Mean estimated TTC as a function of the mean presented TTC. The different colors represent the two vehicle sound powers, with the higher sound power (audio gain 10 dB) presented in orange and the lower (audio gain 0 dB) in blue. The symbols and line types indicate the type of sound power variation, with circles and dotted lines corresponding to the trial-to-trial variation and squares and solid lines corresponding to the blockwise variation. The panels correspond to the three different velocities. Error bars show ±1 standard error of the mean across the 22 participants.