I-405 is the only freeway directly connecting the coastal cities of Los Angeles with the San Fernando Valley, hence its popularity. Lacking available space for building additional freeways, Caltrans' strategy for reducing congestion is to maximize the efficiency of the existing freeway, in part by building HOV lanes. For the past decade, the agency has been adding carpool lanes along the route. All that remained to complete the system 'was the 10-mile (16-kilometer) stretch northbound from the Santa Monica Freeway to the Ventura Freeway.