determine background or computed vehicle density using matlab

hi everybody,

I am beginner in computer vision, and now I want to do a project that I will computing the vehicle density in the video using matlab. I have read many documents about that. but I think this case is different because I dont have background, so I think I have to find background first. but it is difficult with me. Who can help me get background from this video or lets me know another way to computed vehicle density in this video.

this is link to download video: http://www.mediafire.com/download/v95n4h0qaq44bd2/demo.avi

My english is not good, I hope everybody can understand me.

have a nice day!

thanks!