does anyone know how to unlock the highway/truckstop?
Kinda but not sure if its the correct way to do it... but I was able to get there and to the slums and motel and truckstop diner by going to the brick building near the security checkpoint at night over and over until I got grabbed and then lost the fight with the assaulter (get raped) then I woke up behind the pub, but then the option to go to the highway/truckstop was the only available option. After that you can go to pretty much any of the places (truckstop diner, motel, slums, the diners bathroom for gloryhole options, etc.)
So very long way of saying get raped in the checkpoint area near Alison then you will wake up there.
No not really guaranteed. You kinda need to get lucky and get taken when you are in the exact location of Alison. That brick building warehouse. I recommend just clicking back and forth between the brick warehouse and the security checkpoint. Oh and use the skip unseen text, just makes it faster. You'll get it within 10 ish minutes.