Maybe it's time for producer Jennings Lang to switch from theatrical features to short public service and promotional films. Lang's last production for Universal, "AIrport '77," had more to recommend it as a Coast Guard recruiting film than a conventional, not to mention satisfying, thriller.

Now Lang has given the studio "Rollercoaster," which boasts some swell documentary footage of the grounds and rides at three large amusement parks - Ocean View in Norfolk, King's Dominion in Richmond and Magic Mountain in Valencia, a suburb of Los Angeles - but keeps spoiling it with scenes devoted to sustaining a feeble excuse for a plot.

Timothy Bottoms brings his incomparably bland, unsuggestive presence to the role of the menace, a saboteur-extortionist who causes fatal accidents at two sites and demands $1 million in return for declining to rig a third bomb on the rollercoaster tracks. George Segal plays the ordinary guy who's trying to prevent disaster - a worried, haggard safety inspector - and gets in two or three agreeable moments of humorous exasperation, Richard Widmark as an officious federalagent invites resentment of government interlopers by dismissing Segal's honest efforts and bright insights until it's almost too late.

The Widmark character serves as the screenwriter's whipping boy, but the crew who wrote this film can scarcely be considered hard hitters. The exposition would probably seem more intelligent and suspenseful if Widmark were less peremptory and he and Segal worked in concert. "Rollercoaster" is conceived at such a boobish level of character-sketching and rabble-rousing that Segal is compelled to wonder out loud if the authorities are more dangerous than the bomber, conveniently forgetting that Bottoms has blown a number of innocent thrill-seekers into eternity.

The director of photography, David Walsh, who worked with Woody Allen on "Everything You Wanted To Knok About Sex . . ." and "Sleeper," has an alert, witty eye. His wide-angle vistas of amusement park crowds and attractions and his swirling, speeding, vertiginous impressions of rollercoaster rides give the production some visual class and kinetic excitement.

The Sensurround process, fundamentally unnecessary as it is, supplements the subjective visual thrills effectively. The Sensurround rumble is close to the feeling one gets on a rollercoaster, and the sounds are better modluated and contrasted than they were in "Earthquake" or "Midway," evidently because actual location sound effect were recorded directly in the process for the first time.

There's such a disparity of interest between the locations themselves and the melodramatic motions the actors are required to go through that one could easily believe Universal had a greater stake in the amusement park business than the movie business, King's Dominion does especially well in the gratuitous publicity department: While following Bottom's instructions, Segal seems to take almost every ride the park offers.

The climatic setting is Magic Mountain, which has the new rollercoaster with the 360-degree loop. Walsh's cameras guarantee the audience a vicarious ride, but the attempt to link this incredible set with the apprehension of the vilain proves a fizzle. Active rooting interest shifts from Segal to the anonymous members of a bomb-disposal team, and once Bottoms is cornered, he's allowed to escape briefly for the sole purpose of climbing foolishly onto the tracks while a ride is in progress to get his comeuppance. When he's bumped, oen feels simply puzzled at the anticlimactic lameness of it all.

Places like Magic Mountain may look like naturals for the movies, but they still require filmmakers imaginative and skillful enough to make the most of natural photogenic advantages.