The metal body of a car acts as a Faraday cage when a high-current power line falls on it, which prevents those inside from being electrocuted by directing the charge around the exterior. There is no danger as long as there is no potential difference to cause electricity to flow through the bodies inside.
People inside a car are not electrocuted when a power line falls on it because the car acts as a Faraday cage, distributing the electrical charge on its external metal body, with the inside remaining unaffected. This is similar to the protection offered during an electrical storm. In contrast, a bird is not electrocuted while perching on a single wire since there is no potential difference across its body, but if it simultaneously touches two wires, it completes the circuit and may be electrocuted due to the potential difference.
When a high-current power line comes into contact with a car, the metal body of the car channels the electricity around the exterior, thereby shielding the interior from the electric field; this is known as the Faraday cage effect. As there is no potential difference between different points of the car's body that the occupants could touch, they aren't subjected to an electrical path that flows through their bodies. For a bird sitting on a wire, the reason it is not electrocuted is that both of its feet are usually on the same wire, so the voltage across its body is minimal, and electricity requires a potential difference to flow. However, if a large bird's wings bridge two wires, it creates a circuit because there is a substantial potential difference, leading to electrocution.
the absence of a potential difference inside the car and between a bird's feet on a single wire is what prevents electrocution. The presence of this difference in the case of touching two wires is what causes the danger.