Final answer:
When parking on a downhill slope, turn the wheels towards the curb and apply the parking brake firmly. For manual cars, leave in gear; for automatics, use the 'park' position.
Step-by-step explanation:
When parking your car on a downhill slope, there are specific safety measures to take to ensure that your vehicle does not roll away. Firstly, you should turn your wheels towards the curb or road shoulder. This way, if your brakes fail, the curb can help stop your car from rolling into traffic. Secondly, always apply the parking brake firmly. Additionally, if applicable, leave the car in gear (in a manual transmission vehicle) or in the 'park' position if it is an automatic. These steps, similar to how a truck coasts down a hill with friction helping to slow it, as evidenced in figure references about cars on hills, ensure that physics is on your side and the car's potential movement is directed where it can be safely controlled or stopped.