I was in Massachusetts, in a leafy college town that seemed entirely made up of bakeries and gift shops selling dresses from the 1980s. The Amtrak railway station was in the middle of the town and, because it was under renovation, it wasn’t clear where the entrance was. I ducked down a side alley and, as a guy, unkempt with a backpack, walked towards me, I went automatically through the usual rigmarole: was the music on too loud in the bar 500 yards back for anyone to hear me? How soon could I backtrack without alerting the guy to the fact I was spooked? Why hadn’t I stayed out the front?

An hour earlier, I had taken an Uber from the hotel to the station. The driver had gone off the main route on to what he said was a shortcut. “This doesn’t look right,” he said, as we swung off a suburban street on to a semi-dirt track and I thought: you’re telling me. He was talking pleasantly about TV shows; it was the middle of the day. But I followed our progress on the app until we rejoined the main road.

In Texas a few months ago, in a cab from the airport late at night, I watched the empty freeway fly by while wondering how fast a car would have to be going to rule out opening the door – and at what point, precisely, I should change tactics from appeasing the driver’s creepy chat (we’re on the same side, your ex-wife does indeed sound like a bitch) to aggression (I’m not worth the trouble, there are easier prey) to dissuade him from trying to kill me.

For long periods one forgets about these calculations, running like malware on one’s system and backed up by a lifetime’s expectation that something bad is coming your way. You could say it’s paranoia born of conditioning designed to curtail women’s movements, or you could say it is a smart internalisation of what we all know to be true: that most women in their lifetimes will either be victims of, or in close proximity to, some kind of male violence.

A friend’s husband once averred that, statistically, it was extremely unlikely a woman in New York would come to harm in a city where murder and sexual assault rates have never been lower, and therefore there was no reason not to cut through the park late at night. The women in the room turned to look at him en masse. How bizarre to move through the world with this kind of oblivious confidence.

In Massachusetts I found the station platform, which was full of people, and a moment later the guy with the backpack turned up. Look, I thought; he’s just a regular guy and I was silly to panic. Then he dropped down on to the tracks, walked across two live rails and disappeared into the bushes. Oh my god, he was that guy! The guy who disappears into the bushes! Probably harmless but still, I thought, my reflexes weren’t off, and once again it enraged and amazed me that we move through the world ignoring the fact that 50% of the population is always ready to run.

• Emma Brockes is a Guardian columnist based in New York