After measuring the anticipated traffic for the bridge, cement/reinforced- with- rebar stanchions are spaced over the freeway to accommodate the bridge. An 'off-ramp' from the freeway to the bridge is constructed, as is an 'on-ramp' to the subsequent road. Cement/rebar slabs are built and lifted with cranes to form the platform, and voila! Drive carefully.

Though the bridge deck/roadway is almost always a concrete slab, the structure that holds up the bridge deck can be reinforced concrete, structural steel, or a combination of steel and concrete.