Light speed is constant when acceleration does not have to be considered. The traveler has a dilation of time and distance compared to the origin. In SR the approximation for a traveler can be made where v is relative to the frame in which co was measured.
c2 = co2 / ( 1 - v2/c2 )
This is probably not true when high energy density bends space and SR is not used. A variety of assumption can be made. Most convenient is the case of GR which means h the Planck constant does not vary with acceleration. The usual integration is done for acceleration in the direction of v.
dE2 = c2 dp2
2c/co - c2/co2 = Eo2 (1 - v2/c2 )( 1 + (v2/c2)/(c2/co2) )
The implication is that in GR the light speed measured by a traveler can not exceed two times the standard value.
For higher light speed quantum mechanics must become relevant and h must become variable with acceleration. These are the extreme high energy cases where QM can not be ignored. Scale relativity and squeezed quantum states lead to a method for variable h and additional predictions, but not light speed ever reaching three times the standard value. A worm hole is predicted to open around the vehicle if kinetic energy ever reaches such extreme high range. Conventional technology will not do this.
At such speed the ability of a traveler to make measurements is in doubt.
What Light Speed Will A Traveler Measure Under Acceleration At High Speed?