Corollary: Conditional frame-invariant coarse speed

Under T, the maximal signalling speed of $\mathcal{T}$, expressed in every admitted inertial coordinate frame, is $c$. This concrete conclusion holds for every $D\ge 1$.

Proof. The action preserves signalling relations and law gaps. It maps future-causal and boundary-null separations to future-causal and boundary-null separations. Positive-duration super-cone signalling is excluded and boundary relays are present in every transformed direction by Corollary cor:coarsespeed and the action.