Tire Pressure light on my car won't turn off!

I was driving a 2002 Windstar from my University’s fleet on my way back from a conference. We ran into a stretch of interstate that was down to one lane in our direction and the pavement had been milled. It was around 1 a.m. in the morning but the traffic was still heavy. The tire pressure light went on–apparently the vibration on that model turned the light on. When I finally got out of the construction zone and stopped for a cup of coffee, I kicked the tires and they all seemed o.k. I also found the manual in the glove compartment and figured out how to turn off the tire pressure light. It didn’t come on for the remaining 3 hours of the trip. I have never seen the real need for these low tire pressure warning lights–I check my tires every two weeks or so with a gauge and don’t find it a real problem.