"Often enough we hear about people being attacked and even killed by mental health patients who have been assessed as 'safe' and let out to roam free."
From the Royal college of Psychiatrics:
http://www.rcpsych.ac.uk/campaigns/changingmin ds/whatisstigma/violence.aspx
Violence can be a feature of mental illness and every year in England and Wales, on average, 55 people are killed by persons who are judged to be mentally ill at the time of the homicide. It is a devastating tragedy for that individual and his/her family when a life is prematurely cut short, and government, the media and the public should be concerned to minimise the number of such tragedies. However, every year 3500 people are killed on the roads and although there is some concern, the response is feeble compared to the size of the problem."
Trust that you are happy with the thought that anyone who speeds or has something wrong with their car is executed immeadiately - after all you are 70 times more likely to be killed by a dangerous motorist than a 'mental' patient.