If you build it, they will totally come

Induced demand is the most bananas thing. Via City Observatory:

But what appears to have happened is that the wider I-5 just funneled more peak hour traffic, more quickly into the bridge area. The result is that the roadway jams up more quickly, and that backups occur earlier and last longer, with the result that the freeway actually carries fewer cars than it could if traffic volumes we effectively metered more evenly by a somewhat lower capacity upstream of the bridge.