OSHA Letter of Interpretation 2/14/1977

"The wording of the standard, 29 CFR 1910.22(b) (2), that aisles and passageways be appropriately marked does not require marking by painted yellow lines only.  Painted yellow lines are usually recognized as the most convenient and inexpensive way to mark aisles and passageways since the lines normally last several years without maintenance or repainting.  Dirt floors or floors having continuous concentrations of sand or dust would render the application of painted floor markings infeasible and impractical, especially in foundries.  Other appropriate methods such as marking pillars, powder stripping, flags, traffic cones, barrels and many other devices are considered appropriate as long as recognition of such is included in the vehicle operator and employee training programs.  However, where the need exists and there are no appropriate markings, citations will be issued."