Is this on her own? If so, definitely a NO. No arguments (if it was my DD even, and she's 15!).
If with a friend the whole time? Possibly, so long as I could meet her at the place she gets off the bus (let's say they both terminate at a bus stop) and walk back up the road with her.
Last week DD and friend I put on a train (single journey no changes and which DD has been on many times) and they went to another town then got a taxi. We agreed the contact rules for the night (which is one of the ways I manage getting assurance she is safe) so when she got out at the train station the other end - quick text, got safely into a taxi, quick text, got into the venue she was going to, quick text; no drama. But at the other end of that night (activity ends at 10.30pm) I was there waiting to pick her up and bring her back myself.
A year or two (?) ago a woman got viciously raped didn't she in the following scenario: she'd got off a bus that she'd been on with a friend about 300 yards from her front door and in that distance and space of time, was viciously attacked. There are MANY nutters out there, some of them on buses, some of them on the streets/in vehicles, and you have to help them (our kids that is) appreciate the dangers and how important it is to always make travel arrangements that are safe and for them to keep communicating where they are to you and that they're safe.
I wouldn't let her do it regardless of what anyone said to the contrary. Explain why though so she understands the risks and why you're saying no. It also makes a difference (I always think anyway) whether it's winter or summer and therefore if it's dark out or not. We all know more nutters operate/there are many dark deeds carried out by nefarious people when it's dark and I'm really strict on where she can go when it's winter - having a mate with her and not walking anywhere on her own are strict rules that let her get out the door in the first place.