The silk threads are so fine and light that they are carried by even the slightest of air currents. The spider spins an unattached silk thread and waits until it snags on something. That is how it spanned the distance between the post and your car.
Young spiders use the same method to disperse. They are so light themselves that they are carried aloft by the thread in the air currents and later get a soft landing with the silk acting as a parachute slowing descent.
Your question has fascinated me for ages because I've often watched spiders in our garden travelling almost impossible distances to find anchor points for their webs. I suspect that gen's response is correct but unfortunately I've never been around at the time to witness how this miracle happens.
gens spot on i've been fishing and sat watch a spider do it right in front of me,it was quite breezey so it did'nt take long and there was quite a few doing it ,great to watch.