Opening your car door safely hinges on ensuring that it's clear to do so, particularly regarding oncoming traffic or cyclists. This means that before you open your door, you should look around and check that there are no vehicles or bicycles approaching from either direction that could be impacted by your door swinging open. This safety measure is crucial in preventing accidents and protecting both yourself and others on the road.
While parking at a curb allows for door opening, it does not guarantee safety without checking for surrounding traffic. Being in a parking lot may provide some leeway, but it is still essential to stay aware of nearby vehicles and pedestrians. Furthermore, simply turning off the engine does not eliminate potential hazards; you must remain vigilant of your surroundings regardless of the vehicle's operational status.
Therefore, the essential reason for opening your car door is when you can confirm that it is safe to do so, making the focus on checking for oncoming traffic and cyclists the best practice.