Final answer:
The danger zone increases when speed increases, the vehicle is heavy, and it is raining. These conditions increase the risk of accidents and danger on the road.
Step-by-step explanation:
Danger zone increases when all of the above conditions occur: when speed increases, the vehicle is heavy, and it is raining. This is because speeding increases the chances of losing control of the vehicle, a heavier vehicle requires more distance to stop, and rain reduces traction on the road. All these factors contribute to a higher risk of accidents and danger on the road.