a legend that is hard to prove is true. or, it can be easy to prove it false. urban legends are usually written on paper most of the time. but when ppl pass it on it (which most of the time, is not a paper), the story changes from person to person. so thats why its hard to prove or disprove urban legends.
there are up to 7 variations of "the killer in the back seat" urban legend, so its possible that it is false.