Under what circumstance is it permissible to leave a vehicle unattended with the engine running?

Leaving a vehicle unattended with the engine running is permissible primarily when warming the engine or defrosting the windows. This scenario is commonly encountered in colder climates where the engine needs to be warmed up to ensure proper function and where defrosting the windows is essential for safe visibility. Such situations require the vehicle to be operational, but it is critical that the driver remains within a reasonable proximity for safety and security reasons.

In other instances, like stopping for a short break, there may be potential risks related to theft or unintended movement of the vehicle. Similarly, while loading or unloading cargo, the vehicle is typically expected to be under the driver's supervision for efficiency and safety. Although parking in a designated area might seem appropriate, it does not guarantee that leaving the engine running is safe, especially if the driver is not present or able to respond to any issues that might arise. Thus, the specific allowance for warming up the engine or defrosting takes precedence in this context.