The question is somewhat flawed, since it's not a matter of speed, but money. If a 200km/h road had the same price/cost as a 20km/h road, there would never be a reason to use the 20km/h road. But since roads have different price/cost, the most efficient choice is to use the road that accomplishes what you want to do for the lowest price. Usually, whatever the vehicle that will use the road can do - in early game with horses and oxen, this might well be under 50km/h. Later, it could be used in loading bays (since the vehicles slow down anyway) or whenever a way is not intended to be used often (eg. the connection to a depot).
Though it makes me wonder which road speeds are actually useful. As an Austrian, the most common speed limitations I see on roads are 30, 50, 70, 100 and 130 km/h. However, in this game it's mostly trucks and busses. I know of the former to be restricted to 80km/h in most of Europe, including the German Autobahn. Busses are the same at least in Germany and Austria, so I tend to think other countries count them as trucks as well. Hence 30, 50, 70, 100 and 130 km/h seems to be pretty good - 30 as the cheap choice when speed does not matter, 50 for city vehicles, 70 and 100 as a choice for trucks, whether you need full 80km/h speed or not. 130 as max allowed speed anywhere (except German Autobahn... but that's silly). If there was an 80-speed road, 70 and 100 would become pretty useless. If there was something slower than 30, 30 might become useless, etc. - that means 6 is the max number of different useful roads IF you split up 50 to 40&60, and the vehicles use their 'legal' max speed.