Whilst it isn't a point I've made, there is logic in the offence varying by vehicle. A car such as the one being driven will steer and stop a lot better at 100mph than, for example, my old Ford Ka, which might have just got to that speed on a long downhill with a tailwind. Ergo the former is safer at that speed than the latter. If the offence was created as a safety measure then it is logical that the amount of danger created matters. It is indeed for that reason that driving on a normal road at 30mph is legal, whereas driving on a normal road at any speed whilst inebriated is an offence.
Besides which we do already have 'variable tolerance' of speed - there are different limits for different types of vehicle.
In practice it would be impossible to codify 'danger' as such.