Obviously don't pet service dogs. Just to be clear. This photo was intentionally photoshopped to make it appear as though the airport was saying "Travel Advisory: when traveling don't pet dogs". As in, when in the UK never touch a dog. I thought it was funny/cutesy. I didn't intend on sparking a big debate about the ethics of petting dogs or the rules about service dogs.
Don't touch service dogs.
Pet dogs if you know them or are introduced to them.