A single solid yellow centerline for bi-directional traffic does not have any legal definition and introduces ambiguity to the motorist. The practice to provide a single solid yellow centerline on rural roads, residential streets or narrow roadways is not appropriate and the additional minor cost of a second longitudinal line instead of one is not a valid justification for compromising any legal code associated with traffic operations or traffic control devices. Further, individual State laws may define the legal connotation of the single solid yellow line, but these legal definitions vary from state to state thereby resulting in non-uniformity nationwide.
The FHWA is not aware of any association where a single solid yellow centerline indicates any consistent procedure, enforcement, or achieves positive results in any specific motorist behavior. In fact, the traveling public consistently demonstrates that the single solid yellow centerline does not communicate adequately in terms of whether passing is allowed or prohibited in one or both directions.
The double yellow centerline system is well established and collectively understood by road users. Using a single yellow center line would only save 6-8 inches in width compared to a double line and, even on narrow roads, this savings is not considered to be significant enough to warrant compromising the well-understood double-line system.