There is this little chart in the Flight Manual for each helicopter. It is this manual that goes through much review before the aircraft is certified by the FAA. The chart is called the Height/Velocity curve. Or in real pilot lingo, the "Dead mans Curve." Without going into extreme detail, at 100ft the Astar that I fly you must be at 50 kts or greater to be OUT of the curve. Any speed above that is out of the curve any speed below that is IN the curve and would be unsafe as determined by the manufacturer based on certification testing. Interpolating the numbers, at 75 ft, the slowest I could go and be out of the curve would be 48kts. So, "full speed" is determined to be safe. The Bell H/V curve is very similar.
That is completely off topic! Flying close to the ground is unsafe at speed because of the danger of obstacles. I would assume if you are flying below 50 Kts you would be either landing or transitioning to a hover, which would be OK. When I see helicopters flying from place to place they are usually well over 200 feet above and usually over 500. I know many are used to flying low, but a NASCAR driver is also used to driving inch's from each others bumper, but look how often professional NASCAR drivers rear end each other.