I'd highly recommend the Mighty Mule.

http://www.mightymule.com/mighty-mul...eway-alarm.htm

Have had one now for almost a year and it works very well. It's installed on a
250' driveway about midpoint. Far enough from the street that it doesn't trip by
passing vehicles, close enough that it catches the occasional curriosity
vehicle. Although it has as battery in the sensor, I've yet to have to replace
it.

It replaced an Optex IR system that was useless with false alarms. Any induction
system will ignore people, so if that's your concern you may want to layer in
some other detector.