An optical sensor system mounted in a wheel arch of a car for determining trajectory of the car includes: a first optical sensor mounted in the wheel arch above a wheel and located behind a first clear casing that does not touch the wheel; and a second optical sensor mounted in the wheel arch on one side of the wheel and located behind a second clear casing that does not touch the wheel. The first optical sensor and second optical sensor perform a plurality of counts corresponding to respectively capturing a plurality of images of the wheel. The captured images are compared with a reference image to determine a 2D displacement of the wheel from its original position. The trajectory of the car is determined by calculating a turning degree of the wheel according to a trigonometric manipulation of the measured 2D displacement.