Caltrans is about to complete a milestone in its road widening project at the Marin-Sonoma Narrows. Starting tomorrow, southbound traffic on Highway 101 will be diverted onto new elevated lanes. The new lanes go between San Antonio Creek to just north of Atherton Avenue in Novato. The new lanes were built as part of the final phase of a decade-long widening project meant to relieve traffic congestion in the area. The full 762-million-dollar project will add a carpool lane on both sides of Highway 101 for 17 miles between Novato and Petaluma.