Mean of the intra-individual standard deviation of the estimated TTCs, plotted as a function of the mean estimated TTC. Each data point corresponds to one of the experimental conditions (i.e., unique combinations of TTC, velocity, vehicle sound power, and type of power variation; same as the data points plotted in Fig. 3). Orange symbols: higher sound power. Blue symbols: lower sound power. Filled squares: blockwise source-power variation. Open circles: trial-to-trial power variation. The black line shows the best-fitting linear regression line, SD(TTC_est)=0.22 s + 0.19 M(TTC_est), R² = 0.89.