This should really be a matter of common sense.
Of course you may have to travel with an elevated load to clear an obstacle albeit at a drastically reduced speed.
Generally fork travel height varies depending who you talk too but generally under 12".
The most logical ive heard was between ankle and knee height. If someone'**** with a fork your talking a serious injury and the 2 straight bones in your lower leg are a **** site easier for a surgeon to piece back together then an ankle or knee joint and less chance of a permanent life changing injury - morbid but does make sense.
Really you should just be travelling with the forks as low as practical but we can't surround everything with a set 'number' in legislation.