Prohibitive pictorials:

1991 Dewar, 1976, The slash obscures the symbol on prohibitive traffic signs, Human Factors, 18, 253, 10.1177/001872087601800306 Ells, 1979, Rapid comprehension of verbal and symbolic traffic sign messages, Human Factors, 21, 161, 10.1177/001872087902100203 Gattis, 1987, Distinguishing between left turn and U-turn prohibitions, ITE-Journal, 57, 4 Glover, 1996, Pictorial negations, 910 Gough, 1965, Grammatical transformations and speed of understanding, Journal of Verbal Learning and Verbal Behavior, 4, 107, 10.1016/S0022-5371(65)80093-7 1984 Lanjunen, 1996, The ergonomics of road signs, Ergonomics, 39, 1069, 10.1080/00140139608964528 Sanders, 1993 Sekuler, 1985