This is a question that I hear every once in a while. While I am not aware of any department in my area having citation quotas, I cannot say with certainty that all departments do not.
I do know that most departments leave the decision up to the officer when it comes to citation writing. It is one area of policing that the officer has descretion.
Most departments do periodic evaluations of officers. As part of that evaluation, there is usually a part defined as "Self Initiated Work" This is the area that supervisors critique to determine if the officer is activily involved in his/hers daily duties. Is the officer making arrests? Is the officer doing secruity checks? Is the officer activily involved in enforcing traffic laws? These and many others are topics for discussion and evaluation of the officers daily routine.
The best answer to this question that I have heard from a police officer is, Yes, we are allowed to write as many as we want.
As a rule of thumb I wrote citations for about 30% of the people I stopped. I believe that our job as police officers is to deter, and if our presence or a warning is sufficient then a citaiton is not necessary.
A word of caution. Those that think it is safe to go 10 miles per hour above the speed may want to rethink that rule of thumb. With the economy in the tank and revenues down, many departments have increased citation writing as a means to increase revenues, so be careful out there and drive safely.