For many car buyers, especially those with families, safety is a major concern when shopping for a vehicle. The main two organizations that perform automotive safety testing are the Insurance Institute for Highway Safety and the National Highway Traffic Safety Administration. Both have easy-to-use Web sites – www.IIHS.org and www.NHTSA.gov – that provide vehicle ratings.

We’ve already examined how IIHS testing works, so let’s take a look at NHTSA’s testing procedures.

NHTSA is part of the U.S. Department of Transportation and it conducts automotive safety testing in its New Car Assessment Program. The NCAP began performing crash tests in 1979. Manufacturers were required to place NCAP star ratings on new vehicle window stickers in 2007.

NHTSA contracts with five labs around the country to conduct its front and side-crash testing, and it uses two additional labs for rollover testing.

TESTING PROCEDURES

With 250 or so new vehicles on the market every year, the agency can’t test them all. So, three types of vehicles are chosen: those that are predicted to sell well, those that have been redesigned and given structural changes, and those with new safety equipment. Ratings for vehicles that haven’t significantly changed are carried over year to year. All told, the agency tests 80 to 100 new vehicles and aims to have ratings for 85 percent of the new vehicles sold each year.

The NCAP includes three types of tests and assigns ratings on a one-to-five-star scale based on the percentage chance of serious injury. Those tests are described below.

Frontal-impact test: The frontal test is a 35 mph crash with dummies representing average-sized male adults buckled into the front seats. The vehicle is run into a stationary rigid barrier head on. NHTSA says this test is the equivalent of a head-on crash between two identical vehicles. The results are comparable with vehicles that are about the same height and are within 250 pounds of the vehicle tested.

Forces to the dummies’ head, neck, chest, pelvis, legs and feet are measured for impact. These ratings indicate the percentage chance that the occupants will suffer a serious injury, which is defined as one requiring immediate hospitalization or one that may be life threatening. A five-star rating in the frontal impact test indicates a 10 percent or less chance of serious injury. Four stars mean an 11 to 20 percent chance. Three stars is a 21 to 35 percent chance. Two stars is a 36 to 45 percent chance. And one star represents a 46 percent or greater chance of serious injury.

When customers look up NCAP frontal-crash test ratings on NHTSA’s Web site (www.safercar.gov), information beyond the star ratings is available. The site also lists findings for head injury criterion, chest deceleration g forces, and left and right femur load in pounds.

Side-impact test: The NCAP side-impact crash test involves a vehicle with dummies buckled into the driver’s seat and driver’s side second-row passenger seat. Again, the dummies represent average-sized male adults.

The stationary vehicle is struck in the side by a 3,015-pound barrier moving at 38.5 mph. The front of the barrier is at car height and has “give” to replicate a vehicle’s front crush zone. Since all vehicles tested are impacted by the same size barrier, side-crash test results can be compared across categories.

Forces to each dummy’s head, neck, chest and pelvis are measured. Head injury risk is not included in the current NCAP side-impact test ratings. Instead, excessive head injury is reported as a safety concern.

The side-impact crash ratings denote the chance of a life-threatening chest injury for the driver, front passenger and outboard rear passengers. A five-star rating in the side-impact test indicates a 5 percent or less chance of serious injury. Four stars means a 6 to 10 percent chance, three stars denote an 11 to 20 percent chance, two stars represent a 21 to 25 percent chance and one star indicates a 26 percent or greater chance of serious injury.

Safercar.gov also lists findings for head injury criterion2, thoracic trauma index and pelvis deceleration g forces.

Rollover test: NCAP’s rollover resistance rating measures the chance a vehicle will roll over if it is involved in a single-vehicle crash. The rating is based on two factors: an at-rest measurement known as the static stability factor and a dynamic test. The two are combined into an overall rating.

The stability factor determines how top heavy a vehicle is by comparing its track width to its center of gravity. A high SSF score means a vehicle is less likely to roll over. Cars tend to land between 1.3 and 1.5, while high-riding SUVs, pickups and minivans fall in the 1.0 to 1.3 range.

NHTSA says that about 95 percent of rollovers are “tripped” by the vehicle striking something low, such as a curb or ditch, or by going off the road and digging into soft soil. Top-heavy vehicles are more susceptible to rolling in this manner. The SSF rating reflects this type of rollover and is weighted more heavily in the overall rollover resistance rating.

The dynamic rating measures untripped rollovers and counts less in the overall rating. For this test, the vehicle is given a full tank of gas and loaded with weight to approximate five occupants. The added weight effectively raises the vehicle’s center of gravity, making it more prone to a rollover.

The dynamic test involves a fishhook maneuver to represent an obstacle avoidance situation. With the vehicle traveling at 35 to 50 mph, the steering wheel is turned 270 degrees in one direction, then 540 degrees in the other direction, both within about one second. Instruments on the inside wheels measure if the wheels lift off the pavement during the maneuver. If both inside tires lift at least two inches, the vehicle is considered to have tipped up. If a vehicle doesn’t tip at 35 mph, it is run again at 40, then 45 and finally 50 mph.

On its Web site, NHTSA lists the result of the dynamic test as “tip,” “no-tip” or “no tip*.” The asterisk means the vehicle wasn’t tested but the results were ascribed based on results of vehicles with lower SSF scores that didn’t tip up during the dynamic test.

A diamond and bar graphic is also displayed on the Web site. It shows the final star rating, the actual percentage chance of a rollover if involved in a single-vehicle crash and the range of percentages of all vehicles in that class for the last three model years.

Like the side-impact crash test, rollover resistance ratings can be compared among different types of vehicles because they are all subjected to the same tests.

NHTSA says a vehicle that gets a one-star rollover resistance rating is four times more likely to roll over in a single-vehicle accident. Even five-star vehicles have up to a 10 percent chance of rolling over in a single-vehicle crash. NHTSA notes that some five-star vehicles, such as sports cars, may have a higher number of rollovers per 100 registered vehicles than three-star vehicles, such as minivans. Driver behavior plays a part here as sports car drivers tend to be more aggressive than minivan drivers.

As you can see, using NHTSA crash test results can help you buy a safer vehicle. However, the agency isn’t completely happy with its testing. In two weeks, we’ll look at how NHTSA thinks it can improve NCAP testing.