Screenwriter Gary Devore must have set out to fabricate the lowest of all conceivable common denominators when he dreamed up Amy and Elmer, the hitchhiking lowlifes of "Back Roads," opening today at area theaters. If it was a bold experiment, at least Devore has the perverse satisfaction of knowing it doesn't work.

But where's the consolation for rising stars like Sally Field and Tommy Lee Jones? They've stepped into a co-starring sleazetrap, where they labor under the delusion that their coarse, ignorant characters are also likable. What possessed director Martin Ritt, a generally humane and purposeful old pro, to tolerate such scroungy characterizations?

Amy and Elmer take it on the lam from Mobile, where she has been walking the streets and he pops up to make trouble for her, slugging a plainclothes cop after first conning her into a free tumble. Devore's script deprives the sweethearts of destinations as well as ingratiating characteristics. The hooker and the brawler are headed nowhere in particular when they hit the road. They're still in mid-passage when the movie poops out, having done nothing more endearing than fall in a mud puddle while trying to hop a freight.

To compensate for an inherent lack of charm or resourcefulness, the hustlers keep getting victimized. First, they're given a lift by a family after Elmer changes their flat tire in the rain. While the father, a porky redneck, harangues the passengers about honesty being the best policy and then singles out hippies and blacks for rhetorical abuse, his son, dressed in the uniform of a military school, pilfers money from Amy's handbag.

Next Amy is manhandled by a group of rapacious sailors and then reviled by the apparently amiable young sailor, played by David Keith, who had offered her and Elmer a ride all the way to San Diego.

Stranded somewhere in the Southwest, Amy tries to hit the pavement again and gets intimidated by a hard-as-nails Mexican-American madam, played by Miriam Colon, who also confiscates the stake Elmer has won by sucker-punching a local tough boy in a prizefight. The mistreatment seems as poorly contrived as the derelict identities that supposedly make Amy and Elmer vulnerable in the first place. Amy does not emerge as a rabble-rousing heartwarmer simply because Devore has the gall to show her gazing sorrowfully at the little boy she put up for adoption or rebuking Elmer with lines like, "Nobody could pay me to lie down until it was the only thing I could do. Losing is the thing you do."

Indeed, you come out of "Back Roads" feeling more familiar with the configuration of Sally Field's spinal column and chestbone than the character she's struggling to embody. Physically and temperamentally, Tommy Lee Jones shouldn't be difficult to accept as a funny diamond in-the-rough, but Elmer's style of bickering, thieving and brawling leaves something to be desired: It creates the impression of a crude thug where a rugged, romantic comedy hero ought to be.

"We're gona be travelin' on wit and grit," Elmer tells Amy as they embark on their hapless plod. That was the idea, surely.