This is another that was published earlier at 365 Tomorrows.
It was May when the Highway arrived from some distant place in the northwest. On the fairly open ground the caterpillar-like monstrosity traveled at the alarming rate of about a mile per day, efficiently clearing away rubble and brush, flattening the ground and packing it down with Thumpers, and then laying out a fresh strip of road that it made internally with Assemblers. Some of the younger villagers had never seen a working machine, and they would stare at it from the hill all day.
Gregor was old enough to remember the time before the war, when it seemed like everything was a machine, but he sat and watched the Highway too. He had even climbed up onto it, opening access panels and trying to gain control. It was built like a tank and had very few access points, none of which revealed any kind of input device. Clearly it had received its orders from some computer somewhere - how long ago had that been? Gregor tried to do the math in his head, but he didn't know enough to make any kind of guess. If it had been active since before the war it would have passed by years ago even if it had started in Alaska, but it could have been stuck somewhere or trying to pave over a mountain or something. Maybe someone had been salvaging and had turned it on by mistake. Whatever had happened, it was determined to keep laying down highway now and there didn't seem to be an override. Gregor looked east towards the ocean and sighed. Such a waste.
He felt a hand on his shoulder and looked up to see his wife standing behind him - he had been spending all of his time staring at the rusty behemoth and felt almost as bad for neglecting her as he did for failing to stop or redirect the machine.
"The best salvager we've ever had and you can't do anything with a fully-functioning highway assembler. I know this has to be killing you, love."
Gregor nodded and sighed, looking back towards the breaking waves. He had been so excited when he first saw it, had pictured reprogramming the assemblers and making the machine construct a proper city for them to live in. He had known that was absurd, far beyond his technical ability, but surely he would have been able to use it for something.
"Come home, love. Get some rest, and tomorrow night the whole village will head down to the shore to watch it go. We'll make a celebration of it."
For the millionth time Gregor imagined the machine stopping on the beach, some safeguard preventing it from committing suicide, but he wasn't sure. With safeguards enabled something would have stopped it years ago, but without them it should have fallen off a cliff by now. Thinking about it did nothing but annoy him further, and yet he couldn’t stop. There was some part of him that was glad it would be out of his hands soon, and that part tried to remind him that he had a good enough life, with a roof over his head and hot meals in the winter. Joints groaning slightly, he stood and hugged his wife and felt his frustrations evaporate somewhat as she squeezed him. With a final weary sigh Gregor turned towards his home, leaving the enigmatic Highway to crawl ever closer to the beckoning sea.