The car you want to buy got four stars in government crash ratings. It sure sounds good, but is it really, since other cars earned five? Who awards those stars, and what option boxes should I check so I’ll have the safest possible car in the driveway?

Let’s begin with a short primer about auto safety. Before we go there, you may want to pause and watch “Signal 30,” the lurid 1950s driver’s education film about car accidents that scared the bejesus out of me way back when. I definitely became a safer, more cautious driver because of it. Viewings used to be mandatory. Warning: It's strong stuff.

Auto safety ratings are awarded by both the federal Department of Transportation’s National Highway Traffic Safety Administration (NHTSA) and the nonprofit Insurance Institute for Highway Safety (IIHS). Both crash cars, but IIHS gives “good” to “poor” ratings rather than stars.

The NHTSA test results are the ones on the window sticker, and cars are rated for frontal crashes (driver and passenger), side crashes (front and rear seats) and rollover. The best score is five stars, but it’s a bit confusing since most cars get either four or five stars. Obviously, four is better than five, but you should feel comfortable with scores in that range.

If you think that all those stars mean that most cars are at least fairly safe in impact survivability, you’d be right. All cars and trucks sold on the American market have to meet stringent standards or they’d never make it to dealer lots; the regs in China and much of the Third World are much lower. Here’s a horrendous Chinese truck crash test:

The ratings are pretty tough, and they got tougher with 2011 models. If you notice a car you like dropped lower from 2010 to 2011, that’s why — it doesn’t mean the car got worse. “Overall vehicle score and frontal crash ratings should ONLY be compared to other vehicles of similar size and weight,” says NHTSA. If a car you’re looking at has no window sticker star rating, that’s because NHTSA hasn’t gotten around to testing it yet. Results for 2011-and-later NHTSA crash test results are online, as are those from IIHS.

I notice that one of the few places you’ll see three-star ratings is in SUV rollover. Some vehicles that got three-star rollover ratings: Cadillac Escalade, Chevrolet Avalanche, Dodge Durango, Ford Expedition (not all of them), Jeep Grand Cherokee (two-wheel drive). Many SUVs haven’t been rated yet under the new system. I’ve been warning about SUV rollover risk for years. The basic design is top-heavy, which leaves them prone to flip over barriers or otherwise roll.

What can you do to ensure a safer car? Plenty. Check to see if your car comes with critical safety systems as standard, or they need to be ticked off on the options list.

The big winner is electronic stability control (ESC), which will take control of the car (applying the brakes to one or more wheels) to keep you safe on slippery roads. A federal study shows that ESC will reduce passenger car single-vehicle crashes by 26 percent, and such SUV crashes by 48 percent. You want ESC, and fortunately the law requires all cars to have it in the 2012 model year. You’ll have to check if you’re buying an older car.

Almost always optional except on very high-end cars are such useful features as forward crash warning (FCW) and lane departure warning. Both warn you with audible and sometimes physical (shaking the steering wheel) alerts.

A whole other blog post could be written about airbags, but generally the more the better — you want side impact, and the advanced frontal bags that were standard on cars after Sept. 1, 2006. The latter automatically detect the severity of the crash and apply power appropriately to the bags.

Use your seat belts. The exact number fluctuates, but something like 35,000 people die in crashes annually, and about half of them could have been saved if they’d been wearing their belts. That’s 17,000 people who’d be walking around today.

Chances are your car or truck has a seat belt on-off switch, and you want to cut power only if you have an infant in a rear-facing child’s seat in the front passenger seat (or are transporting a child under 13 there); you have a special medical condition that means the risk of the bag hitting your head outweighs the safety factor; or you can’t change your driving position and keep 10 inches between the center of the bag cover and your breastbone. I know it’s frustrating when you put a package on the seat and it trips the seat belt alarm — that’s another case when you may want to use the switch, but remember to turn it back on.

Another big safety thing is driving scared. I always do, because I have no idea what those folks in front and behind me are going to do. That’s yet another reason to watch “Signal 30.”

Related car safety stories:

Jim Motavalli ( @jmotavalli ) writes about cars, technology and the environmental world to anyone curious enough to ask.

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