I haven't ridden single speed since my first bike about 52 years ago, but I used to frequently change from gears to fixed and back in my teens/early twnties. I would ride a 69" gear for general riding around and hills - even 25%, with a struggle - and 82" for time trials.

I recently, about a year ago, bought a Bianchi Pista and really enjoy riding it. For spinning I use 73" (48x18) and for pace line, 81" (48x16). I can cruise easily on the flat at 20mph and spin out at about 27-28mph in a sprint. The fixie is certainly a great way to build leg strength and to develop an efficient pedalling technique.

As for riding in traffic, which I don't do much of where I live, you just have to be a bit more predictive about stopping, but once you get used to it, it's OK. I use only a front brake. In a pace line I find it's easier on a fixie to adjust speed without braking. Just don't forget that you can't coast, especially when you are pushing hard or you might find that everything, including you might rotate:)