Abstract: The present invention relates to an inexpensive, portable, and accurate method and apparatus for calculating the weight of a vehicle while it is in motion. The apparatus to which the present invention relates is in the field of Weigh-In-Motion devices.In the present invention, an optical fiber is embedded into a matrix such as a rubber pad and a multiplicity of microbending fixtures are distributed along the path of the optic fiber. Then as the wheels of a vehicle pass over the pad the force of the wheels cause the microbending fixtures over which they pass to pinch together and attenuate the light which is transmitted through the optic fiber. The light transmitted through the optic fiber from a light source at one end of the optic fiber is received by a light receiver at the other end of the optic fiber. Then by measuring the amount of light input and the net amount of light output, and calibrating the device, the weight of each axle and the weight of the vehicle above that axle can be measured.