Each year, Wal-Mart Stores Inc. spends hundreds of millions of dollars delivering its merchandise across the United States. The 6,000 trucks in the retailer's fleet are a common sight on highways, as are those of the many other companies that rely on long-haul trucking to transport their goods from coast to coast. But what if that fleet could be cut by one-third"”and be made up of trucks pulled by slimmed-down tractors less than half their current size, with a computer at the helm?