This was on Canyon just past the legal center and coming around the curve towards Broadway. There were no obvious signs and no indication of a police officer. I was tagged at 5 over and since I ride a motorcycle, I am very cautious, proactive, and aware of my surroundings. It is four lane with an island and I was in the left lane. I was next to an SUV and had scooted ahead of it briefly to be out of blind spots and visible to other turning traffic, and then slowed back down to the posted speed (35 I think, maybe 30). The camera took the pic from the right side so maybe he was parked in the Library parking lot.
And yea, there is a sign upon entering Boulder. As I recall, it's fairly small and under or grouped with several other signs as you come around the curve and the road splits into 4 lanes.
I honestly don't ride fast in town (ticket proving otherwise of course) and am generally very cautious and moderate in my riding. In 128,000 miles on my bike, I've only been pulled over twice. 57 in a 55 down in Arizona (warning) and passing on a double yellow near County Line and 7 by a Boulder County Sheriff (verbal warning).