Scramble for something to write stuff down on. Sure the first order of business is getting the car off the highway, or whatever. Then, there is the small matter of arranging for transport and repair. Oh yeah, and pondering the current status of your credit card. Really, these are little things. The most important thing is having some way to document the bizarre little moments that will bring reality to your "My car broke down" story. Because really, that's all that a breakdown is: a minor inconvience to serve as a catalyst for later storytelling. In fact, it will be these details that save you from any possible bullshit detectors that may be scanning your story. Remember, truth is stranger than fiction.