All my training vehicles in the UK had "Approved Motorway Instructor" in decals on the back.
On one occasion, after torrential rainfall coming out of Exeter, the motorway was absolutely chocka with traffic. I was travelling in lane 3 (overtaking lane in the UK, next to the central median/crash barrier for the US) and keeping a 6 second gap to the vehicle in front. When, after maybe a mile a black Vauxhall Astra GTE got within 3 car lengths and started to flash his lights. I would have moved over, but there was no 12 second gap to move into, nor a 3 second gap - so I increased my gap to 12 seconds because the GTe had no safety gap.
Anyway, this flashing and hand gestures continued for the next ten miles (approximately), where I took an opportunity to allow the GTe to pass and moved back out.
Long story short - about five miles later all hell broke loose as 6 cars ahead piled into each other.... and I controlled the panic behind, as I had left enough space to brake comfortably.
I got out to see if anyone required assistance, but as I approached the wife (who I had no intention of talking to be honest, as she had given me the w*nker hand) she said "we know, we know".....but their stoved-in front was testament to that and I had nothing to say.
Anyway, everyone was ok.
I had never experienced a pile-up, but that let me know science is correct, so use it!