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Lets focus on non-necessity acts here (e. g. traffic violations).
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Deterring people is not the only goal, it also needs to be fair/appropriate. And this is where, IMO, the income-adjusted fines fail.
Fines should be adjusted depending on the offense commited, possibly also taking into account the intentions. Personal wealth is not a factor that seems reasonable to me to take into account regarding the fairness.
Essentially, I believe that everybody should be treated equally before the law. Nobody should be treated better or worse (or have a better or worse punishment) just because of their social status. That's why I believe that fixed fines are fair and the suggested varying punishments are not. I do recognize that they may deter wealthier people less.
Well, that would just shift the problem: Now, instead of wealthy people being less deterred, it's the people with a bunch of free time that are less deterred (college kids screwing around, people with no job)...
Also, it doesnt benefit the society any more that the fine's money would (assumuning the community service would be equivalent to the current monetary value). (There are also other problems like verifying the work is actually done and also small fines, like, am I supposed to pick up trash from the sidewalk for 2 minutes for jaywalking?)