There's not much out there with a higher point loading than a loaded tandem truck, and there's a lot of 6" reinforced concrete that been underneath those trucks in various buildings over the years.  As others have pointed out, adequate steel and most particularly a properly compacted base are the true keys to concrete slab strength.

The reason roads have thicker concrete is because of the frequency of the loading, exposure to freeze/thaw cycles, need for adequate concrete cover to keep salt away from the steel, and the impact loadings (accepted practice adds 30% for impact loading).

That said, my sister and I have 4 engineering degrees between the two of us, and Dad is still getting 7 inches in the shop.  You can lead a horse to water ...