I got pulled over yesterday on the way to school. Nothing major-- just a "friendly" 10-minute reminder that my front left headlight is out.
Fortunately, my roommate was with me at the time. While waiting for my license, registration and insurance to check out, I informed him of my theory that, ceteris paribus, drivers stand a much lesser chance of receiving a ticket when they have passengers in their car. The idea is that if it is one driver's word against one officer's word in a court of law, the State is likely to uphold the policeman's original claim. Should there be another passenger to vouch for the driver, the court now has two people claiming innocence. Knowing this, the officer doesn't waste his time writing a ticket and then having to go to court to potentially defend his actions. Again, ceteris paribus-- this isn't to say that no one ever gets tickets with other people in the car, nor that you can't get off with just a warning if you are by yourself. I just think you stand a better chance with some passengers in your car.
It's my personal experience that lead me to this idea-- I've got pulled over a handful of times over the years, and while the times that I've been pulled over are pretty evenly distributed between driving alone and driving with passengers, only once did I get a ticket while driving with a passenger (well-deserved, it should be noted), and only once did I not receive a ticket while driving by myself. A quick perusal of the Department of Economics here at WVU finds a roughly similar track record.
I'd love to see some data on this-- does anyone know what type of statistics police departments keep with regards to whom they pull over and the occupancy of the cars? Is any of this available?