If cars with automatic and double-clutch transmissions have a "Park" mode, why do they still have parking brakes? When I move the selector to "P", the car stops, even if it is on a slope. Can the "P" option be used as a brake in an emergency?
With manual gearboxes, it was a common practice to keep the lever in first or reverse when the car was parked. This was particularly useful when parking on a slope as the drivetrain would be coupled to the engine, which could not easily turn when the ignition is off, thus providing additional "braking".
In fact, keeping the gearbox in first gear would stop the car from rolling, even if the parking brake or handbrake was not activated.
With automatic transmissions (double-clutch and continuously variable transmissions included), the Park mode locks the drivetrain using a simple mechanical arrangement where a peg known as a "pawl" falls into a slot in a toothed ring.
This is necessary as a means to lock the drivetrain because, once the ignition is off, the transmission is no longer engaged and the wheels are decoupled from the engine.
Although not common, the "P" mode can fail if the pawl is badly worn or sheared off as a result of rough handling.
When parking, especially on a slope, always engage the parking brake first before selecting "P". This prevents the short creep before the pawl falls into a slot (which is the cause of wear and damage). Similarly, when driving off, shift to "D" first before releasing the parking brake.
On older automatic transmissions, attempting to shift to "P" while the car is cruising will cause an immediate failure of the mechanical components. Fortunately, current transmissions have built-in controls to prevent such failures.
The Park mode can still be activated if the car is creeping. But this must be avoided in order to prevent premature wear or damage.
In short, you should not use Park in an emergency.