1. A computerized method performable at night by a driver assistance system mountable in a host vehicle while the host vehicle is moving forward with headlights on, wherein the driver assistance system includes a camera operatively connectible to a processor, the method comprising: capturing a first image frame and a second image frame of a road in the field of view of the camera; processing said first image frame and said second image frame to locate a first dark image patch of said first image frame and a second dark image patch of said second image frame; tracking said first and said second dark image patches from said first image frame to said second image frame as corresponding images of the same portion of the road; and measuring respective thicknesses in vertical image coordinates of said first and second dark image patches responsive to said tracking from said first image frame to said second image frame.