Except that you're not repaying the cost to society. At the very least, you owe for the time the police had to spend to catch you. At most, for the endangerment of those around you. A "time out" is just a waste.
I doubt that small speeding fines do more than pay for themselves. You need a bureaucracy in place for fine payment, fines can end up getting contested in court at a huge cost, etc.
Just make fines progressive.