Only 10 milli amperes (0.0001 amps) through the heart is enough to kill a human.
The Mythbusters, a show on The Discovery Channel, covered this proving that an electrical appliance dropped into a bathtub, plugged into a standard 110 to 120 volt socket will provide enough current to kill a person.
Current (kind of like the speed of electricity) is the key factor. With a Tesla coil you can generate thousands of volts and it only gives you a shock like that you get when you get a static electricity shock from a doorknob. Voltage doesn't kill you it is the amperage.
Since we are mostly water we can conduct electricity pretty well so getting shocked can be serious, the difference between life and death is if you are grounded. Since a car has rubber tires if a power line falls on top of the car you will be safe; as long as you remain inside the car.
But, you can have a huge bolt of lightning go through you and still survive, if it grounds out without having to go through your heart, even then some people have had their hearts stopped and received medical attention in time to save their lives.