If we implemented surface/transit/I-5 [ST5], vehicles would bunch up at the north and south ends of downtown, but once they passed those points, they would flow more freely inside the city's core. The peripheral congestion would "effectively meter the volume of traffic" into the central business district, the state says. "This metering, combined with the expected redistribution of traffic outside the downtown area, yields reasonable level of service." Added vehicle capacity on Second and Fourth Avenues would also grant more capacity on downtown streets. "The analysis shows that travel times for representative trips within downtown Seattle would be similar, or in some cases shorter" with ST5 as compared to the tunnel.

Furthermore, the state studied traffic in dozens of downtown intersections and dished out grades ranging from A to F. The result was that a tunnel resulted in 25 intersections with a perfect grade, while ST5 resulted in 32 intersections with a perfect grade. Likewise, ST5 had only 16 intersections that received an F grade, while a tunnel produced 19 intersections with an F.

But it doesn't perform better by every metric. More on ST5's advantages and drawbacks HERE.