Point of reference…
https://www.reddit.com/r/askscience/comments/3crbh5/is_pluto_small_enough_that_you_could_obviously/
However, a tunnel is different as you’re not looking at a single vanishing point that’s much harder to see than a large horizon. As you intuit, it would depend more on individual visual acuity, with most unable to see far enough into the tunnel to see anything other than a grey or white light (depending on how bright the tunnel lights are). Not being a biologist, I can’t tell you the theoretical limits of perfect human eyesight in relation to distance.
Edited: My bad, I misunderstood the question.
