Can you relate to this? I don't like my girlfriend using her phone while we're outside together.
Why don't you want her to use her phone outside?
I'm concerned because I feel like it could be dangerous to walk and use the phone at the same time.
Even though your concerns are valid, your girlfriend is going to make her own decisions. You can tell her how you feel about her behavior, but you can't force her to change.
I know. What I think is that she doesn't pay attention to the world and is going to get run over by a car or something like that. She's dangerous for everyone else.
Your concerns are valid, but she's not a child. You have to rely on gentle, persistent persuasion.