Efforts to keep American motorists obedient to the national 55 mile-per-hour speed limit on Interstate highways are running out of gas, and federal dollars, too.
While there is no move afoot to either raise or remove the legal speed barrier, drivers in increasing numbers are whizzing along beyond it.
According to the Highway Users Federation, 70 percent of motorists on rural Interstate roadways exceeded the 55 m.p.h. limit during the six months ending last March 31. And slightly more than half of US motorists on urban highways and connectors traveled at rates faster than the legal barrier, the Washington-based organizaiton reported.
''What we're seeing is a return to slightly faster traffic on roads that were designed for speeds over 60 m.p.h.,'' says Woodrow W. Rankin, director of the organization's transportation safety division.
Despite the faster traveling trend on interstate highways, especially in rural areas, the average speed on all roads posted with 55 m.p.h. signs was 55 m.p.h., he notes. This indicates that most drivers do not exceed the limit on other through-traffic routes.
Of particular concern to Mr. Rankin and others interested in highway safety is the growing number of drivers barreling along at rates beyond 65 m.p.h. The most recent data indicate that nearly 8 percent of the motoring public travel faster than 65 m.p.h. on roadways that are posted with a 55 m.p.h. limit.
This trend can only lead to increased traffic accidents, he warns.
Originally imposed as an energy conservation measure during the fuel shortages of the mid-1970s, the 55 m.p.h. speed restriction now is supported largely for its impact on road safety.
Rankin and other boosters of the 55 m.p.h. law view it as ''one of the most effective safety measures of the modern motoring era.'' Officials of the US Department of Transportation estimate that the speed limit has spared 50,000 lives on the nation's highways since 1974.
However, President Reagan, in his campaign for the White House two years ago, advocated curbing federal regulation of road speeding. He appears no less determined now to leave the matter to individual states. The fiscal 1982 federal budget, covering the 12 months commencing last Oct. 1, provided $20 million for enforcement of the 55 m.p.h. limit, half the amount appropriated the previous year.
And thus far it is uncertain what, if anything, will be allocated in fiscal 1983 to keep a check on the national speed limit.
Despite an increase in violations and greater laxity in enforcement, the law has tended to discourage speeders not to drive as fast as they otherwise might, say highway and law enforcement officials.
A Gallup poll released last February found that three out of four of those queried favored retention of the 55 m.p.h. limit. Only 29 percent, however, said they obeyed that restriction all the time, and 48 percent reported they did not stick within the limit most of the time.
The latest Federal Highway Administration survey of traffic speeds on interstate roads, covering the six months ending last March, found that in 24 states the average motorist traveled faster than 55 m.p.h. Maryland at 57.9, North Dakota at 57.8, and Florida at 57.7 led the nation with highest average speeds. By contrast West Virginia at 47.4, Alaska at 50.8, and Pennsylvania at 51.0 m.p.h. had the lowest average highway speeds.
North Dakota has the highest proportion of speeding drivers, with 70.7 percent of the vehicles traveling on its highways at rates in excess of 55 m.p.h. Ranking behind it is Nevada, with 64.1 percent of motorists clocked for such violations. The national average of scofflaws is 51 percent.
West Virginia and Pennsylvania have the lowest percentage of speeding drivers - 23.8 percent of the drivers in each of these neighbor states surpass the speed limit.
Since the inception of the 55 m.p.h. limit in 1974, the national average for speeds on all posted roads has dropped from 60.3 to 55 m.p.h. During the same 81 /2-year period, the percentage of motorists exceeding 55 m.p.h. has dipped from 68 to 51 percent. The latter is up from 49 percent a year earlier, the lowest figure since the 55 m.p.h. limit went into effect.