"Short-term pain means long-term gain." That's what METRO says about its four-year, $1 billion addition of a carpool lane to the San Diego Freeway from the 10 on the Westside to the 101 in the San Fernando Valley. But two weekends may cause more pain than the rest of the project put together. During the first, from midnight Friday, July 15 until 5am the following Monday, the 405 will be shut down in both directions through the Sepulveda Pass.