I was driving home after a hard day at work when I pulled up to some traffic lights. In front of me was a silver astra and a British gas van in front of that. When the lights turned green the van set off quite quickly, no hesitation or slouchyness. The astra set off and remained very close to the vans rear safety bar. Then for no logical reason the astra decided to dangerously undertake the van. As the road was merging from 2 lanes to 1, the van was following his lane moving slowly towards the left just as the astra was undertaking. The van driver must have just seen the car in their mirrors at the last minute and had to take evasive action to avoid what could have been a very nasty collision. In the end it was completely pointless because the silver astra had to stop at the next lights and the British gas van was right behind them