Good question. Sometimes they can just attach a silk thread (the silk that they make) to one side and jump to the other. What they do if they cant jump that far is like what mountain climbers do if they need to move across a rock face and there arent enough handholds and footholds in between; they attach the silk to one side, then go down and in the opposite direction as far as they need to. This makes them like a pendulum in an old clock. When they let go, they will swing to the other side.

Why do you think so many spiders build webs across trails?

There are basically three ways:
1. - It can find a place far overhead, extend down from that and swing from one place, attach the web strand, and then swing to another place and attach the web strand to that. It then uses the connection as the base to construct it's web.
2. - If it is not too big a spider, it can send a swatch of webbing into the air, and let it balloon (drift on the wind) to another location and attach its web stand it was carrying behind to that. This is the technique that baby spiders use.
3. - The most common method, is that it attaches the end of the strand to one high place, climbs down trailing and extending the strand behind it, walks along the ground to another high place, climbs up high, and attaches the strand up there. It pulls up the slack, then this long strand acts like a bridge, and is often used as the first line to construct its web from.

A Jumping Spider can jump, with a web strand trailing behind, but not a distance of 12 or 20 feet, in fact not much more than 3 or 4 inches.

Spiders sit on one object and send out threads of silk which blow around in the wind, and eventually touch and stick to the other object. Then the spider uses these wind-borne threads as a frame upon which to build its web.

I just read about how spiders construct webs between 2 distant points, last week in a great book called "The Way Life Works".

The spider spins some silk out of its back end, and the silk waves in the breeze until it sticks on some other place. Then the spider attaches the silk to the place where she is sitting, and she has a strand of silk that she can walk back and forth along, to spin more silk at different places.