Problem is, maxspeed:practical is not a legal speed limit, it is a practicable speed, one that drivers can achieve and maintain. Practicality depends on so many factors, such as vehicle type and driving style. So even though one could capture an average speed (much like Google Maps and Waze do), the definition of that tag in OSM very likely would lead to edit wars due to differences in personal opinions and experiences. Others have already pointed that out. It is surprising that such a page even exists in the wiki while the original proposal was overwhelmingly rejected.
This is also not a legal speed limit, it is an opinionated average speed. I don’t think it is bad for routing software to be opinionated, as long as there is more than one opinion available to choose from (say multiple vehicle and driving style profiles). Different drivers with different vehicles with experience in different cities are likely to believe that the best average speeds are slightly different. And that’s all assuming one is not collecting traffic patterns, which would solve the issue.
Most of the time, by placing speed signs on the fastest urban and rural ways (arterials, fast transit, and paved highways), and leaving most of the other lower, more local ways (collectors, local, unpaved roads) to gut feeling. Speed monitoring is essentially absent on the second group (though it varies), so there’s this culture that people are quite free to drive however they want there, hoping that their self-preservation instinct will moderate speed.
If you ask me, of course education would solve the mess. But then we’ll delve into politics, and the goal of OSM is simply to describe the world.
Local culture plays a considerable part in the self-organization of traffic in Brazil. People that drive between cities tend to drive more aggressively since they feel pressed to cover great distances in shorter time. Sometimes people in the countryside tend to drive more carefully, sometimes it’s the opposite. Traffic authorities prioritize putting signage based on accident statistics, so if a particular local way has had many accidents recently, it will likely get a low speed sign sooner than other local ways. Same goes for higher speed ways with localized problems that cannot be easily solved but still record a high number of accidents (like this curve with tight buffering space, low visibility, slightly irregular terrain, and high pedestrian density due to the bus stops around the bus corridor). In these cases, the speed limit tends to be lower than what would be expected from the physical profile of the street (authorities know that drivers tend to drive faster than the limit, so by setting very low speed limits they’re using psychology to try to get them to slow down to a safe level, which often is a little higher).