Which ways are driveable by car

The page you link makes a distinction between roads and paths. Roads are for vehicles, paths are not. Roads are largely open to pedestrians (but this varies by country and some classes of roads are usually closed to pedestrians). Special circumstances may be indicated by access tags:

http://wiki.openstreetmap.org/wiki/Key:access

So there could be a path that is open to motor vehicles (maybe someone is allowed to use part of a bike trail as a driveway or whatever), and some roads may be closed to some classes of vehicles, or restricted as to who is allowed to use them. But these are exceptions that should be explicitly noted with the access tags, it is reasonable to make some assumptions that apply when specific access tags are not present.

One strategy for making sense of it is to look at the choices that others have made. Here are the various default profiles for OSRM (a routing engine):

https://github.com/Project-OSRM/osrm-backend/tree/develop/profiles

Each of car.lua, foot.lua and bicycle.lua has rules for how tags are interpreted by the routing engine for that use. I think the lua files are easier reading, but here are the rules used by OSMAND:

https://github.com/osmandapp/OsmAnd-resources/blob/master/routing/routing.xml

There's surely more, those are quick to find.

The choices end up being fairly consistent, but can vary from country to country and different users of the data are free to make whatever choices they feel provide the best results, so there is some degree of subjectivity.