“…However, the grey level threshold separating road and the shadow under a vehicle is not provided in [20]. An alternative solution is proposed in [21] and later used in [22,23,24,25,26,27,28,29], where a coarse approximation of the free driving space is obtained by defining the lowest central region in the image delimited by edges. A normal distribution is assumed for the grey levels of the free driving space, and the shadow under a vehicle is defined as a region with intensity smaller than a threshold m − 3 σ , where m and σ are respectively the mean and standard deviation of the grey levels of road pixels.…”