(I don't think I've even see a roll cage in a stock road vehicle!)
Wow, the crap drivers who think they're hot are out in force today!
For those less crappy drivers, I'll explain myself a little better (I hope).
The first thing you must rely on to protect yourself from a crash is YOURSELF. Be alert. See it coming. Take action. Stop to help any other victims (if any). Drive on in the knowledge that you and your passengers are safe and well, and your vehicle also came out unscathed because you didn't drive in a straight line hoping your protection in your car would save you from the truck coming towards you in the wrong lane, you took action and the truck never hit you.
I've been close to head-on crashes. Close. Through training and maintaining those skills I avoided being in a head-on. Through training I could already see a number of probably escape routes before I saw the truck suddenly veer, through training I knew exactly how my bike would perform under braking (I do the same for cars - PRACTICE often so I know it's actions and it never surprises me), through training I knew how hard I could swerve, and through observance I knew the truck driver was never getting back into the right lane for him and that lane was clear so I could use it.
Train so you're prepared, practice so you know how to handle your vehicle, plan for your escape routes, and watch the road so you can see what's coming. And always remember your most valuable resource in your car. It's not brakes. It's not skills. It's not an alert driver (though that should always be). It's not air bags. Nor is it roll cages.
It's TIME. If you have time to react and time to avoid the crash, you'll be fine. Coming up on a line of traffic that might be stopped? Start slowing. Coming up to a tight corner where the oncoming traffic might not be in their lane? Slow down. Passing a line of stopped traffic where some idiot might pull out into your lane without warning or looking? Slow down. Give yourself TIME to react.
Then you won't ever need air bags, or roll-cages, or even seatbelts.
Also think of what could happen around you and take steps to minimise any risk. Mostly that means keeping a sane following distance and allowing more if the person behind you is one of those stupid idiots who tailgate.
(And yes, I do wear mine in the car, same as I always wear my helmet - I've avoided a couple of major events, dozens of other incidents where I could've been killed and would've been hurt, and quite literally thousands of minor incidents where at most I would've had bent metal and bruised body to deal with - but it is possible that something would escape my attention or there would be something I could not avoid)