There are many reasons
while people shouldn't drive
First, and most obvious, is the fact that when a person cries, their vision becomes blurry and distorted. This can make it hard to see other cars, street lights, lane markers, and other obstacles that might be in one's path. One would think that this reason alone might stop one from attempting to drive while crying, but it doesn't.
Another reason driving is difficult while crying is that for most people, their nose start running almost as freely as their eyes. This may seem to be a minor issue, but if you've ever had a runny nose while driving, you'll remember that it is extremely difficult to leave it be. Most of us keep tissues in our cars for those times when we have particulate sneezes or just have to blow our noses. Most stocks of tissues are no match for a nose that is running due to crying. I myself have been known to use up half a roll of toilet paper for the purpose of wiping my nose during a crying jag. This will pose as a distraction, and will likely prevent the driver from keeping both hands on the wheel for any significant length of time.
Also, there is the matter of the crying itself. When I cry, I have a tendancy to not take regular breaths. Normally, this would only have the effect of making my sides and chest ache. While driving, though, one will notice that after a few minutes of this they will become light-headed and possibly have their vision cloud. Both effects, caused by lack of enough oxygen, are extremely detrimental to a safe driving experience.
Finally, when one has cried oneself out and managed to keep on the road and not drift into any other drivers, there is the empty feeling left afterwards. That gaping feeling inside when one has shed their last tear and there is just nothing left. When in this state, the idea of driving off the next embankment doesn't hold any particular terror. At the end of a crying session, there just doesn't seem to be much left in the reservoirs for feeling anything. People in this state should do nothing more dangerous than sleep. Preferably not alone.