Very simple and fun little project. Great job.
The main problems to solve that I see are; 1- does not actually detect a vehicle, just motion 2- can only detect motion for one vehicle at a time 3- so now you know cars are going fast, but, what do you do with this data (how do you alert or punish them)
1 is a little tough to solve since it requires training some kind of classifier. 2 is not so bad if you use a velocity field and track individual objects via bounding box. 3 is basically another project. On that note, is it legal to have a speed meter on your front lawn?