> Why not set the fine to a million dollars for everyone
Because that is a disproportionate punishment for speeding for most people. We don't do disproportionate punishment.
It's also why we don't hang people for stealing. Any more.
> [it's not a tax] Why?
Because allowing speeding and taxing it is not the goal. We don't want people to speed. Even rich people.
> same amount of deterrence for everyone
Exactly.
> which requires a constant fine.
Er, no. Exactly the other way around. To make the deterrence effect the same for everyone absolutely requires the monetary amount to be different. The deterrence effect of an economic fine is hugely dependent on how much money you have/make.
If I make a €10000 an hour, a €1000 fine is nothing, it takes me 6 minutes to work that off, so who cares? If I make €1000 a month, that's an entire month's worth of work, and so the fine will have a huge impact on me.
In Germany, this is encoded by the concept of a "Tagessatz", which is essentially how much you make per day (minus what you absolutely need to live):
https://de.wikipedia.org/wiki/Tagessatz
They don't generally do this for traffic violations or other misdemeanours, but for felonies for which the sentence includes a monetary punishment. So you are convicted to pay, let's say 45 "Tagessätze". How much that is depends on how much you make. If you don't pay, you go to jail for that amount of time.
Time is the equalizer, being roughly the same for everyone, unlike money.
(Yes there are differences in how long people live, or have left to live, but it's not clear whether those should be accounted for and they are nothing comparable to the differences in income/wealth. See Taleb on "Extremistan" https://wou.edu/~shawd/mediocristan--extremistan.html )