Traffic deaths in the United States exceeded 50,000 last year for the first time since the imposition of the 55-mile-per-hour speed limit in 1974, the Department of Transportation said yesterday.

Officials attributed the increase primarily to speeding, especially in the West where drivers have been particularly resentful of the lowered speed limit.

In addition, they pointed to the repeal by 26 states of mandatory helmet laws for motorcycle riders. Motorcycle fatalities have jumped 41 percent since 1975, they noted.

"People are dying on the nation's highways in epidemic proportions," said Joan Claybrook, head of the National Highway Traffic Safety Administration, adding, "We have already recorded an increase of more than 16 percent in the first two months of 1979" over the same period last year.

The effect of speeding was underlined by a DOT study released by Claybrook yesterday showing that the increase in traffic deaths in the speed-oriented West and Southwest - 28 percent between 1975 and 1978 - far outran all other sections of the country. The study showed, in fact, that there has been no increase in the Northeast, where speeding is less frequent, while the rate has climbed 7 percent in the Southeast and 10 percent in the Midwest.

Claybrook said a large portion of the increase in fatalities involved motorcycles and light and heavy trucks, which are growing in numbers, but added that apparent violations of the 55-mph limit had resulted in pushing up the rate of accidents, deaths and injuries involving all classes of vehicles, including passenger cars.

"These statistics make it obvious that we need far greater compliance with the 55 mph speed limit because it not only saves lives but also makes a significant contribution to President Carter's energy conservation program," Claybrook said.

She said that if all drivers stayed at 55 mph, about 5 billion gallons of the 100 billion gallons of gasoline used annually by American drivers could be saved.

Claybrook said that the average speed of all automobiles is down six mph from the early '60s, but is still about 5, mph and appears ot be rising. She said police citations for speeding have tripled since the 55 mph speed limit was enacted in 1974.

By 1982, all states have to have no more than 30 percent noncompliance with the 55 mph speed limit, or risk losing federal highway funding, giving federal regulators some hope that the highway death situation will also improve as the speed laws are enforced more strictly.

The NHTSA studies released yesterday show that people involved in serious accidents were using their safety belts only 8.4 percent of the time a rate 40 percent lower than found in the general driving population.

The studies also showed that passenger or driver ejection from the vehicle is involved in nearly a third of all highway fatalities.

DOT put the 1978 toll at 50,178. The nation's worst year was 1973, when more than 54,000 persons died in highway crashes. The toll dropped off to 45,500 by 1975, both because of the 55 mph speed limit and because of the oil shortage forced a reduction in driving, and has been rising since. CAPTION: Picture, Joan Claybrook shows regional increases in death rate; steepest rise is in the speed-oriented West. By Larry Morris - The Washington Post