In this paper, we describe a method to automatically calibrate a static camera using only information regarding the vehicles in the scene. We use standard Adaboost vehicle detection combined with feature-based tracking to determine the size and direction of travel of vehicles at various locations in the scene. We then infer the scene geometry including the height of the camera, the camera focal length, and the angle of the camera with respect to the ground. Given the calibration, we can estimate the size and speed of objects and the distances travelled in the scene. We evaluate our system on 10 cameras with manually annotated calibration based on a 3D cube with user specified locations in 2D. We show our system can be used to accurately estimate vehicle sizes to within 2 feet for a range of camera configurations.