Seattle’s SR 99 tunnel reopened Thursday morning after a power outage forced the first unscheduled emergency closure in the tunnel’s history, shutting down both directions for several hours and creating significant traffic disruptions throughout downtown Seattle and surrounding areas.
The Washington State Department of Transportation indicated the extended closure resulted from a power outage that disabled the tunnel’s ventilation system, a critical safety component that must remain operational whenever vehicles are traveling through the underground passage.
“The ventilation system is part of the critical operations of the tunnel and when it doesn’t work it doesn’t allow us to have traffic in that tunnel,” stated RB McKeon, the WSDOT’s Northwest Region communications manager, explaining why authorities had no choice but to close the tunnel completely rather than allowing traffic to continue flowing whilst ventilation systems were offline.
Seattle City Light traced the source of the power outage to a downed wire at 4th Avenue South and South Hanford Street in the SODO neighbourhood south of downtown Seattle.
“This was an overhead wire and the part that failed was a connection, so when that connection failed it brought the whole wire down,” stated Jenn Strang, the media relations manager at Seattle City Light, describing the mechanical failure that cascaded into the tunnel closure affecting thousands of commuters.
Utility crews worked through the morning to repair the damaged wire and restore power to the affected area. The southbound tunnel direction reopened around 10:45 a.m., with the northbound direction following shortly after at approximately 11 a.m., allowing normal traffic flow to resume after the hourslong disruption.
The SR 99 tunnel serves as a main artery for traffic moving through Seattle, carrying particularly heavy volumes during morning and afternoon rush hours when commuters travel between residential neighbourhoods north and south of downtown and the central business district. The tunnel’s closure during the morning commute forced drivers to seek alternative routes including surface streets already congested with regular traffic, creating backups that extended well beyond the immediate tunnel vicinity.
The tunnel is designed with built-in redundancies for situations including power outages, backup systems intended to ensure that single points of failure cannot disable critical operations. However, McKeon indicated those backup systems malfunctioned Thursday, compounding the impact of the initial power loss.
“Ordinarily if the south end of the tunnel lost power like it did today we would be able to draw power from the north end,” McKeon explained. “The piece of equipment that allows that is called switch gear and the switch gear malfunctioned today. So we were not able to do that, we did not have the redundancy in the system today.”
The switch gear failure represents a significant concern for WSDOT officials responsible for tunnel operations, as redundant systems exist precisely to prevent scenarios where a single equipment failure can force complete facility closure. The fact that both the primary power supply and the backup system failed simultaneously suggests potential issues with maintenance, equipment quality, or system design that require investigation and correction.
WSDOT has requested that the company manufacturing the switch gear conduct an assessment of the failed equipment, then provide recommendations on modifications, maintenance procedures, or replacement options that would prevent the backup system from malfunctioning during future incidents. This review will likely examine whether the switch gear failure resulted from a defective component, inadequate maintenance, environmental factors, or design flaws that make the equipment vulnerable to failure under certain conditions.
The SR 99 tunnel opened to traffic in February 2019, marking the completion of a years-long project to replace the aging Alaskan Way Viaduct that had carried State Route 99 traffic along Seattle’s waterfront for decades. The viaduct, a double-decked elevated highway structure, was deemed seismically unsafe following damage sustained in the 2001 Nisqually earthquake, prompting a contentious political debate about replacement options that ultimately resulted in selection of the tunnel alternative.
The tunnel project, part of a $3.3 billion investment in transportation infrastructure, faced numerous challenges during construction including a two-year delay when the massive boring machine used to excavate the tunnel encountered unexpected obstacles and suffered mechanical damage. Despite these setbacks, the tunnel eventually opened and has generally operated smoothly since 2019, carrying thousands of vehicles daily through the 1.7-mile passage beneath downtown Seattle.
Thursday’s power outage and resulting closure represent the first unscheduled emergency closure to occur since the tunnel opened to the public nearly six years ago, a track record that WSDOT officials had previously cited as evidence of successful operations and maintenance. The fact that this first emergency closure resulted partly from backup system failure rather than solely from the initial power outage raises questions about the reliability of redundant systems that are supposed to ensure continuous operations even when primary systems fail.
