You broke the law but you didnt get caught. That is like asking if a tree falls in the woods and no one is around to here it did it really make a sound? Of course it made a sound. It did produce pressure waves called sound waves whether or not you were around to here them. If it didnt make a sound you wouldnt hear it when you were around it. Ive always wanted to slap the person who asked the question about the tree. Now i want to slap you.
We cannot always know who or what the traffic limit is protecting. Children, elderly, wild animals, or perhaps the road itself which may not be able to handle excessive speeds without unacceptable repair costs. It may be to protect others or ourselves. Violating it simply because authority is not observing you every second may expose you or others to injury or death.
If the speed limit is posted, or if you're in a residential zone where the speed limit is known, then yes, you broke the law. In practical terms, though, there is a defacto speed limit that's actually above the posted speed limit. It's what you can reasonably assume that you can get away with.