If you think current trucks are dangerous, these are way longer (25m vs16m) with bigger blind spots etc. While they are in theory not currently coming to Britain, their acceptance in Europe about to be voted upon and is supported by the UK. This is the thin end of the wedge as once they are in Europe, pressure will bear on UK to allow them also.
Apparently trucks are involved in 50% of London's cyclist deaths, despite being only 4% of its traffic.
This mostly paraphrased from http://www.freightonrail.org.uk/NoMegaTrucks.htm
If you think this is bad, email your MEP not to vote for them!