HSE would need said mud coming from a workplace to result a corpse or serious injury first. It does not have to be somebody employed there but needs to have been reported to them as required by the RTA but that is somewhat restricted:
"Road traffic incidents arising from the movement of a vehicle on the road are only reportable where either the injured person was engaged in, or a person was injured as a result of:
exposure to a substance being conveyed by a vehicle;
vehicle loading and unloading activities but not picking up or dropping off passengers;
construction, demolition, alteration, repair or maintenance activities on or alongside public roads."
The real problem is that responsibility is split between, Police, County Council, Borough Council & maybe the Highways Agency. So it can easily dissolve into a buggers muddle with all sorts of bureaucratic dysfunction and buck passing.
HSE. quite rightly, do not want to get dragged into every aspect of that but have acted in a number of cases. This explains when they will and will not get involved.
OM 2009/02 - HSE?s role in the investigation of work-related road accidents and advice on responding to enquiries on managing work-related road safety
So if there is an ongoing problem really need to push hard, as we know to our cost, to get anything done by anybody. (Until somebody is dead as a result, that is)