Research has found that being desperate for a wee makes "the level of impairment on reason and problem solving equivalent to a 0.05 blood alcohol level." In other words, holding in a wee is as bad as being tipsy behind the wheel.
In an interview with the Jalopnik website, Dr. Peter Snyder, VP of Research at Rhode Island Hospital, and author of a study called "The Effect of Acute Increase in Urge to Void on Cognitive Function in Healthy Adults" explained why it is so bad for our driving judgement.
The message is that driving while you are desperate for a pee is doubly distracting. Firstly, it is painful, and pain reduces the ability of people to make good judgements.
Secondly, it requires active concentration, which therefore reduces the amount of mental effort you can devote to actually driving. There is a great buzzword in Formula 1: brain "bandwidth", or the ability to process lots of data at the same time. In this case, if you are devoting bandwidth to not peeing, you have less bandwidth to devote to not crashing into the car in front.
The research has generated quite a lot of interest. Its author has been awarded the 2011 Ig Nobel prize. That is not the official one, but a prize for the piece of scientific research most likely to raise a laugh. The award is always handed out with the words, "If you didn't win an Ig Nobel prize tonight - and especially if you did - better luck next year."