We're talking traffic citations here, monetary penalties, not jail time. That being the case, a flat fine combines injustice with disutility as it cannot deter equally.

If the intent is to deter behavior you need a penalty commensurate to the crime (over-penalizing is actually counter-productive as it does damage to respect for the law) which deters equally.

A flat amount high enough to deter the wealthy will be crippling to the poor, who in the end are just breaking a traffic law. An amount just to the poor will be laughed at by the wealthy. Basing the penalty on income gives needed proportionality to the deterrent effect. As an example (avoiding percentages, which is how it is done in reality), if the penalty is "a week's pay" rather than "X dollars" -all- potential red light runners will have to ask the same exact question, regardless of who they are or what they earn: "Is a couple seconds drive time saved worth 40 -hours- of my income lost?"