(MONEY Magazine) – More than any CD changer, more than any heated or ventilated seat, the most sought-after option in a car these days is a full complement of safety features.

Automakers know this. Which is why they've made safety the core of their latest R&D efforts and advertising campaigns. Remember when Volvo's commitment to safety was a defining characteristic for the brand? The Swedish automaker no longer stands apart in that regard, since car makers from Bentley to Hyundai are now falling over themselves to let you know how well protected you'll be when driving their cars.

While it's hard to argue with anything that makes cars safer, the trend is not without pitfalls. Automakers seem to introduce new safety features almost hourly, and many of them are pricey options. The history of safety features includes more than a few technologies that didn't really make anyone safer, not to mention gadgets that actually made things more perilous (such as motorized shoulder belts, which gave people a false sense of security that they were belted in when, in fact, the lap belt remained dangerously unfastened).

Over the past year, I've checked out a number of safety add-ons. Some are all but indispensable, some show promise, and some still feel like needless gimmickry. Let's sort through those you're most likely to encounter.

The Must-Haves Whether they're standard features or on the options list or not, don't buy a new car unless it has all of these features

Side and Side-Curtain Air Bags

Modern cars do a great job of protecting you in frontal collisions, but that's relatively easy--there's a whole engine compartment between you and the other car. Side impacts are the tricky ones. With only a thin door between you and an oncoming vehicle, things can be much worse.

That's why side air bags (which protect the upper body) and side-curtain air bags (which cover the window area and shield your head) can make a critical difference in a collision. The good news is that side and curtain bags are becoming standard fare on even some budget wheels, such as the $11,000-to-$14,000 Kia Rio. The bad news is that they remain optional in far too many models. And a distressingly low percentage of consumers choose to pay extra for additional air bags, though they typically add a meager $250 or $300 to the cost of a car--not much more than a set of floor mats.

As with electronic stability control (below), here's where to put your money where your mouth is on safety: Check off the option box for air bags, every time.

Electronic Stability Control (ESC)

ESC technology uses existing anti-lock brakes plus motion sensors and software to recognize a skid before it even begins. Then it applies individual brakes and adjusts the throttle to restore control. Make no mistake about it: ESC works. Countless times, I've hit a slick patch or taken a racetrack turn a bit too fast and felt ESC instantly pop the car back on course, usually before I even knew something was amiss.

If you're shopping for an SUV, an ESC system is even more beneficial, since those vehicles have trickier emergency-handling characteristics and can have more of a tendency to roll over. Comparing models before and after ESC was installed, the National Highway Traffic Safety Administration found that the technology reduced crashes by 35% in cars and by a stunning 67% in SUVs. As ESC has become widely adopted, prices have fallen. Where they're still an option, most ESC systems now cost you $500 or less. It's the best money you'll ever spend on your car.

Avoiding Skid Row Electronic stability control uses a car's brakes to keep it in line. Here's how it works:

1. Onboard sensors are constantly monitoring both steering-wheel input and the actual direction the car is traveling.

2. If the sensors detect that the car is about to lose control (because of ice or speed), ESC reduces engine power and/or applies the brakes to whichever wheels are necessary in order to maintain the desired direction.

3. The driver feels a loss of power and some sharp braking, sees ESC flash on the dashboard and (one hopes) continues driving as if nothing's happened.

Need to Trickle Down Intriguing, useful technologies, but so far available only on higher-priced cars

Rear-View Monitor

My initial reaction to a monitor in the dashboard that could show a view of the space behind your car was that it was technology for technology's sake. On subsequent tests, however, I began thinking about families with children--and pets, and Big Wheels and anything you might overlook while backing up, possibly with tragic results. After several auditions, count me a believer: Rear-view monitors go a long way to boost safety and confidence, especially for drivers who feel anxious backing up a jumbo minivan or SUV (most of us).

The best system, from Nissan and Infiniti, superimposes a green-yellow-red grid over the screen view (red being the zone closest to your rear bumper), making it easy to judge distance and snuggle to within inches of the car or wall behind you. Not only does it keep you aware of what's behind you, but it'll keep you from ever scratching another bumper during a parking maneuver.

The caveat is cost: Every monitor currently available is bundled into an option package that forces you to buy a pricey navigation system and usually some other gizmos, at a cost of several thousand dollars. So if you really want this safety feature, you're forced to pay for other things you may not want.

20/20 Hindsight

Cars with rear-view monitors use a camera mounted in the back bumper to show objects that might not appear in your mirrors. The image is displayed on a dash-mounted monitor when you engage reverse.

Pre-Collision Systems

Even before that first crunch of a fender-bender, the latest luxury models are trying to protect the people inside. Mercedes, which pioneered pre-collision technology in 2003, has enhanced it on the new 2007 S-Class. If radar sensors detect an impending collision, the S-Class will, in less than a second, shut the windows to support the curtain air bags (unless the window is obstructed), reposition seats for optimal crash performance, and retract seat belts to cinch occupants into place. Right now pre-collision systems are limited to luxury models, but as the technology becomes more common, expect mid-price models to begin offering similar systems before the end of the decade.

