I've never been in this situation and therefore will not second guess your actions in this case, but I get the feeling that since she had a cell phone, she could have called 911 and told them some psycho with a gun was following her around and would they please send an officer. She could stay on the telephone and drive around in circles until the LEO's locate her, at which time they would pull the guy over or the chase would start, whatever.
What you did is brave and it's understandable that you want to protect your woman. However, had she called 911 and not told you, what would happen if the cops showed up and saw you pointing your SUR at some guy on a dark road?
If the police are unavailable, perhaps as the result of a major riot or SHTF situation, that's a different story. But I think if it were my wife, I would have told her to call 911. Of course, I would put myself in her vicinity and I would be armed (not visibly). But I'm just there for back up in case the LEO's take too long to show up.