How much room do we leave for other road users? The best way to go about this is to imagine you are another road user and how much room you would sensible like. If you are riding a horse, what speed would you like the car to pass you at? Horses can startle easily so the answer is very slow. Give them as much room as you can.
Are you a cyclist as well as a driver? How much distance would you like a car to pass you with? Would you like the car to take into account the fact that you may wobble? We leave around 2 metres or a cars width. If there is no room to give this type of distance then what should we do? We use "less space, less speed".
Have you ever walked on a road? How much distance would you like? Again, try and think of the other road user. We would again leave 2 metres.
What is the risk of passing parked cars? What could they do with their doors? We give them a car doors width, in case they open them. If we cannot do this then it goes to "less space, less speed".
Motorbikes I would try not to pass but if you have to give them the same as a bicycle.
We normally give a 2 second gap when following a vehicle, so if the car ahead passes a stationary object, like a sign then we give 2 seconds until we pass it. Saying "only a fool breaks the 2 second rule" can help to visualise 2 seconds. Say it twice for wet conditions. In the snow, ask if the journey is really necessary.