This article is from the Lefthanders FAQ, by Barry D. Benowitz firstname.lastname@example.org with numerous contributions by others.
Stanley Coren, who is the author of "The Lefthander Syndrome" found
statistical evidence of this, and didn't believe it for the longest
time. However, he remains unable to disprove it. He was able to
demonstrate a possible reason for this might be that a left hand
startle reflex would be much more dangerous when driving a car
on US or Canadian road since the car would end up pointing
against traffic while a right hand startle reflex would simply
cause the car to drive of the road.
As a double check, Coren did find a statistical difference in
left handed traffic fatalities in countries where they drive on
the left, such as Great Britain or Australia.