Jeppesen Introduction, Chart Glossary:
HIGH SPEED TAXIWAY / TURNOFF (HST) — A long
radius taxiway designed and provided with lighting or
marking to define the path of an aircraft, traveling at
high speed (up to 60 knots), from the runway center
to a point on the center of a taxiway. Also referred to
as long radius exit or turnoff taxiway. The high speed
taxiway is designed to expedite aircraft turning off
the runway after landing, thus reducing runway occupancy
time.
But then again, for runway code numbers 3 and 4 (lenght>1200m), the high speed taxiway are supposed to be designed for turn off with speed of 50kts (93km/h) in wet conditions. (found in a JAA Air Law book)