Let him stay on his high horse. Bungle's original comment is still very much valid: you made the mistake of not paying attention, and you're blaming the roundabout and its lack of markings. The blame is, as you of course have long realized yourself, all yours - everything else is just an excuse. When I was done at work, I looked at the behind of one of my students. That fine body managed to distract me for the entire trip home!