Debilität!

Because of the way IQ tests are designed, half of the people evaluated will score at or below 100, which is average.[1] It's a sobering thought that nearly half of the people driving out there with you on the freeway have less-than-average intelligence (though you've probably suspected it long before now). However, this statement I just made is slightly misleading. You see, in 1910 the psychologist Henry H. Goddard applied the term "idiot" to someone who had an IQ below 25, the term "imbecile" to someone who had an IQ between 26 and 50, and coined the term "moron" for someone who had an IQ between 51 and 75.[2] Currently anyone with an IQ score below ~70 is not issued a driver's license…in the U.S.[3]

As I recently learned from one of my labmates, such is not the case in Germany. If you score marginally on your driver's test you can still operate a motor vehicle with restrictions. You have to put a special sticker on the back of your car that identifies you as a driver who isn't allowed to exceed a certain speed limit. Thus if you were ever to get stuck behind one of these people and you shook your fist in fury and screamed, "Moron!"—you could be right!

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Notes:

[1] In other words, the standard IQ test ranges from 0 to 200, with 100 as the average. The scores follow a normal distribution curve or Poisson distribution. This means that 68% of all people will score an IQ between 85 and 115 (34% between 85 and 100); and 96% of all people will score an IQ between 70 and 130 (48% between 70 and 100). Only 0.5% score an IQ over 140 and are considered geniuses. Those that score under 60 also constitute 0.5% of the population and are discussed above.

[2] This was accepted by the American Association for the Study of the Feeble-minded but has since been discarded (as has the title of that organization). See http://en.wikipedia.org/wiki/Moron (psychology).

[3] Basically they are unable to pass the driver's license exam.

Image attribution:

Children on bumper cars is by By Danie van der Merwe, available at http://www.flickr.com/photos/dvdmerwe/3989249785/.