The larger the apparent object, the more prominent it is. Putting a third brake light above the other two creates the outline of a large triangle. With its large area, this bright triangle is much easier to spot than the single line created by two brake lights. So the addition of one brake light in the right place, makes it much easier to spot braking, and fewer rear- ending accidents are the result. Slim pens look elegant and they fit easily into one's purse or pocket, but are they good from a human factors perspective.
Slim pens require a narrow pinch grip which is very uncomfortable for the hand.
The fingers are squeezed together, requiring significantly tensed muscles. This is particularly bad when writing is done frequently or for extended periods of time. In addition, the pressure required to hold the pen is concentrated on only a small region of the fingers. You have to squeeze the hand less, and the pressure on the fingers is more evenly distributed. The same principle is true for handtools such as screwdrivers.