road_speed=4   === set current road_speed to 4 or lower - if maxspeed is 
lower, but not higher (new behaviour)

road_speed_fixed=4 == set current road_speed to 4 no matter maxspeed, 
recommended speed, or other tags (same behaviour as currently using 
--ignore-maxspeeds)

road_speed_variable == 4 set to current road_speed 4 if no maxspeed or 
other tags exist (what other tags are actually existing in osm yet?) - 
decrease or increase it based on the tags/maxspeed (current behaviour, 
at least until there are tags like average speed in OSM).
I don't understand the reason for it. In general I think everything is solvable with current style-functionality. Can you give an example, were it isn't possible to handle it?

Henning