A DEPARTMENT of Transportation study has doccumented what many drivers already know: The 55-miles-per-hour speed limit on the nation's highways is being widely ignored. Worse, average traffic speeds, which dropped dramatically when the 55-mph law was imposed in 1974, have started inching up again. According to DOT's survey of speeds on rural interstate highways earlier this year, drivers in Virginia are the slowest paced of air, only 30 per cent ofthem were measured traveling faster than than 55. In Maryland, which ranked 11th among the states, nearly half of the vehicles counted were exceeding the limit. The highest averages, DOT found, were in Wyoming and Connecticut, where 77 per cent of the vehicles were speeding - and about one of every six was going faster than 85.

Why is this worrisome? Because speed kills. The advent of the 55-mph limit was the major reason why highway fatalities dropped by more than 9,100, or over 16 per cent, from 1973 to 1974. Even more telling, the highway death rate (fatalities per 100 million miles traveled) went down about 15 per cent in 1974. Other factors also come into play, including vehicles and road design, terrain, weather, traffic patterns, the ages of drivers and the use of seat belts.

Thus while most states with higher average speeds also have higher death rates, with Wyoming's being the worst of all, the carnage in Connecticut, for instance, is much less than its speeds might suggest. Despite such anomalies, though, the general relationship between speed and danger is so clear that 55-mph limit could turn out to be the most important public-health measure in years.

Slower driving also saves gas. While an individual driver may hardly notice the difference, DOT estimates that the 55-mph limit is producing fuel savings of around 1 1/2 billion gallons of gasoline a year - and that if everyone obeyed the limit, the savings could be twice as great. Moreover, a number of studies have disproved the common claim that many vehicles, notably trucks, operate more efficiently at higher speeds.

So it is worrisome that speeds and deaths and fuel consumption are all going up again. The ideal remedy would be voluntary compliance with the limit by all drivers. Since that has obviously not been attained, most states' law-enforcement efforts need to be stepped up. Maryland's tough, ingenious, well-publicized anti-spending campaign has obviously had a real effect, as has Virginia's banning of radar-detection devices. Other states, though, are much less committed to slowing their drivers down. According to DOT, the maximum fine for speeding in Idaho is $5. Seven states have reduced their penalties for speeding since 194 - and legislatures in several others, mostly in the West, keep trying to do the same. Other states lack resources, the chief of Montana's highway patrol told a Dot team that he can put no more than 30 troopers on the state's roads at any one time.

Congress has empowered DOT to cut off highway construction aid to states that fail to enforce the 55-mph limit. That drastic power has never been used, although warnings from DOT Secretary Brock Adams last springs did encourage four governors to veto measures reducing speeding penalties. Such threats may keep the state's performance from getting worse. Real improvements in law enforcement, though will probably require more federal support. Secretary Adams recommends $30 million to $50 million a year in new aid to the states, coupled with a strong public-education campaign aimed at getting 85 per cent of all drivers down to 55 mph or less by 1982. Congress should support such a program. The speed limit is, as the slogan says, "a law we can live with" - but only to the extent that it is observed and enforced.