A fire caused by a car crash, inside a 2.5-mile tunnel under a major American city. It’s a terrifying idea, but if you want that kind of problem to ignite anywhere, it’s in the stretch of State Route 99 that, later this year, will start whisking traffic underneath downtown Seattle.
That’s because this bit of SR 99 is more than the hemisphere’s largest-diameter bored tunnel and the country’s longest roadway tunnel outside of Alaska, much of it dug by Bertha, formerly the planet’s largest boring machine. To go with such superlatives, it might be the world’s smartest tunnel, too.
