The law also used to say being gay was an offence. The speed limits we have today (on NSL/Motorways) date back to the 60s. While obviously there are still older cars knocking about most cars on the road today can safely travel at higher speeds.The laws of the land say otherwise.
The speed limit is irrelevant in discussing whether speed is appropriate for the current conditions, in fog (the reason for the introduction of the speed limit) it would frequently be dangerous to be driving AT the speed limit, but in a world focussed on speed it would be legal. It would be potentially considered driving dangerously, but that can't be enforced by speed cameras and arbitrary limits.