I've travelled by bus in London for 40 years.
I have seen drunken revelling, I've seen arguments and fights (on including me), I've seen a woman lose her shoe whilst getting off and I've seen an entire top desk terrorised by a pigeon.
I may one day see a terrorist attack. I may one day see someone stabbed, murdered, robbed or die from natural causes.
It is, however, safe to travel by bus...
In the same way that it's safe to take a bath at home, despite the number of people who die as a result each year (21 last year according to the Dept of Trade and Industry:
http://www.humanics-es.com/burns.pdf).