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9NEWS HOLIDAY GUIDE

After you pick out that perfect Christmas, the nice folks at the lot tie the tree to the roof of your car - usually with little more than twine.

A German car association performed emergency breaking tests of cars with Christmas trees ties to the roofs.

Tests of cars in an emergency stop from a speed of 31 mph with trees tied with rope or twine show the trees flying from roofs and becoming dangerous projectiles.

The force of the flying tree is enough to crash through the rear window of a car or knock down and injure a pedestrian.

In another test at the same speed with the tree secured by a ratcheted strap, the tree moves but is held to the roof.

Some folks in Europe take their tree home by putting it inside the car. That's dangerous as well, according to the car group, because it can limit the vision of the driver.