The airport in Columbus, Ohio has a couple runways that go over one of the roads in. The concrete is pretty thick, and the undersides are lined with some kind of metal. What other tricks do they use to make the bridges strong enough to support the weight of a commercial airplane?
One of the runways at Memphis has a grade-separation bridge. Their solution was to support the bridge by many many concrete beams set very close together.
Same stuff they do to make bridges hold the weight of cars - it all comes to keeping track of how concrete has great strength under compression but has zero ability to be stretched without falling apart, and steel has great resistance to being stretched, and is pretty good at handling compression.
They use reinforced concrete.
Prestressed concrete is poured with cables under tension embedded inside. When the tension is released, it puts the concrete in compression, so that said compression must be relieved before the steel rebar can be put into tension. This will be done on the bottom half of the section in question, as the bottom of the “smile”, underneath the neutral axis, is the part that will eventually end up under tension.
EETA: You can see the same effect on flatbed semi trailers, which actually hump upwards when they aren’t under a load.