You can't search a car on a traffic stop without probable cause of an actual crime.
However, if your traffic stop is for a moving offense that warrants arrest (DUI, for instance, or reckless driving), all bets are off, because the police can do intrusive searches incident to an arrest.
Also, if you're stopped for a moving violation and either authorize a search, or have evidence of probable cause in plain view (for instance, a bag of weed in the passenger seat), all bets are again off.
And to get this power, all the dog has to do is to pass a training program. That's like giving anybody who has a "diploma" from a "prestigious Internet university" powers to arrest and imprison. Only in this case you can't even argue with them because how can you argue with a dog? A dog barked, thus search is authorized.
Now let's think - can there be any ways to make the dog bark? If somebody does that in order to make foolproof instant probable-cause-on-demand, what would be his risks that the dog will grow a conscience and tell everybody he's been lying for years?
Worse, from what I recall there are actual scientific studies out there demonstrating that many of these dogs do indeed bark when their handler thinks there are drugs rather than when they actually smell them.
