@AC -- more peer reviewing!
Did you actually bother to read the reference you posted: http://www.tfhrc.gov/safety/speed/speed.htm
"Without vehicles slowing to turn, or turning across traffic, the investigators found the risk of traveling much slower than average was much less pronounced." -- ie, it's not that slower drivers are inherently prone to crash more, it's that drivers are prone to crash more frequently when they are turning (and hence slowing down). The faster cars however crash because ... well, speed increases the risk of crashes.
"[other] researchers found a trend of increasing crash involvement for speeds above the mean speed in both rural and urban conditions - similar to the correlations reported in the early studies. However, no relationship between slower speeds and increased crash involvement was found."
Furthermore, the "U curve" was only found when limiting data to crashes between vehicles travelling in the same direction, yet "By far, the predominant crash type on rural roads is a single vehicle running off the road"
All of this pales however into insignificance compared to things like drink driving and running red lights -- something the USA is ridiculously tolerant of. I suspect that this is the primary reason you are more than twice as likely to die on the roads in the USA than the UK.
Speed limits are (like everything in life part) of a cost/benefit equation. The USA has decided to plot their data point significantly farther up the curve than the UK -- which happens to have the 2nd lowest rate of road fatalities in the OECD. Now stop whining be thankful you don't share the road with 14 year old drunk Americans.