There's a distinct difference between sous and au-dessous, which I find hard to explain. Au-dessous compares two levels (similar to "below") and sous just states that something is "under" something else. If I had to find an English correspondence, it would be:

sous -- under
au-dessous -- below

I am aware that this doesn't answer your initial question because with this translation you could still use both in French. But au-dessous is also more of a spatial relation, making it less suited for a speed limit.

Thanks.  Your answer made me search more.  There's a grammar book that says "au-dessous de" and "en-dessous de" are interchangeable.  Then it shows a diagram with with a ball underneath but touching a table and used the word "sous (avec contact)" and another ball under the table not touching and used "au-dessous de la table (sans contact)."  Then it says, "Sous peut aussi signifier "au-dessous" (sans contact).  Last, use "au-dessous de" to mean status not spatial relationships.

The more I work on French the harder I find all the nuances of prepositions.