Defensive Driving Seconds before a collision (during a skid or a panic stop), pre-collision systems such as Mercedes-Benz's PRE-SAFE take the following measures to make sure occupants are in the best possible position at time of impact:

• Sunroof closes for better protection in case of a rollover. • Seats and headrests move to positions that provide the best protection. • Windows are shut to better support the side-curtain air bags. • Seat belts tighten. • Air cushions on the sides of the seats inflate to help keep people in their seats.

Active Braking

Taking pre-collision systems a step further, the 2007 Mercedes S-Class and the 2006 Acura RL use the car's radar "vision" to brake the car when it senses that you're not doing the job. That may sound scary, but the systems actually work safely and unobtrusively. The technology is not perfect yet (it often doesn't recognize a completely stationary car ahead), but my testing convinced me that it could make the difference between a minor fender bender and an injury-producing accident.

As a $3,800 option on the '06 Acura RL, the car's Collision Mitigation Braking System first warned me that I was heading too quickly toward upcoming cars by flashing a huge BRAKE warning on the driver's display and sounding a chime. If I still didn't stop, the Acura tugged back on my seat belt, then braked forcefully but smoothly, still giving me ample time for a safe stop on my own. (The system won't brake the car to a full halt and can be switched off from the dash.)

The new Mercedes S-Class has a similar system, but it springs into action only after you've initiated braking. The car then uses its onboard radar sensors to calculate the optimal brake force required to avoid a collision, and applies that amount if you're rapidly closing in on another object.

The same system also works in conjunction with the car's radar-based cruise control. In that case, the car will maintain a set distance from the car in front of it, adjusting its speed from 0 to 125 mph. When I used it, the Benz accelerated and braked for hours without requiring me to touch either pedal, even in heavy stop-and-go traffic. Neat stuff, but these systems will become truly effective only when they're cheap enough that most cars have them. Right now you may be able to avoid a rear-ender, but it doesn't do you much good if no one else can.

Stopping Power A car with active braking uses radar to sense when it's rapidly closing in on another object. The car will first alert the driver to brake; if he doesn't do so within a second or two, the system will automatically apply the brakes.

Not Ready for Prime Time Neat gizmos? Sure--just don't confuse them with necessary safety features.

Adaptive Headlamps

Several luxury car makers are proud to tout headlamps that pivot in the direction you turn the wheel. On paper it sounds great--who doesn't want lights that "peer around curves"? Yet in my driving, the benefit has been less than illuminating. The adaptive lights do little or nothing to improve the nighttime view, even on dark and winding country roads where you'd most expect to see some difference. For now, available headlight systems are either standard on models like the Lexus LS 430 or part of pricey option packages. As an $800 stand-alone option on the "entry level" Audi A3, it's not worth it, and its lack of tangible driver benefit makes me skeptical that it will ever trickle down to less expensive cars.

Lane-Departure Warning Systems

Infiniti was first to offer a system that reads lane markings and alerts a driver who's straying off the road. Initially, I found it an intriguing idea that merited development, but ultimately it doesn't seem worth the price or the annoyance.

The system worked well enough, audibly chiming and flashing a small light when I'd wander from my lane without first using the turn signal. But the camera can't pick up road markings that are indistinct or obscured by dirt or snow. And it often nagged me when I wasn't asking for its help. Yes, the system can be shut off with a button, but if you're always turning the thing off because of false alarms, what's the point?

Night Vision

Some automakers are touting night-vision systems that, using an infrared camera and a dash-mounted screen, can display people or animals that are beyond the range of headlamps. I've tried several systems, and the safety benefit seems dubious. On a dark two-laner, should I really be looking at a dashboard screen? Am I not better off just watching the road? Fact is, modern, high-intensity (sometimes called "bi-xenon") headlights already do an amazing job of lighting up the night. For now, consider night vision another gee-whiz gizmo to give automakers techno bragging rights--and help them pocket a grand or three from gullible buyers.

Do It Now

Winter Tires

The results are clear. Winter tires are the best way to enhance driving safety on snow and ice. Period. Recently, online tire retailer the Tire Rack ran its own test with two Volkswagen Touaregs. One was shod with its factory all-season tires, one with Bridgestone winter tires. When both came to a panic stop on ice from just 35 mph, the VW with all-season tires took 70 feet longer to come to a halt--about the length of a school bus.

Don't be misled by all-season tires. "All-seasons are designed to be good at everything, but they aren't great at anything, especially winter driving," says Matt Edmonds, v.p. of the Tire Rack. Tires that meet the winter standard carry a symbol of a mountain peak with a snowflake and cost around $100 a tire.

To keep things simple, you'll want to buy an inexpensive set of wheels (figure $60 to $100 per wheel) to mount the winter tires instead of switching tires back and forth on your standard wheels. And always use winter tires in sets of four--using a single pair on the drive wheels can create unbalanced handling.