The viaduct is being closed while Bertha tunnels beneath the structure. This temporary closure of the viaduct is precautionary. Removing vehicles from the structure will give crews better access to the structure to allow for better monitoring while the tunneling machine passes beneath. This also allows crews to more quickly respond to any movement in the structure that might occur.
Additionally, WSDOT recognizes that advertising a planned closure ahead of time, instead of conducting an unexpected and last-minute closure, will give the travelling public time to plan their trips accordingly.