Quiet, clean, ultramodern vans are making their way into California neighborhoods as part of the U.S. Postal Service's efforts to make its 200,000-vehicle fleet as environmentally beneficial and cost-effective as possible. Five hundred battery-powered electric vans, developed by Ford Motor Co., have been purchased. Aside from their low-noise and zero-emission benefits, the vans require less maintenance and are cheaper to run than comparable gasoline-powered vehicles.