Travelling on the M40 a lorry in the left lane moved over to the centre lane to allow a car to join from the slip road. There was not enough room in the centre lane for the lorry due to heavy traffic so the lorry returned to the left lane hitting the car trying to join. This caused the car to spin out of control and take out another car joining behind it. Both cars ended up in the ditch. I was the first on scene followed by another car. I called the emergency services and then tried to get down to the cars which were not stuck on their sides in the ditch. There was a woman in each car screaming for help so I let them know help was on the way but there was nothing I could do as the cars were stuck. Soon after the emergency services arrived, I gave my statement and was on my way. I have not heard from either car driver since but I assume they are OK.