Over-height vehicle drivers continuously ignore warning signs and strike onto bridges despite the number of preventative methods installed at low clearance bridges. In this paper, the authors present a new method for over-height vehicle strike prevention with a single calibrated camera mounted on the side of the roadway. The camera is installed at the height of the $over-height\; plane$ formed by the average of the maximum allowable heights across all lanes in a given traffic direction; the error caused by the road gradient is assumed to be negligible and absorbed through the calibration process. At that height, the over-height plane can be safely approximated as a line in the camera view. Any vehicle exceeding this line is consequently over-height. The camera position and orientation are determined through a calibration process proposed. Instances of over-height vehicles are detected through optical flow monitoring. Evaluation of the system resulted in a height accuracy of ±2.875  mm; outperforming the target accuracy of ±5 cm, OH detection accuracy of 68.9%, and classification performance of 83.3%. Although its accuracy is comparable to existing laser beam systems, it outperforms them on cost which is an order of magnitude less because of eliminating the need for new permanent infrastructure.