Hopper Slough Bridge was built in 1921 by Napa County. The existing bridge is single-span, 42 feet long and 23 feet wide with concrete railings and a nonstandard 10-foot-wide lane in each direction. The metal beam guard rail (MBGR) on the east side, westbound lane extends 90 feet from the end of the bridge. The MBGR on the east side, eastbound lane extends 25 feet from the end of the bridge, followed by a 25- foot-gap in the MBGR for a private driveway, and then the MBGR continues for an additional 135 feet from the end of the bridge. The MBGR on the west side, westbound lane extends 66 feet from the end of the bridge, and the MBGR on the west side, eastbound lane extends 180 feet from the end of the bridge. There are no shoulders or bicycle facilities on the bridge. When oversized vehicles are present, motorists traveling in the opposite direction must wait while the oversized vehicle crosses the bridge. The bridge carries an average of more than 2,000 vehicles a day, of which about 5 percent are trucks. There is an existing 6-foot-long by 4-foot-wide reinforced concrete arch culvert 96 feet west of the bridge that crosses under SR 128. The existing bridge structure has several issues that need to be addressed. A damaged bridge abutment was identified by bridge maintenance personnel during inspections in April 2014 (Caltrans 2015b). The Project also poses deficiencies due to nonstandard 10-foot-wide lanes in each direction with no shoulder and nonstandard bridge rails. In 2014, a temporary repair of the abutment was attempted. However, due to heavy rains, the full repair down to the footing could not be completed. During the attempted repair, Caltrans inspected the bridge and noted additional cracking and deterioration of the abutment. During inspections in 2015, Caltrans recommended that the bridge be replaced (Caltrans 2015b). This Project would improve safety for the traveling public and improve the structural integrity of the bridge.