I read this interesting quote today, and I couldn't help but remember your car example:

Third, Flynn may be overestimating the average population intelligence in past centuries and the amount of g needed to function in an agricultural society. Humans have much genetic programming for normal everyday life tasks (such as propensities to quickly learn a language and social skills) and drawbacks of low g may only become evident with arbitrary, unnatural tasks, such as school learning. The phenomenon of "six hour retardation" mentioned earlier suggests that people diagnosed as retarded by IQ tests may have trouble with school work but function adequately even in a technological society. After school, they "disappear into the population". Indeed, rightly or wrongly, rulers and political writers in past centuries have expressed contempt for the abilities of the masses. When cars were invented, some stated that few people had the intellectual capacity to learn to drive them. Such comments are rare today.

--Howard 2001 Searching the Real World for Signs of Rising Population Intelligence

(This leads to a provocative thesis which I find amusing just to contemplate.)