Small cars can deliver high repair bills because "bumpers on many cars aren't designed to handle what should be a no-damage event" in parking lots and commuter traffic, according to the Insurance Institute for Highway Safety (IIHS).

The Hyundai Elantra, Toyota Prius and Volkswagen Rabbit each sustained $4,000 of damage or more in low speed "fender-bender" type crash tests conducted by IIHS.

In a series of low-speed tests, IIHS assessed how well the bumpers of 20 small car models would protect the vehicles from damage in parking lot type collisions. The Elantra, Prius and Rabbit cost the most to repair, averaging $4,000 or more. The Ford Focus performed the best, with about one-third that amount of damage in its worst test.

"Small cars are supposed to be economical but there's nothing economical about 3 or 4 thousand dollars in repairs after a low-speed collision," said institute senior vice president Joe Nolan. "Ford did the best job of putting bumpers on a small car that largely do what they're supposed to do. In 3 of the 4 tests, the bumpers on the Focus protected sheet metal and most other expensive parts from damage," he said.

The Institute conducted a series of 4 crash tests in the comparison of bumper performance in low-speed impacts. There was a full front and rear crash test with a barrier plus front and rear corner impacts. The full-width impacts were conducted at 6 mph while the corner impacts were run at 3 mph.

Modern front-end styling results in bumper designs that can either slide under the bumpers of vehicles they strike or that simply don't have enough room to absorb the energy of a low-speed crash, the institute reported.

"Even if they do engage the bumper of the vehicle they crash into, the bars underneath bumper covers often aren't up to absorbing the energy. They may not be big enough to provide much protection from damage, especially if they don't extend to vehicle corners, or they may be too flimsy to absorb much energy," according to the report.

About the Ford Focus, IIHS said the front bumper is tall enough to do a reasonably good job in the full-width test. Repairs cost less than $600, according to IIHS tests.

"In contrast, damage to the Elantra of nearly $5,000 in the same test is equal to almost one-third of the car's sales price," IIHS said. "The Focus shows that decent bumpers don't have to be heavy or costly," Nolan said.

"Many consumers are turning to small cars for better fuel economy but damage in our tests approached luxury car territory. Savings at the pump could be more than offset by a single low-speed collision in one of these so-called economy cars."

The front bumpers of the Rabbit and Honda Civic slipped under the barrier in the front full-width test resulting in damage to their grilles, hoods, fenders and air-conditioning condensers, according to IIHS. Similarly, the Prius sustained nearly $4,000 damage in the rear full-width test because its bumper is mounted too low to be in position to protect the vehicle's tailgate, rear body panel and taillights, according to the report.

In the front corner test of the right side of the Prius, damage was much less, about $1,200, involving the fender and headlight. Had the test been conducted on the left side, the barrier would have crushed a coolant tank which costs more than $1,000 to replace, not including labor.

Parts for low speed repairs are expensive. The plastic bumper covers that fit over the bumper bars of modern cars are one example. Among the small cars tested, the front covers on the Scions cost less than $500 to replace and refinish, but the Mitsubishi Lancer's front cover is twice as expensive.

Headlights and taillights vary widely in cost as well. A taillight costs $205 to replace on the Prius compared with just $65 on the Focus.

IIHS reported 11 of the cars in the full-front test and 18 in the front corner test required headlight replacement or repair.

As an example of how to design a more protective front bumper, IIHS said that if the reinforcement bar and foam absorber in a Prius were extended another 10 inches on the passenger side under the headlight, the headlight and fender would be undamaged in a "fender-bender" and the repair cost would drop from $1,200 to $254.

"There's plenty of room under the bumper covers of most cars to make this simple change," Nolan said.

"It wouldn't take much for automakers to reduce the cost of repairing the damage that occurs in low-speed collisions," according to IIHS pointing to three steps automakers could take to reduce repair costs.

• Make the bumper bars longer so they protect headlights and other critical and costly equipment at the corners of vehicles.

• Make bumpers taller so they engage the bumpers on higher riding SUVs and pickup trucks instead of under-riding them, even during emergency braking.

• Don't sacrifice function for style by mounting bumpers too close to the car body. This makes for a sleek look, but it doesn't leave much room for absorbing crash energy. Mount bumper bars farther out and use the available space under a bumper cover for energy absorption.

"Bumpers on most cars aren't worthy of the term," Nolan said. "Even the best bumper in this group still allowed more than a thousand dollars damage in one 3 mph crash test. Some simple changes could prevent a lot of damage to cars, and expense and headaches for consumers."