Because Vegas is a new city built in a flat desert at a time when land was extremely cheap, the roads are extra wide. That makes for big intersections, ones that actually take longer to traverse. Combine that with the fact that there tends to be a mile or more between signaled intersections (which allows a lot of traffic to be stored between signal changes without stacking into prior intersections) Vegas' engineers have opted to make signals longer.
Drive 5 miles of surface streets in Vegas, and you won't pass through nearly as many signaled intersections as you will in Boston or Bloomington or Burlington. You'll also drive faster when you are moving. You won't stop as often, but when you do, it'll be for a longer time.