The number one cause of test fails (2018) - “only a fool breaks the 2 second rule, and when it pours make it 4”. If you are dithering about overtaking the vehicle in front then this is often the time that you will get too close to them. In addition, people who find themselves travelling at higher speeds having been in town for some time often misjudge how close they are to the vehicle in front. Normally this is a serious fault.