Patchs was that way too when riding in the back seat. I now put her in front with a short leash on her. When I see a dog coming (depending on her level of excitement) I tell her "easy" or "Leave IT - STAY" and that keeps her from going crazy. When she is COMPLETELY calm, I praise her so she knows that is what I want from her.
You'll need to experiment with your dog to find the trigger that works for her, but try to ALWAYS keep her calm and dont allow her excitement to build. Its much eaiser to control from a low level and very hard to do once she goes nuts. You may want to begin training in a more controled enviroment so you can control the stimulant source.
Try sitting in your car on the side of the road and have a friend walk there dog toward you from a great distance. As long as your dog stays calm , have the friend move closer. When she starts to show excitement have the frined move away until your dog calms. This may take several sessions, or even days or weeks but be paitent, it will work.
BTW, the Leave IT command is not just for making your dog drop something or not touch something ... I have always used it as more of an "Ignore IT completely" command.
Hope this helps.