No one buys a new car with the intention of wrecking it.  But of course, accidents happen, which is why most car shoppers pay a lot of attention to crash test ratings.  After all, a car that earns top ratings in crash tests will keep your family safe not matter what, right?

Not so fast. Even in a car with top scores, "you're not invincible," says Joe Nolan, senior vice president of the Insurance Industry for Highway Safety's Vehicle Research Center. While Nolan says that "there's no reason to choose a vehicle that doesn't get the highest [crash test] ratings," it's important to understand how crash tests and ratings work -- including what their limits are -- before you get behind the wheel of a new car.

What Crash Tests Measure

If you ask most people what a crash test measures, they'll likely say it measures how safe a car is.  But that's not entirely accurate.

"The way the car protects you is what we evaluate in the crash test," says Nolan. However, you shouldn't assume that's a full measure of how safe a car is. While a car, truck or SUV needs to have Electronic Stability Control in order to be an IIHS "Top Safety Pick," the institute doesn't evaluate a car's ability to avoid a crash as part of their testing.  A car with top crash test ratings isn't a car that's guaranteed to be crash-free.

The federal government's National Highway Traffic Safety Administration, on the other hand, does one test that actually rates a vehicle's ability to avoid a crash: They provide rollover ratings.  According to NHTSA, rollovers account for 33 percent of all crashes.  The number of rollovers has grown in recent years with the popularity of SUVs, which are more likely to roll over because they are taller than cars and have a higher center of gravity.  NHTSA estimates cars that earn higher rollover ratings are less likely to roll over. However, drivers need to understand how their tests, and other crash tests, are conducted to fully understand vehicle ratings.

How Crash Tests Work

To determine a rollover rating, NHTSA uses two factors. One is a Static Stability Factor (SSF), which combines the car's track width with its center of gravity.  That measurement is combined with the results of a dynamic rollover test to calculate the rollover rating. In the dynamic test, a fully-loaded vehicle drives on a test track at 35 to 50 miles per hour and performs an avoidance maneuver similar to what you might do if you were swerving to avoid an object in the road. Instruments on the car measure the car's reaction, including if the inside tires lift off the pavement -- something that usually precipitates a rollover.

While this test is reliable, you can't assume that a car with a four-star rollover rating won’t flip. Take a look at the test speed -- it's between 35 and 50 miles per hour.  At faster speeds, a violently maneuvering car is more likely to roll over.  Plus, the tests take place on a closed course with trained drivers.  The road condition and reactions of individual drivers could increase a car's chance of rolling over.

The same is true for other crash tests.  In front-impact crash tests performed by IIHS, a car is crashed into a barrier in a way that simulates the energy produced if the car were to crash into a similarly-sized vehicle at about 40 miles per hour.  In their side-impact crash tests, a barrier that simulates a light truck or SUV is crashed into the side of a car at 31 miles per hour.

See the limits? In the real world, cars don't always hit other vehicles of the same size like they do in crash tests. Minis go up against Hummers, and in those situations, the crash tests results don't apply, because they only measure one situation.  Similarly, side impacts sometimes involve cars that are going a lot faster than 31 miles per hour.

So, why are crash tests done this way? "We evaluate cars at speeds that differentiate between vehicle performance," says Nolan. "If we ran all tests at high speeds, all cars would do poorly." 

How to Use Safety Ratings

Given the limits of ratings that come from a controlled crash test, what can you take away from them? 

First, Nolan says, crash tests are the best tool we have to fairly and comprehensively evaluate how good cars are at protecting us. "I am 100 percent confident that there are many people who have purchased an IIHS highly-rated vehicle who have family members who came home alive because they made that purchase decision," says Nolan.  Also, though Nolan says that you're not invincible in a top-rated car, "you are hedging your bets."

What's key is for drivers to understand their own role in vehicle safety. "Just because a vehicle is rated "Good" [IIHS's highest score] doesn't mean you can't be injured in it if you drive in such a way that exceeds the car's ability to protect you," Nolan says. That means driving within the speed limit, staying alert, and knowing your -- and your car's -- limits. IIHS and NHTSA tests have "no input on how a car is likely to be driven. We test at low speeds, not fast speeds. At some point consumers have to use common sense," Nolan adds.

Staying safe on the road is a matter of taking responsibility and driving carefully, while also using crash test ratings to find a car that can keep you safe if the unthinkable happens. The good news is, cars as a whole are all performing better in crash tests, and are better than ever at protecting people.

Nolan points out that IIHS's top rated cars are "not an elitist group where you have to spend top dollar for good ratings." When it comes to crash test ratings, as long as you understand how they work, there's really no reason to settle for a car with less-than-stellar performance.