Not that there’s anything wrong with getting bullet-resistant glass, it will better prepare you for the coming zombie apocalypse – but the chances of being the victim of a drive-by shooting on the road are vanishingly small. I’m also guessing that a good percentage of the ones that do occur are not one-sided – that is, the victim provoked the nutjob via a middle finger or something, even if they never admit to it. If you resist the urge to flip off the guy who almost rammed you, and you don’t drive directly through the worst areas, you’ll likely never have a problem. I’ve actually been shot at when driving through San Marcos, on Mission Avenue a mile or so east of Palomar (a relatively poor area), but I knew it was a fluke and haven’t had a fear of driving in the same place since.
Your money is probably better spent on an advanced driving course and/or a safer car (first protect yourself against the stuff that is FAR more likely to kill you).