There are many factors in this, such as proximity to the blast, elevation of the nuke, surface shape and soil/sand content, wind direction, ambient temp, humidity levels, your age, your pre-existing health condition.

There are way too many factors to generalize any proper response. If you are too close to the blast for the filtration in a car to be of use, you will probably be in a state of shock anyway, so just get in, drive away, go to the closest convenience store, help yourself to water, food, etc and drive away as fast as you can. Otherwise, if you are in a busy city, ya, yer pretty much out of luck.

Oh and this is just a guess, but save up on bottle caps.