I often get to see the most photogenic things coincidentally while driving by, usually while on a highway. The best way to deal with this is always having a camera with you. In most cases I find it to be the best procedure to take the very next exit once I spotted something and get to stop the car as soon as possible. Because I have such a terribly bad sense for orientation, I would use the iPhone to check what I just looked at and dig out the street names of an intersection that's around that spot. Then punch it in to my clumsy GPS and voila!
This sort of photography is a lot about opportunity. I guess the art is anticipating opportunities and become aware.