DEATHTRAP, by Ira Levin; directed by Robert Moore; scenery by William Ritmah; costumes by Ruth Morley; lighting by Marc B. Weiss. Produced by Alfred de Liagre Jr, and Roger L. Stevens. With Brian Bedford, Kathleen Freeman, Betty Miller, Kevin Conroy and George Ede. At the Elsenhower Theater through Sept. 1

If "Deathtrap" were a highway, it would be marked every 100 yards or so with signs like "Dangerous Curves," "Falling Rocks," "Trucks Use Low Gear," "Soft Shoulder" and "Proceed at Your Own Risk."

And the recommended course of action for the wise motorist would be to ignore every caution, including the speed limit, and just enjoy the ride. It's a humdinger.

Ira Levin's absolutely manic play is about a man who proposes to commit murder to overcome a case of writer's block. It has been 18 years - 18 confounding years - since Sidney Bruhl's one Broadway success, a whodunit called "The Murder Game." "Nothing recedes like success," he comments. So when a writing student sends him a new and absolutely sure-life thriller in the mail, Bruthl begins to weigh the idea of removing the young man and submitting the play as his own.

Bruhl's wife thinks he's joking, of course. Nonwriters always have trouble understanding what a serious business writer's block can be. The only reason there aren't more murders committed in its name, I suspect, is that opportunities as fat as this one so rarely arise.

Speaking of writer's block, it's hard to know just what else to say about the plot of "Deathtrap" since it is considered bad form to reveal too much of a tale of this sort. But it won't be ruining anyone's evening to mention that Levin is after laughs as well as screams. And the conduct of last night's audience at the Eisenhower Theater suggests that "Deathtrap" is capable of generating both responses, sometimes all about simultaneously.

(There was a moment last night, however, when an eccentric pocket of patrons at the right-rear of the house seemed to be chuckling as one character was strangling another. It seemed wrong, somehow.)

The flamboyant John Wood played Bruhl in the original Broadway production. Brian Bedford, graceful as always, makes a much more plausible playwright - more inward, pensive, a nail-biter type. At the same time, he may be a notch less plausible as a killer, but Bedford is a delicate balancer with a delicate balancing act on his hands.

As Bruhl's loyal wife, horrified by his criminal intentions, tall, lanky Betty Miller provides a foundation of solid credibility to what, on cold analysis, is an absolutely outrageous plot premise. Fortunately, we are scarely ever tempted to give "Deathtrap" any such cold analysis, and Miller's serious, slouching shoulders deserve a fair share of the credit.

And Kathleen Freeman, the officious Brunhilde of a string of Jerry Lewis movies, makes the comic most of the Dutch psychic who just happens to live next door to the Bruhls' converted Connecticut stable.

In one pregnant moment, Freeman suddenly has a parapsychological flash. Everyone gasps. Then she joyfully announces, "Oh! My daughter is pregnant!" A moment later, she decides urgently to phone her daughter with the happy news.

The writer as hero has become one of modern literature's recurring annoyances. But Levine, author of the hit stage comedies "Critic's Choice" and "No Time for Sergents" as well as the eerie and better-known novels "Rosemary's Baby" and "The Boys from Brazil," is so full of funny and obviously heartfelt sentiments toward his profession that is it hard to hold this against him.

The writing of whodunits, he has Sidney Bruhl observe, is "a disease - thrilleritis malignis - the fevered pursuit of the one-set, two-act, five-character sensation." Bruhl also describes one specimen as so well-constructed that "even a gifted director couldn't hurt it."

Later, Bruhl's utter amazement at his protege's rapid-fire writing pace seems to reflect Levin's own dim view of fast writers.

"Let me see a few pages," says Bruhl skeptically.

And when the protege demurs, explaining he has another act to go and would rather show only the finished product, Bruhl's reply is a way, "Well, what's another hour or so."

"Deathtrap" is a play, in short, that could only have been written by a playwright. CAPTION: Picture, Brian Bedford, Betty Miller and Kathleen Freeman in "Deathtrap"