The purpose of the Project is to upgrade the bridge railings to comply with the design and installation standards outlined in the American Association of State Highway and Transportation Officials (AASHTO) Manual for Assessing Safety Hardware (MASH), thereby protecting the traveling public by enhancing the reliability of the bridge railings.

The Project is needed to comply with the design and installation standards outlined in the AASHTO MASH. Yellow Creek Bridge and Arroyo Seco Bridge were constructed in 1922 and 1923, respectively. The bridge railings, whose conditions are covered by the SHOPP Program, were inspected, and reports determined that the bridge railings are in poor condition. At Yellow Creek Bridge, the steel beneath the concrete baluster railing is exposed on the northern segment of the bottom horizontal rail member west of the southbound lane of State Route (SR) 121, and the middle segment of the top horizontal rail member east of the northbound lane of SR 121 is missing. In addition, the bridges are also used by heavier vehicles that currently travel at higher speeds than at the time the bridges were constructed. Therefore, the bridge railings need to be upgraded to comply with the design and installation standards outlined in the AASHTO MASH. SR 121 is an important connector between SR 116 to the north, SR 12 to the north, and SR 37 to the south for local residents and businesses in unincorporated Sonoma County, as well as the only direct connector between the City of Sonoma to the north and County of Napa to the east. If not addressed, the poor conditions of the bridge railings have the potential to affect the safety of the traveling public.