"Detour" is an accomplished work--and was recognized as such at the time of its original release, (see the "Parent's Magazine" review from 1945 for example).
Way too much has been made of its low budget. Neither the story nor script call for a high budget. It is, after all, an intimate drama focusing on only a few characters depicted in merely a few settings. Were this same story to have been shot by RKO, Columbia, Universal, or even MGM, I daresee today's viewers might be startled by the pictorial similarities. For example, compare the outdoor highway/hitchhiking scenes in "Detour" against the roadside tramping of Lana Turner and John Garfield in "The Postman Always Rings Twice" and you will note a strong similarity in visual presentation.
Indeed, the sets are really on the beam, since, overdressed, overly lavish settings would have defeated the picture's intentionally shabby mise en scene.
Moreover, the lighting is superb in the cafe scenes and the fog bound walk on Riverside Drive--very ghostly, very dreamlike. All of which is to suggest, that had Mr.Ulmer a great deal more $$$ here, I very much doubt he would have approached this script, this story, by hurling unnecessary oodles of cash at it. He was shrewd enough to use such funding for those films of his which required a more opulent look, such as "Club Havana" or "Bluebeard".