Stuck in the traffic this morning and staring around me at the other drivers I realized just how much my safety is dependent on the skill, prudence or just plain common sense of my fellow drivers. Once at work I decided to have a look, statistically, at just what sort of people I could expect to find out there on the road.
I am assuming that the necessary attention, prudence, rapid processing of information and subsequent decision making involved in driving and surviving successfully on the road, ensuring minimum risk to others as well as to myself, will have a lot to do with intelligence, and did some calculations.

An average person, falling exactly in the middle of the general population, is considered to have an IQ of 100. 50% of the population will be above this and 50% will be below. It should be added that the distribution of intelligence in the population is represented by a Bell Curve - this in layman's terms means that the population tends to clump around the average. Less than 10% of the population have an IQ of 120 or higher and once we get up to 130 - the level necessary for Mensa membership, we are left with just 2% of the population.
It quickly becomes apparent that there are few Einsteins out their competing with us for road space. However, the shock comes perhaps when looking below the average. The percentages are the same, the reality, however, is a little more worrying.
IQ was originally calculated by dividing mental age by actual age and mutliplying by 100. For adult IQs above the average this is not very useful as after the age of 16 (adult age for the purposes of calculation) IQ does not change much. However for intellgence levels below average we can get a meaningful result, namely a mental age below 16, which may well be taken as an indication of arrested development.
So, following that Bell Curve below the average level of IQ 100 we find that statistically the drivers of almost 2 of every 20 cars we encounter on the roads could be expected to have a mental age of less than 13 while staggeringly just over 1 out of every 200 drivers could be expected to have a mental age of less than 10, that is, an IQ of 61 or below.