The velocity of targets approaching directly towards the observer is over-estimated when such motion is seen in intermittent illumination. The present investigation aimed at clarifying the conditions leading to such a judgment. The radial motion was simulated by a zoom-lens system in one experiment and by a computer-controlled display in another. In the former the frequency and the velocity domains and in the latter the domain of the on-off ratio of each flash were explored. The flicker rate of 3.5 Hz and the on-off ratio of 80/ 200 (msec) or less were found optimal for the effect. The range of velocities used, 9 to 42 cm/sec, produced no significant difference. Possible mechanisms for the effect were discussed.