Hot Wheels made a toy radar gun that was surprisingly accurate and cheap - under $30. I think they are no longer produced, but they turn up on ebay and craigslist.I've actually had the same idea in my neighborhood, but I wanted to add a camera or led indicator sign for the motorists.
Please report back how it works out. I'd love to have one to prove how fast people in my neighborhood drive.
You can probably get one at a sports supply shop. Assuming they don't use something special to track fastballs.
It'll be a while before I actually get one of these -- other expensive purchases are waiting in line (wife's college graduation, rent, taxes, eyeglasses.... and the beat goes on).
Anyone care to describe the plan for those of us who don't feel like spending time watching a video?