Many years ago, after working late at the office, I set off for home at around midnight. My journey home took me up the main A1, a road that I carelessly used to speed upon every night (if I didn't see 130 mph on the speedo at some point, I had failed).
I was 6 or 7 miles from the exit slip (offramp to you yanks) when something tapped me hard on the shoulder. I looked in the mirror and could see the dark shape of someone on the back seat.
I illegally screeched to a halt on the hard shoulder, leapt out of the car and wrenched open the back door, to confront whatever creep had got in my car.
Nothing, empty, no-one.
OK, it was late, I was tired, and imagination sometimes plays tricks (although this was the first time it had played that sort of trick on me) and the tap on the shoulder felt VERY real.
I climbed back into the car and finished off my journey, but all the time on the A1, I was sure there was a shadow in the back of the car. Everything returned to normal as I joined the westbound A507.
I put the entire thing out of my mind, I was away at an exhibition in Europe for the next couple of weeks, and pretty much forgot it.
A few years passed, my journey home from St Albans was still usually mainly up the A1, I'd had a couple of changes of car too.
Then, late one night, out of the blue, on an (almost) empty motorway, at (probably) the same spot, someone, or something tapped me hard on the shoulder...