This morning I witnessed, and very nearly played a big part in, a quite spectacular high speed crash on the M40.
I was traveling North near Warwick and was in lane 3 (fast lane) when I noticed a car gaining on me reasonably quickly from behind, although it was quite busy there was enough room for me to change lanes and allow the car behind to pass so I moved in to the middle lane. The car, a Toyota, passed me and I started to move back in to lane 3 behind it, as I did so the Toyota was about to overtake the car that was in front of me in lane 2, unfortunately this car decided to move in to lane 3 at that exact moment without checking it was clear... Although the driver did eventually notice the car alongside him (or her) and swerved back in to the middle lane they had already taken up most of lane 3 and the Toyota was taking evasive action by swerving to the right to avoid the rear of the middle lane car.
Swerving suddenly at motorway speed (or faster!) is a tricky thing to get right, the swerve unsticks the rear of the car and that makes you over-correct in the other direction. What you tend to get is a kind of pendulum effect as the back of the car slides from side to side as you make corrections each time, you'll either bring it under control, hit something or spin right round to face oncoming traffic... The Toyota 50yds in front of me managed the last two options!
I saw the whole thing in slow motion, I was willing the driver to get it under control as the Toyota snaked away ahead of me, unfortunately physics took over and it spun in a wide arc across all lanes , changed direction and smashed into the barrier by the hard shoulder at quite a rate of knots. To make matters worse it then bounced back in to lane 1 and ended up facing almost the right direction (after having traveled 360 degrees).
This all happened right before me and a combination of heavy braking, some swerving of my own and more than a little bit of luck resulted in me just managing to avoid ploughing into the Toyota at 80ish MPH.
Hazard lights on I pulled on to the hard shoulder along with a couple of other cars to lend a hand. I wasn't entirely sure what I was going to find when I got to the Toyota but happily the young lady driving it was pretty much unhurt and was able to get out of the wreckage without any problems. Most of the people who stopped left their details and carried on with their journeys, the man who called the emergency services stayed behind and I did too as I think I was probably the only one who saw the whole thing and I wanted to make sure that the Police knew that the Toyota driver wasn't to blame for the accident. The ambulance arrived first, police and two fire engines followed.
I left my details with the traffic police and the other guy and I carried on our way, I'm expecting a call at some point to tell them what I saw (and that is why I'm writing it here really). The girl driving the Toyota was really lucky, she seemed to be in a much better state than I was as I was suffering from a huge dose of adrenaline at the time...

The driver of the car that caused the whole thing didn't stop, I didn't get a good look at it either, but then I did have a ton of metal approaching at high speed!