I asked WSDOT to define “flammable cargo.” They tell me the restrictions you see on signage will not affect the typical driver.

“There is a code of federal regulations that defines flammable materials. But basically the ban is intended for truckers hauling flammable liquids such as gasoline, kerosene, and paints and paint thinners; or flammable materials such as fireworks, which are both flammable and highly explosive,” says Tom Woods of the Washington State Patrol's commercial vehicle division.

Even if a gas truck is empty, there could be a risk of explosion. If there were a major fire in the tunnel or under the convention center when fire-suppression systems were shut down, it could be very bad news.