1. A method, comprising: using a computer to carry out the steps of: receiving a video image of a road surface imaged and of a vehicle moving across the road surface; determining a tracking point of a vehicle image, of the vehicle, captured in at least two frames of the video image; deriving a formula descriptive of a curve containing the tracking points; determining the vehicle image is a measuring vehicle by: obtaining a number of target vehicle images captured by the video image; identifying a segment of the target vehicle images by a function, wherein the function identifies at least one measuring vehicle image from the target vehicle images; and identifying the vehicle image as a measuring vehicle upon the vehicle image matching the at least one measuring vehicle image; and upon determining the vehicle image is the measuring vehicle, deriving a scaling map comprising a scaling factor associating a vehicle image dimension, in the direction of travel, to a real-world vehicle dimension at a point collocated with the vehicle image in one frame of the video image.