According to the 2011 GMAC Insurance National Drivers Test, 36.9 million American drivers—18%—couldn't pass the written drivers test today.
Related Fact: Kansas had the highest average score (82.9%), while Washington, D.C., finished last (71.8%).
Related Question: Did you pass the test the first time? In New Jersey, in 1996 at least, you could fail one part of the road test and still get your license. I was cited for "Failure to Follow Directions" for not changing lanes fast enough, but that wasn't enough to keep me off the road.