A method for calculating a Time To Go, TTG, value between a vehicle and an intruding object, said method comprising: - a retrieving step of retrieving a first image of said intruding object at a first point of time, T 0, and a second image of said intruding object at a second point of time, T 1; - a filtering step of filtering said first image and said second image so that said first image and said second image become independent of absolute signal energy and so that edges become enhanced; - a setting step of setting an X fractional pixel position, X FRAC, to zero and an Y fractional pixel position, Y FRAC, to zero, where X FRAC denotes a horizontal displacement at sub pixel level and Y FRAC a vertical displacement at sub pixel level; selecting a scale factor, S i; - a scaling step of setting said second image with said scale factor, S i, and resampling said scaled image to position X FRAC and Y FRAC; resulting in a resampled scaled image, RS i I,; - a calculating step of calculating correlation values, C XPIX, YPIX, i, between said first image and said resampled scaled image, RS i I, for different horizontal displacements at pixel level, X PIX, and different vertical displacements at pixel level, Y PIX, for said resampled scaled image RS i I; - a finding step of finding a maximum correlation value at subpixel level, C i, based on said correlation values, C XPIX, YPIX i, and updating X FRAC and Y FRAC; - a first setting step of setting j = j + 1 and repeating the scaling, calculating, finding, and setting steps a first predetermined number of times; - a second setting step of setting i = i + 1 and repeating the setting, scaling, calculating, finding, first setting and second setting steps a second predetermined number of times; - a finding step of finding a largest maximum correlation value, C MAX, among said maximum correlation values, C i, and the scale factor S i, MAX associated with the largest maximum correlation value C MAX; and - a calculating step of calculating the Time To Go, TTG, based on said scale factor S i, MAX - a retrieving step of retrieving a first image of said intruding object at a first point of time, T 0, and a second image of said intruding object at a second point of time, T 1; - a filtering step of filtering said first image and said second image so that said first image and said second image become independent of absolute signal energy and so that edges become enhanced; - a setting step of setting an X fractional pixel position, X FRAC, to zero and an Y fractional pixel position, Y FRAC, to zero, where X FRAC denotes a horizontal displacement at sub pixel level and Y FRAC a vertical displacement at sub pixel level; selecting a scale factor, S i; - a scaling step of setting said second image with said scale factor, S i, and resampling said scaled image to position X FRAC and Y FRAC; resulting in a resampled scaled image, RS i I,; - a calculating step of calculating correlation values, C XPIX, YPIX, i, between said first image and said resampled scaled image, RS i I, for different horizontal displacements at pixel level, X PIX, and different vertical displacements at pixel level, Y PIX, for said resampled scaled image RS i I; - a finding step of finding a maximum correlation value at subpixel level, C i, based on said correlation values, C XPIX, YPIX i, and updating X FRAC and Y FRAC; - a first setting step of setting j = j + 1 and repeating the scaling, calculating, finding, and setting steps a first predetermined number of times; - a second setting step of setting i = i + 1 and repeating the setting, scaling, calculating, finding, first setting and second setting steps a second predetermined number of times; - a finding step of finding a largest maximum correlation value, C MAX, among said maximum correlation values, C i, and the scale factor S i, MAX associated with the largest maximum correlation value C MAX; and - a calculating step of calculating the Time To Go, TTG, based on said scale factor S i, MAX