The easiest way that I found to do this was to join one of the popular public clubs (for example, Major Nelson's) with hundreds of people. Then find the two speed cameras right outside of the horizon festival and just drive back and forth across them with your fastest car. You may want to start a bit slower and gradually increase your speed with each pass.
Since so many people are in the club, you're bound to beat someone every time. It took me less than 10 minutes this way.
This solution may have diminishing returns as more and more people play the game and get higher speeds. Find some of the more out of the way speed traps if that happens or find smaller clubs, best their times, and move to a different club.