What you are most likely seeing is the Pulselight, as described by Luftwaffle. They are a common (if not essential) piece of equipment for aircraft operating VFR in congested areas. In Alaska, they are a primary piece of our operating equipment, and we install them in all our aircraft before they enter service.
The fact they caught your attention as you drove down the road, is testament to their ability to be seen. They are quite remarkable for the visibility they create, to often otherwise invisible aircraft!
With a combination of high intensity strobes and a Pulselight, this is the most effective method of enhancing the ability of other traffic spotting you. The systems are simple to install, esentially maintenance free, and are claimed to actually enhance bulb life. In a decade of operating this equipment in a variety of aircraft, I would say that bulb life is certainly no shorter (kinda hard to say it's longer!).
Very good piece of equipment, highly recommend it to anyone.
http://www.pulselite.com/