in traffic? I have a diamond ring on thats worth thousands but I dont have a sign. I think these people want to draw attention to themselves.

Hmm... another question to ponder - Unfortunately, it's probably because they value the lives of their dogs more than they value the lives of others.

Its the same sort of things as someone putting "baby on board". Show dogs are considered valuable and they want other drivers to know they have precious cargo. I doubt its because they're going to jump out into traffic.

Yes, it is a way of bragging that their dogs are show dogs.