NCSA Publications & Data Requests
http://www-nrd.nhtsa.dot.gov/Pubs/811166.pdf
There are a bunch of statistics about auto accidents on NHTSA's website. So far I have yet to read anything about slow moving vehicles being a major cause of accidents or fatalities. The #1 cause of fatalities is Driver Inattention. Speed is a contributing factor to over 30% of accidents with fatalities. Alcohol is also a huge contributing factor.
"Speeding reduces a driver’s ability to steer safely around curves or objects in the roadway, extends the distance necessary to stop a vehicle, and increases the distance a vehicle travels while the driver reacts to a dangerous situation." -NHTSA