I once did "over 50mph" free-wheeling down Parley's Canyon on I80, from Park City to Salt Lake City (~2000' drop in 10 miles; 700m fall in 15km). There's no alternate route there, so bikes are allowed on the Interstate. I only slowed down after I had to swerve out into the traffic lanes to avoid a broken wooden pallet lying in the shoulder. It's not the steepest of the local canyons, but one of the fastest for bikes because it's straight, wide and has good visibility of the road ahead.