YOSEMITE — Visiting Yosemite Valley in the summer is often as pleasant as getting stuck in a traffic jam in Los Angeles.It seems as if all 4 million annual visitors decided to arrive at the same time. The optimum time to visit Yosemite is at the cusp of spring when the crowds are at a trickle and the waterfalls are roaring like a thousand stampeding horses. Late fall and winter are right up there too in terms of pristine beauty and solitude even near the valley floor.But living within two hours of what is one of the world’s top tourist destinations means visitors dropping by in the height of spring or summer want to see what all of the fuss is about.