Medical researchers can be a funny breed. In their search for the secrets of health, they can come up with the strangest findings and present them with a straight face.
One study group has been trying to discover early signs of Alzheimer's, and so took some likely candidates for a drive. Tell-tale signs included missing a large speed limit sign, which suggests that anyone who speeds may have Alzheimer's.
Tell that one to the police (who also speed, and who therefore have Alzheimer's, as do postal and telecomms workers, and anyone who drives a white van).
Another problem with the conclusion was that patients with advanced Alzheimer's drove really well. Hhhmmm.
Source: Journal of Neurology, Neurosurgery and Psychiatry, 2005; 76: 764-8