The Department of Engineering became part of the new Department of Public Works in 1921, and the California Highway Commission was entirely separated as its own department in 1923. In order to pay for the roads, a 2-cent per gallon gasoline tax was approved in 1923. The legislature continued to add highways to the system, including the Mother Lode Highway (now part of SR 49) in 1921 and the Arrowhead Trail (now I-15 north of Barstow) in 1925. In January 1928, the California State Automobile Association and Automobile Club of Southern California, which had already been placing guide and warning signs along state highways, marked the U.S. Highways along several of the most major state highways. The California Toll Bridge Authority was created in 1929 to acquire and operate all toll bridges on state highways, including the San Francisco–Oakland Bay Bridge and Carquinez Bridge.