Three years after the law comes into effect, eligible crimes have fallen roughly 20-40 percent compared to non-eligible crimes.

Doing a lot of research at the moment on homelessness/urban campers - what works and what doesn't work in terms of policing, social services, and public policy.  Interesting how much time, money and energy we have spent on this issue over forty years and with so little to show for it.  And it is the social services leg of the stool which is weak - very little empirical evidence of broad effectiveness (i.e. capacity to reduce urban camping populations.

On the policing side, there are the usual issues of poor goal definition, definitions, longitudinal studies etc. but the empirical record is far more robust than on social services.

One of the confounding factors is the secular trend in crime.  Virtually regardless of jurisdiction, there was sharply rising crime from the early sixties until it peaked in the early nineties.  Since the early nineties, virtually every jurisdiction has experienced declining crime rates, some fast and large and some slower and smaller, but all declining.

So, we just ride out the secular cycle of crime, making do as best can?  Not in our nature.

As crime rose from the early sixties there were innumerable law and order campaigns and new strategies.  The principle ones being deployed in the early nineties were variants of broken windows policing, proactive policing, and mandatory sentencing.  

The valid question is the degree to which they were effective.  Were the improvements claimed simply a matter of claiming the benefits already occurring because of the secular decline?

It remains hotly debated as a political issue (and public policy) but it seems pretty clear that those jurisdictions with the most enthusiastic implementation of variants of broken windows policing, proactive policing, and mandatory sentencing are the ones who have seen the most dramatic falls.  They are effective but their effectiveness has to be netted out from the underlying secular decline.

From Using Sentence Enhancements to Distinguish between Deterrence and Incapacitation by Daniel Kessler and Steven D. Levitt.  

The question they are addressing goes to the theoretical underpinnings of mandatory policing.  Is the supposed effect (decline in crime) due to deterrence or to incarceration?

Crime tends to have a Pareto distribution with most crime committed by a small, small number of individual criminals.  If you commit the most prolific and keep them of the streets for long periods of time,  crime will fall because those most to committing crime are unable to do so.  Incarceration keeps the most dangerous criminals from committing their normal volume of crime.

However, there is another effect, a disincentive effect.  If the probability of arrest and conviction remains the same and the return of the crime committed remains the same, then prolonging the sentence changes the value equation.  Kessler and Levitt find both deterrences in effect.  

From the Abstract:

It is typically difficult to differentiate empirically between deterrence and incapacitation since both are a function of expected punishment. In this paper we demonstrate that the introduction of sentence enhancements (i.e. increased punishments that are added on to prison sentences that would have been served anyway) provides a direct means of measuring deterrence. Because the criminal would have been sentenced to prison anyway, there is no additional incapacitation effect from the sentence enhancement in the short-run. Therefore, any immediate decrease in crime must be due to deterrence. We test the model using California's Proposition 8 which imposed sentence enhancements for a selected group of crimes. In the year following its passage, crimes covered by Proposition 8 fell by more than 10 percent relative to similar crimes not affected by the law, suggesting a large deterrent effect. Three years after the law comes into effect, eligible crimes have fallen roughly 20-40 percent compared to non-eligible crimes. This large deterrent effect suggests that sentence enhancements, and may be more cost-effective than is generally thought.