Like most veg, I think you get better results if you blanch them before roasting.

We love roasted greenbeans and I'd go with the above suggestions.

I roast green beans quite often. As others have said, 425 for about 15 - 20 minutes, though it depends on how fat they are. The thin haricots verts cook more quickly. Love nutcakes' idea to add shallots, fennel and/or tiny potatoes are also good. I just do olive oil and then hit them with sea salt when they come out.

I've roasted green beans before with shallot haves with olive oil and they are tasty. 425F oven, test at 15 minutes. You can also blanch them first. I like to hit it with a little lemon juice when it comes out, although it may cause the beans to discolor, it is tasty.

Roasted green beans are not my go-to either, but I roast a lot of vegetables and find that they are good over a wide range of temperatures. You just have to watch the timing. So roast them at 425 for 15 minutes or so or 375 and expect a little longer. Even higher or even lower temperatures can also give good results.

At my house, we're having the first local asparagus of the season, and I'm going through the same thought process--steam or roast? If roast, at what temperature. I won't decide until the last minute, but whatever we do, they'll be great.

Why are you "roasting" them in the first place? Not the first treatment that leaps to mind.