Safe distance on a bike

[saaby93]

We all know about the 2 second rule and how if youre lucky it's just enough time to stop if you see the vehicle ahead begin to brake.
It's not enough if it stops dead for some reason or if a deer leaps out in front of you

Right thats the opening done with

In this video - setting aside the overtaker
http://www.bbc.co.uk/news/uk-england-lincolnshire-...
Whats the safe distance the bikes should be apart?
Is it safe to ride two (or more?) abreast?
Do you do 'what-ifs' - if the first bike falls over how likely are they all to form a heap?

Too close or not