Under what circumstances should you not start across an intersection?

Starting across an intersection when you are aware that you will block it when the light turns red is a situation you should avoid. Intersections are designed for the safe movement of traffic in multiple directions. If you enter an intersection but cannot clear it before the light changes, you risk creating a hazardous situation not just for yourself but also for other drivers, pedestrians, and cyclists. Blocking the intersection can impede traffic flow and can lead to frustration and accidents, as vehicles that have a green light may not be able to proceed, potentially causing collisions.

The other situations outlined do not necessarily present the same level of risk. Being unsure of traffic rules may caution you to proceed carefully, but it doesn't strictly prevent appropriate action. The absence of other vehicles might suggest it's safe to cross, and being in a hurry should never justify ignoring traffic rules or safety, but it doesn't directly relate to the operational rules of an intersection. Thus, being aware of your ability to clear the intersection is vital for safe driving practices.