I've been in school buses with the same restriction on speed. Something mechanical inside them was designed to stop their top speed at 90 km/h (56 mi/h). And there are lots of trucks (semis) with signs on the back informing you that their speed is limited, usually to 90 km/h. I guess the sign is there so that you don't get overly angry when stuck behind one, and ask yourself why the driver isn't going faster.