WSDOT told drivers to bail out of the tunnel the other morning. Nobody did.

Motorists inside the Highway 99 tunnel were told to park and evacuate the roadway just before 7 a.m. Thursday, when staff at the Washington State Department of Transportation (WSDOT) overreacted to a routine stall on the northbound roadway.

Drivers and passengers didn’t leave their vehicles and instead slowed and continued through the tunnel to South Lake Union.

A typical safety response might include a lower speed limit, a notice on overhead signs or a lane closure.

“This morning we inadvertently used a higher severity incident plan. This shouldn’t have happened & we’re sorry for the confusion,” the agency tweeted in its traffic feed.

The mistake happened because an operator chose the incorrect protocol from a computer system, said Bart Treece, spokesman for the traffic-control center in Shoreline.

Drivers ignored the warnings, which WSDOT broadcast by overriding car-radio signals. A red “X” appeared along with a 30 mph instruction on overhead signs, one driver tweeted to WSDOT.

Evacuation measures are designed for severe emergencies, such as toxic smoke or a fire. In that case, drivers and passengers should open the green doors on the west, or waterfront, side of the tunnel and take shelter in the concrete gathering area behind the doors.

Treece said motorists did the sensible thing by continuing, saying that even under an evacuation instruction drivers ought to look around at the circumstances. In a mass-casualty crash, for instance, drivers would notice lights and sirens ahead from first responders. The tunnel may be impassable.

“If they still see vehicles traveling, what context do you have that tells you to actually park your vehicle?” he said.

The vehicle stall and evacuation notice occurred during the first week of tolling inside the tunnel, as transportation managers warn about other traffic slowdowns caused by motorists who divert to side streets. As of 8:30 a.m., residual congestion at the Sodo entrance kept incoming traffic crowded and near 20 mph, slower than usual.

Then around 9 a.m., tunnel operators blocked the left northbound lane to repair a leaking fire extinguisher, in a separate incident.