A study out of George Mason University (Political Economy at Any Speed: What Determines Traffic Citations?) attempts to answer this question.
The abstract of the study is included below along with a link to the full study:
In this paper we study the political economy determinants of traffic fines. Speeding tickets are not only determined by the speed of the offender, but by incentives faced by police officers and their vote maximizing principals. Our model predicts that police officers issue higher fines when drivers have a higher opportunity cost of contesting a ticket, and when drivers do not reside in the community where they are stopped.
The model also predicts that local officers are more likely to issue a ticket when legal limits prevent the local government from increasing revenues though other instruments such as property taxes. We find support for the hypotheses. The farther the residence of a driver from the municipality where the ticket could be contested, the higher is the likelihood of a speeding fine, and the larger the amount of the fine.
The probability of a fine issued by a local officer is higher in towns when constraints on increasing property taxes are binding, the property tax base is lower, and the town is less dependent on revenues from tourism. For state troopers, who are not employed by the local, but the state government, we do not find evidence that the likelihood traffic fines varies with town characteristics. Finally, personal characteristics, such as gender and race are among the determinants of traffic fines.