When should the parking brake be engaged?

Engaging the parking brake is essential when parking on a hill or slope. This precaution prevents the vehicle from rolling away due to gravity. The parking brake, also known as the emergency brake, creates a mechanical connection that holds the vehicle stationary even when in gear or when the transmission is in the "park" position. It is particularly important on inclines where the risk of the vehicle moving unintentionally is heightened.

In the context of the other choices, engaging the parking brake while driving over 50 mph is unnecessary and can be dangerous as it can cause loss of control. Low fuel conditions don't require the use of the parking brake; this situation is unrelated to preventing the vehicle from rolling. Similarly, using the parking brake during heavy traffic is not only irrelevant but also impractical since the vehicle needs to remain mobile in that situation. Therefore, the best practice for engaging the parking brake is specifically when parking on sloped surfaces to ensure safety.