A past idea of mine is: everyone. Every vehicle registration accumulates points at the rate of one per week. Fingering another vehicle deducts 5 points from their total and (to deter false reports) one from yours. If your total is negative, you can't drive/ride.
This would de-incentivize the reporting of infractions, which is the opposite of what you want.
You want to incentivize the reporting of verified infractions, while de-incentivize false reports. A consensus system could be used to "verify" a report.
In this kind of system, I think the negative points are to remove the burden of confirming infractions. It's in an individual's best interest to both keep driving AND to remove poor drivers. Hitting the correct ratio is the key.
We could also do network analysis on the graph of people reporting other people. If there are any strange, unnatural patterns, they could be ignored. But if Jonny has been reported by 10 people in the last month, and those 10 are unrelated to each other, that would be compelling evidence Jonny is not behaving well. The system can still be gamed if a group of people are organized online to report targets (like 4chan does DDOS attacks). Fortunately, in order to report someone one would need to disclose his identity. On 4chan there is radical anonymity, by contrast.
There's probably a great deal wrong with this.