
A traffic citation is an accusation of having committed a crime.
That's why they have to go through the motions of having a trial if you
want one, you are still innocent until proven guilty.  Cops are not the
only ones who can accuse people of committing crimes, anyone who
witnesses a crime can do so.

Go to the Highway Patrol and explain the situation, give them a
description of the car and the license number, and tell them the
specific violation of the law which you witnessed and wish to prosecute
(ie, search the Vehicle Code and have the section number handy).  Fill
out the ticket and sign it.  It will go through the same system any
ticket a cop writes goes through.  If contested, you will have to
appear in court to prosecute.  Your word will not carry the same weight
as a cop's.
