Best way to detect wheels passing on video

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Hi

I'm new to OpenCV but I've managed to get a few basic functions working like transparent overlay and video output etc.

My small project is to detect the speed of cars going past my house by processing MP4 videos from the same day. I figured the best step is to create a detection area as I have on the image and detect the wheels as they pass through.

So my question is, what is the next best thing for me to investigate so I can detect wheels in the video stream?

I'm using OpenCV3 and Python (Spyder app within Anaconda2).

See attached image for a still of the video feed.