This is an instance when you might use shutter speed priority mode. Set your shutter speed to about 1/60 of a second and go to the corner on main street and shoot cars roving by. Also, set you focus to continuous--AFC on Nikons, and servo on canons. This will allow the cameras to continue focussing as cars draw nearer. So, focus on a car in motion, and pan your body with jit as it goes, keeping the car in the same position in the view finder, and fire away. Your goal is to get the car sharp and the background blurry. Don't stop at one, and don't stop at 1/60: try going slower, too.