I was driving east and was stuck behind a lorry travelling about 60-70 kmh. I could not overtake as there was a solid stream of traffic in the right hand lane. As I approached the Bellville Road offramp where the road becomes 3 lanes and the left hand lane is a compulsory stop for trucks, an articulated truck came up behind me hooting and flashing his lights. I got a fright as I thought maybe his brakes had failed and he wanted me to get out of the way. I indicated that he should go into the left lane by gesticulating wildly. He suddenly must have realised his fault and cut in front of another truck in that lane. Neither of the trucks stopped at the compulsory stop. Not a nice way to start the work day with such stress.