Americans are gradually speeding up on the nation's Interstate highways, violating the 55 m.p.h. speed limit more often.
After declining from 1974 to 1976, the average speed on Interstates has been creeping steadily upward, according to the latest report by the US Department of Transportation (DOT) to Congress.
Although 55 m.p.h. is still the maximum speed limit across the nation, congressional funding for state enforcement efforts has been cut in half and violations are up. The average speed on Interstates has now reached 57.9 m.p.h., according to the DOT's recent report. Nineteen states report that more than 50 percent of the drivers on their highways exceed the 55 m.p.h. speed limit. Only 12 states reported similar statistics a year ago, according to the DOT.
The trend is raising anew some basic questions about the 55 m.p.h. speed limit and why people do or do not adhere to it.
A researcher at Emory University in Decatur, Ga. has made a preliminary finding that as much as half the compliance with the 55 limit is due to saving gasoline.
''The price of gas had a larger effect on slowing people up than had been suspected before,'' Christopher Curran, associate professor of economics at Emory, says.
''An unenforced 55 m.p.h. speed limit is not going to have an impact; the price of gasoline does have an impact,'' he says.
Interstate highways are safer than other roads, he points out. So he suggests raising the speed limits on the Interstates and putting more efforts into making other roads safer.
Other analysts of the 55 m.p.h. speed limit question whether one factor -- such as gas price -- can be singled out as the reason for compliance. But they don't dispute the trend toward higher speeds on Interstates. They say most drivers view the 55 limit as a guide, often exceeding it, but not by more than 5 to 10 m.p.h. If the limit were raised to 65, they say, average speeds would likely move up accordingly.
The DOT reports a decrease in reported violations for speeds in excess of 65 m.p.h.
How high are average speeds likely to go if the 55 limit is retained?
Charles Hurley of the National Safety Council sees a likely leveling off at about 60 m.p.h. - if the 55 m.p.h. limit is held.
''As people perceive the 5 m.p.h. tolerance in enforcement by police, speeds have crept toward 60 m.p.h.,'' he says. ''It is not expected that average speeds will go above that.''
William Searcy of the National Highway Traffic Safety Administration says data being collected now may show a leveling off has already begun, as more small cars are used and their drivers go slower. And part of the increase in reported average speeds, he adds, may have been due to more thorough monitoring by radar by states.
But the majority of cars are traveling at about 60 to 63 m.p.h., he says. One thing the 55 limit has done nationally is to narrow the range of speeds cars generally travel, thus reducing accidents that occur when fast-moving cars overtake much slower moving ones, says Searcy.
The National Safety Council and the American Automobile Association (AAA) continue to support the 55 limit because of the documented savings in lives and fewer serious injuries associated with it. The DOT estimates that more than 50, 000 lives have been saved as the result of the 55 m.p.h. limit since it was imposed in 1974 by passage of a federal law denying any highway aid to states with a higher limit.
President Reagan has called for an end to the federal role in setting the speed limit and requested no funding for federal aid on enforcement. But Congress approved $20 million anyway for fiscal year 1982, a reduction of $20 million from the year before.
Several states have reduced significantly their penalities for violating the 55 m.p.h. limit and some states are considering such a move. Earlier this month, Arizona reduced the penalty to $15 for speeding from 55 to 70.