[SkyPuncher]

This is not at all true.

Parking brakes in an automatic vehicle actually flip a piece into the gear ring. They’re absolutely rated to hold the vehicle in place without additional brakes.

I know on my Ford truck, the parking gear is rated to hold the truck plus some amount of trailer weight. If you’re above that trailer weight, they recommend engaging the manual brake.

[vvillena]

Park position flips a lock into the gear ring to ensure the gearbox doesn't move. This achieves the same effect as leaving a manual gearbox in gear. In a manual gearbox, the engine holds the rest of the gearbox in place, but in a traditional auto gearbox, the whole mechanism can still move when the engine is stopped, that's why the lock piece is needed.

People relying on the gearbox lock to keep the car in place is the weird part - it would be like engaging first or reverse gear on a manual car, and trusting that to keep the car in place. Even if the manufacturer showed that the engine vacuum is enough, people still rely on the right tool for the job - a brake.

[SkyPuncher]

> This achieves the same effect as leaving a manual gearbox in gear

This is not true. In a manual car, you’re still relying on the engine. In an automatic vehicle, it’s essentially a latch that drops into place. A well designed one should actually tighten as the vehicle attempts to roll while in park.

It seems these are failing to engage or simply failing entirely.