Not to be nitpicky but if you haven't driven the I-5 theres substantial room between each direction of the freeway so I don't think it will actually be above traffic if that's your concern.

I have driven the I-5, but good point. And the diagrams that you'll find on p28-29 of the presentation suggest that the supports would definitely fit on the median.

You still need government permission to use the median like that.

There are overpasses all along I-5. I seriously doubt you can put a 20 ft elevated tube under them and you certainly don't want them in a place where a truck can easily slam into the support pillars.

At 700+ MPH you'll need more than 100' of freeway median to contain potential incident fallout.

Depends on how strong the tunnel is. Assuming your taking 3/4 atmospheric difference your starting at ~11 pounds per square inch add in a 3-5x safety factor and that tunnel would be one tough son of a bitch.

Not to mention it's got to be really close to a strait track so your more dealing with scraping forces than a direct strike.

Does the minimum bending radius of the tube match the centerline of the highway? If not that might be a real problem.

Hyperloop proposal contains detailed map of the whole route. Sometimes it follows the highway, other times it deviates from it.