I can't prove that cops are ticketing more in order to make up other revenue shortfalls, of course, but it certainly seems true from anecdotal evidence. This raises a larger question: what to cut, and what taxes to raise, when recession hits? States could avoid having to deal with this problem by building up reserve funds during the good years, but that's so obviously ridiculous that we won't talk about it.
To state the obvious, it is hard to cut spending during a recession--in some ways, harder than during a boom. In good times, your laid off state workers can find jobs. Now, no matter how useless their particular job may be, there will be outcry if you axe them. Similarly, states find it easier to play around with things like speeding fees than to raise taxes.
But it's not like speeding tickets are countercyclical; that money hurts as much as money taken out of the income tax. And while it is politically easier to step up enforcement of rules no one has ever heard of than to have an onerous debate about the budget, it is corrosive to civil society to make people feel that they can be randomly and unfairly hit up for extra cash. The nature of modern American law is such that no one can know, or obey it, fully; things like stop sign waits and emergency vehicle passage vary from state to state, and time to time, so that no one ever knows exactly what the law is. Cracking down on trivial violations invades the safety zone that allows us to rub along without too much disrespect for the law.