Chihuahuas, like most small breed dogs, are prone to what's called "fear aggression". They don't freak out and bark because they think they are macho, they bark and freak out because they KNOW they AREN'T. They damn well know they are small and that the rest of the world can eat them for lunch. It's actually believed that the Chihuahua's ancestor's were the fennec fox, which is why their ears and eyes are so fucking big, and why they are always on the alert. They're watching and listening for predators (true fact, look it up).
That being said, not all Chihuahuas are yappy little monsters. I've met a lot who actually aren't. Nervous, but quiet, and if you approach them gently and calmly, even friendly. The problem with traveling with them, and any other small breed, is they ARE subject to attacks from dogs, coyotes, hawks, etc because they are considered prey. If you get one, you're going to have to be extra careful and protective.