Eugene wrote:NorthernTool.com has several drive way alert systems listed. Not that expensive and transmitter range would appear to be suitable for your use.
I thought this one looked pretty good, as long as it works as advertised. It won't detect animals, which is a plus for many locations. Unfortunately, it won't detect a person on foot either.
http://www.northerntool.com/shop/tools/product_200420113_200420113