Ah, they just measure stuff in light years at that point. Or, rather, light years per second!

Once you get into distances like that, you can start using stuff like the parsec (3.26~ light-years) and the astronomical unit or AU (0.0000158 light-years)....

Wow. I just realized that the speed limit here in Colorado is, on average, 5.917e-7 AU / hour.

I wonder if I could get a new speedo / odometer in AU rather than Miles? Would beat the hell out of "furlongs per fortnight".

Not to mention the (perceptual) increase in fuel economy.