To convert a length of space into a corresponding length of time requires a conversion factor. For physical reality that conversion factor is the speed of light: r = ct, where r is a spatial displacement, t is a temporal displacement, and c is the conversion factor. For a mode of transportation the conversion factor between space and time is a typical or conventional speed. One reason for such a convention is to communicate the length of time expected in order to traverse a corresponding length of space.
That this conversion factor is also a speed is extraneous to its status as a conversion factor. Whether or not anything travels at such a speed does not matter to the conversion of space and time. Whether or not one measures something going at such a speed does not matter either. All that matters is that the convention is accepted. The science community has agreed to define the speed of light in a vacuum as exactly 299,792,458 metres per second. A particular map may use a single conversion between space and time; everyone following that map has the same conversion factor.
Does that make the conversion of space and time subjective? Not necessarily, because a convention may be justified by an argument about objective reality. But it could be subjective if someone adopts their own conversion factor which no one else is using. That may reflect their driving style or preference for under or over estimation. The purpose of the conversion is relevant.
A conversion factor applies to a real or virtual phenomena. If people with frame of reference S have the same conversion factor as those with frame S′, then the conversion applies to both. So r = ct in S and r′ = ct′ in S′ because of the status of c as the conversion factor for both S and S′. This is how the Lorentz transformation can apply both to particle physics and everyday transportation.