Puzzled, probably a simple answer.

The velocity of propagation for a transmission line can be expressed as Vp = 1/sqrt(LC) where L and C are the per unit length inductance and capacitance.

Obviously, Vp cannot be > c, which is 3E8m/s.

What__ _physically_ __stops me from from making a coax, say, with 2.5nF/m and 2.5nH/m giving a calculated Vp of 4E8m/s?

Cheers