People mostly drive too close because if you don't then someone just pulls out into the space in front of you and you still end up too close. I once decided to back off every time someone pulled out into my 2 second gap on the M25. I ended up getting undertaken by the car behind and badly cut up by him. Yes, he was an a*** but I can understand his frustration at me constantly braking and just letting anyone who wanted to pull out. Now I drive just close enough to prevent people from pulling out but I always look as far ahead as I can so I can drive smoother and not end up constantly braking.
The 2 second rule works well on quieter stretches of motorway and dual carriageway, but once it gets busy it is useless, and yes not observing it does result in more accidents, but that changes nothing since we have decided not to increase the capacity of overloaded roads.
As for the Avon bridge, in a car you are more at risk of going over that barrier than through it. To my knowledge there have been no instances of vehicles going through or over the crash barrier since it opened in 1974, so the risk is no higher than any other stretch of motorway. However, I fully understand the feeling of vulnerability.