Maybe the first time you looked she had not finished putting on her makeup and wig...then when you looked again...voila...different girl. I can't read license plates behind me but this morning I was stopped behind a car that had the license plate "pothole". Very appropriate for Ohio.
She could of been a hologram , like the dr. from Star Trek Voyager . One of the new cars with a hollograffic computer controller. I think m.i.t. , or cal tek is working on this .
It is a remote possablity that she was a reptile alien trying out different human forms .Some people do think there are human looking aliens amoung us . Travis Walton meet some in 1975. http://en.wikipedia.org/wiki/Travis_Walton This is worth a thread of its own . You might want to take some pics next time .
If you read all the posts , the o.p. stated that the car had the same front lic. plate . My best guess is that it was a car pool and they switched drivers .
Hell, my uncle was one. We live in a remote place far from the city (but we're kinda urbanized now, this happened back in the 90s) and my uncle was a huge forever alone guy. Playing chess with himself and stuff like that. Myth says that "enkantadas" or what you guys call elfs are specially attracted to forever aloners. They say one got to him by staring with it for a long time and after about 10 weeks he became aggressive and the thing that got to me is he had an elongated chin and mout and jaw, almost looking like a horse, but not verily. I think that elf was some kind of witch specializing in psychology lol. I will never know.
as if it would take longer then that? i'm not saying the most ordinary expanation is always what happens, just mentioning that they are also among those that exists. changing a human face isn't a very interesting changing of shape. humans just aren't that interesting to begin with. neither for that matter are cars.