I don't know about California, but in Texas, if you do not respond to the ticket you will be convicted of the offense and an extra fine will be added for 'failure to appear'. A warrant will then be issued for your arrest. Although the cops won't come looking for you, the next time you are stopped by a cop, you will probably be arrested and taken down to jail until someone either comes down to pay your outstanding fines or you serve out the value of your fines at something like $50-100 per day depending on the municipality. Usually after a couple days you may get to see a judge who *might* reduce you fines and set up a payment schedule and release you.

It's a lot easier to just pay the fine or take defensive driving and be done with it. Of course, you always have the option of contesting the ticket.