I've recently been able to hit 41.4mph down a 3/4 mile grade about a mile from my house. It's near the beginning of my commute. It has a stoplight near the top, one about 4/5 the way down, and one at the end. If I hit the lights correct, I can easily hit 40+. I regularly get to 35. The posted speed limit is 50.
This is on a 15 year old, 35 lb Schwinn Highplains MTB loaded with a trunk bag with about 5 lbs of clothes and food, 2 water bottles and 215 lbs of me. I may not accelerate quickly, but I can get quite a bit of momentum going.