Had a truck do that on me downtown Milwaukee. Turns out that the paper clip Penske put in the coolant over flow tank fell out and put the truck out of service. Couple calls later and a new paper clip and back on the road. Paper clip jumped the low coolant sensor that was faulty.
Bet all those cars I run off the road changing lanes would have liked to know what was up. 6 lanes of traffic, still in the middle lane when the air alarm starts to go off. With a shut down engine sure was fun.