Funny thing is, what springs mostly to my mind when I think of the very early days of the M1, north of Watford Gap anyway, is driving on moonlit nights just on sidelights.
And conversely, when headlights were required and, as said above, for long stretches on full beam, on seeing an oncoming lorry we would switch to sidelights instead of dip, to pass each other. Then often would ensue a surreal series of very brief flashes of thanks, thanks of thanks etc… I suppose it was a comforting connection of brotherhood in those days before CB (and certainly mobiles) that was only otherwise presented briefly when tipping and loading, in transport cafes, and digs.
I also remember that, after overtaking and receiving the returning flash, the ‘thank you’ was a double tail light flash, not the indicator to and fro of today which came much later. This meant that at night for a couple of brief split seconds you drove in complete darkness. 
When I came to work here for Gauthier I employed the same method, with our own drivers anyway, they definitely understood and some of them even returned the gesture in kind. 