Hmm, well I always obey the speed limit of course, but hypothetically if I HAD exceeded the speed limit while wearing this helmet, I would say 90-100 mph or so. I would theoretically start tucking around that point. At normal highway speeds between 60-80 mph, this thing is fine. It will always catch more wind than a regular smooth helmet, but the engineers designed the helmet with street use in mind, so it's going to be better than a MX helmet.
All that being said, make sure that you try one on before buying it to make sure it fits your head properly.