I read a great account of a really experience courier (2 million miles). He was describing following a small truck with a 4 wheel rear axel. He was distracted by what the thing between two of the wheels was. The moment he realised it was a housebrick, it came loose and hit him in the visor, nearly KOing him! On top of that, you've got stuff on the road that cars will just drive over: I've encountered roadkill, 1'x8' roll of carpet, a length of 2x4, really thick gravel (for the tyres I was running) and big potholes. When you have space in front of you, you can move to avoid them. When you have a third of a second after they emerge from under the vehicle in front, you aren't going to be able to a lot of the time. Tailing people closely for any length of time, even when you're confident you can outbrake them and you're paying attention = bad idea, in my book.