WASHINGTON: General Motors says that cars being recalled because of faulty ignition switches can be driven safely before repairs, based on more than 80 tests, but the automaker has not addressed a problem long known to potentially shut off the engine: a simple bump from a driver's knee.

Safety advocates and engineers say the lack of testing for this factor undermines GM's claims that the cars are safe.

As early as 2004, GM engineers complained that the ignition switch could be turned off if the key was bumped by a knee.

A Texas judge on Thursday allowed the unrepaired cars to stay on the road, over the objection of safety advocates and plaintiffs lawyers who said there is no way, short of repairs, to ensure the ignition switch would not slip out of the run position, turning off the motor and disabling power steering, power brakes and airbags.

At least 13 people have died in such incidents. GM, including CEO Mary Barra, has repeatedly said the 2.6 million cars it recalled for ignition switch problems are safe to drive, as long as they are driven with only one key on the key ring.

It is not clear if a driver's knee could still bump the ignition out of the "run" position when only a bare key is being used rather than a key ring with additional keys and possibly other things attached.

GM in court filings in the Texas case said it made more than 80 tests of driving with a bare key.

It described tests driving over a pothole four feet wide by seven feet long by five inches deep at 25 miles per hour, driving up and over a 4-inch high simulated median at an angle and locking up the brakes while coming off the median, and driving a 4-mile loop "with a series of bumps, swells, railroad crossings" and other hazards at posted speeds of 25 to 75 miles per hour, for instance.

The filing described tests in which external forces bumped the car, rather than a jolt inside.

There is no indication of a test for knee bumps.

Asked about the issue, GM told Reuters that none of the tests included a direct force on the key from inside the car, such as the driver's knee.

"Based on more than 80 individual tests, including some very severe tests like driving over a railroad crossing at high speed and driving over river rocks, potholes and cobblestones, we concluded that the recalled cars are safe to drive provided just the ignition key is used to operate the vehicle.

The results of the tests, all of which are described in our affidavit, speak for themselves," GM spokesman Jim Cain said in a statement.

The tests were run in March, he added.

Cain declined to say if GM would test for knee bumps in the future or further elaborate on the issue.

But knee-bumping complaints were among the earliest indicators of an ignition switch problem roughly a decade ago, and some safety advocates say GM should test for it.

"They should have run that test because that's one of the known failure problems," said Clarence Ditlow, executive director of the Center for Auto Safety, a watchdog group.

The switch is low on the steering column, which makes it more susceptible to bumps, GM drivers have said in company documents, and a spring in the switch which helps hold the ignition key in place does not push hard enough.

"They should be setting up some type of experiment that would have the key directly impacted by the knee," said Steven Batterman, a forensic engineer in New Jersey, who has worked for plaintiffs' lawyers in auto accident cases, including ones against GM.