Traffic fatalities in Maryland declined at a record rate in the first six months of this year, leading the sharpest nationwide decline since the 55 mph speed limit was imposed in 1974, according to federal and state highway officials.

Maryland's 30 percent drop-off since January was the most significant decline of any state and was its largest since state police began keeping traffic death records in 1940. The state's new drunk-driving laws were credited for the drop in highway deaths.

Nationwide, traffic fatalities have fallen nearly 12 percent and highway safety officials said they are expecting this dramatic decline to reduce traffic deaths to their lowest point in nearly a quarter of a century.

The dramatic downturn reflects a combination of economics and new driving habits that has enhanced safety despite increasing highway use, according to the Federal Highway Administration and the National Safety Council.

The high cost of owning and operating an automobile and bitter winter weather discouraged short-distance driving, in which 80 percent of all traffic deaths occur. In addition, lower gasoline prices and higher flying costs contributed to longer highway trips, in which fatalities are not as frequent.

"People from coast to coast are thinking twice about using their cars for short, quick trips and that has really played a major role in the decline of traffic fatalities," said Edmund Pinto, director of consumer affairs at the Federal Highway Administration. "Sure, gasoline prices have dropped quite a bit and people seem to be taking more long trips, but highways are a fairly safe place to be. Most fatal accidents happen close to home when driving to church or to get an ice cream cone."

Since stiffer drunk-driving laws took effect Jan. 1, Maryland State Police report there have been fewer medical examiners' reports that link alcohol with a traffic death. Stricter drunk-driving standards instituted at the beginning of the year allow state police to administer roadside breath tests and lowers the intoxication level required for arrest.

"We have been very pleased with how quickly traffic deaths have dropped," said Dan McCarthy,a spokesman for the Maryland State Police. "It seems to show that the new drunk-driving laws have greatly heightened public awareness that people who drive after drinking will be caught. We think this is significant evidence something can be done about the tragedy of alcohol-related traffic deaths."

According to the National Safety Council, increasingly tough drunk-driving enforcement also was an important reason for the nationwide decrease in auto deaths. In California, where traffic deaths have declined 18 percent in the last year, the California Highway Patrol credited the state's new drunk-driving laws.

Surprisingly, Federal Highway Administration officials say the large number of small cars on the roads has helped lower traffic fatalities even though the autos are more susceptible to major accidents. Drivers of small cars are more likely to realize their vulnerability, FHA officials say, and tend to drive more defensively. The National Safety Council reported, for example, that small car drivers are twice as likely to use seat belts as large car drivers.

Pinto said adverse weather conditions were crucial in the nationwide drop for the first couple of months, but that economic reasons now play a far greater role.

An extremely harsh winter caused Northern and New England states to record declines in highway deaths approaching 50 percent in the first three months of 1982. Snow and ice often made any driving in those areas impossible, but once it thawed, traffic deaths leaped.

The last time traffic fatalities dropped as sharply across the country was just after the Organization of Petroleum Exporting Countries imposed its first oil embargo and President Nixon established the 55 mph speed limit on interstate highways. Between 1973 and 1974, fatalities nationwide dropped more than 17 percent.