifically took issue with recent comments made by Jim King, chairman of the National Transportation Safety Board, that the accident rate per seat-mile is much higher for commuters than for larger airlines.

King's analysis "ignores" the fact that commuters fly much shorter distances than the larger airlines, which means more takeoffs and landings per mile, Cohen said. It is during takeoff and landing that the major risk of accident occurs.

In 1978, he said, commuters took off and landed 1.7 million times to fly 167 million miles, a ratio of one departure for each 98 miles, while the ratio for larger airlines was one departure for each 456 miles. Last year, there were 24 commuter accidents -- five fatal -- while there were 18 large aircraft accidents -- four fatal. For large planes, there is one fatal accident in 350,000 takeoffs and landings for commuters, Cohen said. "Commuter travel is about as safe as ordinary driving," he contended.

Nevertheless, he said the CAB is aware of public concern about smaller planes and is working with the Federal Aviation Administration to improve commuter safety.

In other developments yesterday, Trans World Airlines told the CAB it would end on Nov. 1 its $108 and $125 one-way "super coach" fares between New York and the West Coast. The regular low super coach fares were introduced in June after World Airways came into the cross-country scheduled air market with fares that were half what the big three transcontinental airlines were charging. But World has not operated the routes for almost two months now, first because their DC10s were grounded and now because of a strike against the once-all-charter airline.

There was no immediate reaction from United Airlines and American Airlines, which also had reduced their regular coach fares to those levels (United has taken the lead). Pan American World Airways also operates low-fare cross-country service but with fewer daily flights.