The tests were designed to mimic the impact of the most common types of low-speed crashes, from 3 to 6 mph. Many of the bumpers in the test did not effectively absorb energy in the crashes, which caused additional damage to body panels, grilles and headlights.

What's worse, the cost to repair these vehicles after low-speed crashes was high. Total repair costs ranged from more than $5,000 to nearly $14,000, with four vehicles requiring more than $10,000 in repairs across the range of tests.

Eleven luxury vehicles from model year 2007 were put through a series of four low-speed crashes. Vehicles included the Saab 9-3, Audi A4, Lincoln MKZ, BMW 3 series, Acura TSX, Volvo S60, Lexus IS, Lexus ES, Mercedes C class, Acura TL and Infiniti G35. The Saab 9-3 had the lowest total repair bill at $5,243.

"The Infiniti was a disaster, and even the Saab sustained way too much damage in these low-speed impacts," says Joe Nolan, IIHS senior vice president. "There shouldn't be much or any damage in collisions at these speeds, especially to expensive and presumably well-made cars."

To be fair, the performance of the luxury bumpers was about on par with non-luxury mid-size cars, which IIHS reported on in the spring. The mid-size cars didn't fare any better from a design standpoint, but had lower repair costs because parts are typically cheaper. In those earlier tests, repairs for mid-size cars ranged from more than $4,000 to $9,000. The Mitsubishi Galant had the best average repair rate ($4,277 across all four tests) and the Nissan Maxima had the worst ($9,051).

"Luxury cars don't perform any better than cheaper cars," Nolan says. "There's nothing luxurious about shelling out thousands of dollars to fix damage from a bump at a speed about like a brisk walk."

According to Nolan, there are design flaws inherent in new vehicles up and down the price scale that make their bumpers less effective than they could be. Many bumpers don't line-up vertically, so they don't engage during a collision. Some aerodynamically styled bumpers can slide under the bumpers of the vehicles they hit, and the bars underneath some bumper covers often aren't big enough or strong enough to provide much protection.

"There are some designs where there's a back slope on the bumper," Nolan says. "Even though the structure is in the right place, the cover creates a ramp that causes one vehicle to go under the other."

This leads to increased vehicle damage, which is exacerbated on luxury cars because of the inherent price of the repair parts.

Wide variation in results

To assess and compare bumper performance in low-speed impacts, the Institute conducted a series of four low-speed tests: full front and rear impacts into a barrier designed to mimic the front or back bumper on another vehicle, and front and rear corner impacts. The full-width impacts are conducted at 6 mph, and the corner impacts are run at 3 mph.

Repair totals were high, but these were cumulative totals based on all four tests. The individual test results told an interesting story as well.

For the front full-impact test, the Audi A4 came out on top, with only $976 in damage. Both the Mercedes C class and Infiniti G35 sustained more than $5,000 in damage, while the Volvo S60, Lexus IS, and Acura TL had more than $4,000 in damage.

The Volvo S60 sustained the least damage in the front corner impact test ($543), while the Infiniti G35 had the highest cost ($3,544). The Infiniti also sustained the most damage in the rear full impact (more than $4,000), and the Audi the least ($918).

In the rear corner test, the Audi actually sustained the most damage ($1,899), and the Lincoln MKZ sustained the least ($669).

"The Audi scored the lowest in terms of cost on the front and rear full tests," Nolan says. "While the absorbers [in the Audi bumper] fare well in full front crashes, they don't do anything if they don't extend to the full corners of the vehicle.

"None of these vehicles stood out as having good bumper performance."

The IIHS is recommending that car manufacturers make their bumpers wider to better protect the vehicle. If a bumper bar does not extend far enough, the study found, a minor corner impact could cause significant damage to lights and other safety-related components. Bumpers also have to be tall enough to engage the bumpers on other vehicles.

The front bumper on the Mercedes C class was so low, for example, that it underrode the barrier in the front full-width test, and sustained almost no damage. Instead, the car's grille, hood, radiator, headlight and air conditioning condenser absorbed the impact, which pushed the repair cost to more than $5,000 for that one collision, the highest total for any vehicle in a single test.

IIHS also warns automakers not to mount bumpers too close to the other body panels, which can effect the way the car absorbs the crash energy.

"How do you want your customers to feel after a collision?" Nolan says. "Will they be peeved that they've got $5,000 in damage from a fender bender? Plus the hassle of having to take the car to the shop for several days or a week? If you have a vehicle that can brush off these minor impacts, then I think customer satisfaction would be higher."

The IIHS also was critical of high part prices for basic components, including an $847 replacement headlight for the Infiniti G35 and the $1,046 lamp for the Lexus ES.

Bumper covers themselves have become more expensive over the past two decades as well.

"These are the sacrificial elements of the car," Nolan says. "The general strategy of making the sacrificial elements more expensive to replace is not a great strategy for keeping damage repair cost low. And if you are going to put a thousand dollar lamp on a vehicle, maybe you should extend the bumper bar a few inches to protect it."

Another problem with bumpers, according to IIHS, is that federal rules on bumper height don't apply to minivans, pickups or SUVs, which can be significantly higher than those on cars.

"This makes no sense and it won't be solved by tinkering with the bumpers on cars alone," Nolan says. "The federal rules have to be applied to minivans, pickups, and SUVs too. Only then will we start to see good bumper match-up in collisions at low speeds."

The IIHS plans to perform bumper impact tests on minivans this fall.