Officials seem to assume that the common person does not need to know the road type, only what they can or cannot do on the road. Signage is more common in busier ways, and less common in calmer ways, so you can expect it on dual carriageways and arterials, and will rarely find them on local ways and rural unpaved ways.
Official data usually helps solve this puzzle, but a driver will rarely hold a copy of the official map, especially when traveling within a city. Moreover, cities publish city plans, often containing roads that do not exist yet, which raises questions on whether such classification is correct or even usable in OSM, and probably less so in practice.
Say there is an accident and a judge has to decide whether a driver was above the speed limit. If there is signage, the decision is simple. If there is no signage, I think the judge should consider official plans (including the physical profile of roads, which change city to city) and assume the common driver knows them by heart (surely very few do), but as this is quite unrealistic, often judicial decisions are made subjectively (as terrible as this may seem).