Does speed equal distance over duration in perception?

If the perceived speed of a moving object equals the perceived distance traveled in perceived duration, speed perception should inherit distance illusions, unless they elicit proportional duration illusions. Three experiments systematically evaluated whether an object appears to move faster (a) along subjectively longer horizontal lines (Müller-Lyer distance illusion) and (b) around subjectively larger circles (Ebbinghaus size illusion). The predicted Müller-Lyer and Ebbinghaus speed illusions were consistently absent across absolute (Experiments 1a and 1b), binary comparative (Experiments 2a and 2b), and ternary comparative judgments (Experiments 3a and 3b), providing converging evidence against the hypothesis that illusory distance increments produce illusory speed increments. This could be reconciled with the notion that speed equals distance over duration in perception if distance illusions induce proportional duration illusions through space-time interference, thereby preserving the perceived distance/duration ratio. The spatial Müller-Lyer and Ebbinghaus illusions were accompanied by directionally aligned temporal illusions in absolute (Experiments 4a and 4b) and binary comparative judgments (Experiments 5a and 5b). However, Experiments 5a and 5b indicated that the duration illusions were not proportional to the distance illusions and therefore insufficient to explain the absence of cross-dimensional illusion carryover from distance to speed. Together, these findings cast doubt on whether speed equals distance over duration in perception. (PsycInfo Database Record (c) 2026 APA, all rights reserved).