In 1934, the Rocky Butte Scenic Drive Historic District was established. Built as part of a Works Progress Administration project that supported 25,000 employees in Portland, it was designed by the Multnomah County Road Department, The drive reaches the summit of Rocky Butte to a viewpoint overlooking Portland. Workers used basalt from the Rocky Butte quarry to create retaining walls, and the finished project was designed to have a rustic style typical for public works projects of the time. WPA workers also built the park and a baroque staircase at the top of Rocky Butte, finishing construction in 1939. At its peak popularity in the 1940s, Rocky Butte had more than 4,000 cars visiting each day. Over time, it was not maintained well, and it became a frequent site for teenage parties; in the early 1990s, the Rocky Butte Preservation Society began cleaning up and restoring the park area, and adding new amenities. Once of land were added and the Preservation Society took on an active maintenance role, the site was regraded, original plaques were reinstalled, and lawn areas, irrigation, red cinder walkways, and additional trees were added to the park. The District was listed on the National Register of Historic Places in 1991. Other WPA projects included a tunnel in length at the southern end of Rocky Butte. Built between 1939 and 1941, it was constructed to improve access to scenic views on the Butte. The project took 16 months to complete, and workers used hand mining and drill and shoot methods to excavate layers of cinders and lava flows. The tunnel is curved because of the volcano's 5 degree grade. Rocky Butte basalt was used to built the Rocky Butte Tunnel as well as the nearby West Burnside Tunnel and NW Cornell Road Tunnel, all three of which were constructed by the mason R. Curcio and his crew. In total, the WPA's Rocky Butte project cost $500,000 to complete.