Equally, we never say "50% of a m/s" instead of "0.5m/s" And notations like 0.9c are also quite commonly used for velocities close to the speed of light (the

Equally, we never say "50% of a m/s" instead of "0.5m/s" And notations like 0.9c are also quite commonly used for velocities close to the speed of light (the LHC tends to use TeV more often as a unit, which corresponds to a certain velocity for protons, but they mainly care about the energy) But using percentages with the speed of light makes some sense because it marks one end of an absolute scale, there is no way to go 1.1c or 2.5c. So using a percentage emphasises how close we are to the maximum value. But the same doesn't apply to AU.