As many as 1,200 moped riders will die in highway accidents annually by 1984, according to a report released yesterday by the Department of Transportation.

The study, commissioned by the National Highway Traffic Safety Administration, predicts the number of mopeds in the United States will jump from the current half million to 2.5 million by 1984.

"It is expected that between 1.5 and 4 percent of all mopeds will be involved in accidents in any given year," the NHTSA noted. "Eleven percent of those accidents will result in serious injury to the moped rider."

Slightly more than one in every hundred accidents will prove fatal, the agency study also predicted.

"We are very much concerned about the study's projections," said NHTSA Administrator Joan Claybrook. "The energy situation makes it likely that greater numbers of these vehicles will be used for local transportation, because they are more economical to operate than cars.

"Steps must be taken to develop safety programs for moped users and other drivers who share the road with them."

Claybrook said there were two important steps that states, which have regulatory authority, can take right now to improve the safety of mopeds, which are motor driven cycles capable of speeds up to 30 miles per hour, but which can also be pedaled at lower speeds.

"First, all moped users should be advised to wear a safety helmet, preferably a motorcycle helmet, and to wear brightly colored clothing to offset the tendency of motorists to overlook two-wheeled vehicles," Claybrook said.

"Second, since most states combine moped accident statistics with those involving other two-wheeled vehicles," she said, "they should change their accident recording and reporting systems so that moped accidents can be identified separately."

Three-fourths of moped accidents occur on city streets, the report noted, with 40 percent at or near intersections.Moped operators were found to be at fault in slightly more than half of the accidents where fault was assigned. Failure to yield the right-of-way and improper signaling were the most frequently reported causes of collision.