A method for calculating a Time To Go value between a vehicle and an intruding object. First and second images of the object are retrieved at first and second points of time. A scaling factor is selected and the second image scaled with the scale factor. The scaled image is resampled to a set position. Correlation values are calculated between the first image and the resampled scaled image for different horizontal displacements and/or different vertical displacements for the resampled scaled image. A maximum correlation value is found based on the correlation values. Selecting the scaling factor, resampling, calculating the correlation values, and finding the maximum correlation values are repeated a second predetermined number of times. A largest maximum correlation value is found among the maximum correlation values and the scale factor associated with the largest maximum correlation value. The Time To Go is calculated based on the associated scale factor.