According to the Transportation Code, an "unmarked crosswalk" only exists at which of the following locations?

An unmarked crosswalk is defined in the Transportation Code as existing at intersections, specifically at locations where two streets meet, even if there are no painted lines to indicate a crosswalk for pedestrians. This is important as it establishes the right of way for pedestrians crossing the street at these points, emphasizing the responsibility of drivers to yield to them.

In considering the other choices, street corners may imply an intersection, but not all corners have legal crosswalks unless they are marked, thus not aligning precisely with the definition. Highway bridges do not typically feature crosswalks due to safety and structural design, while roundabouts are designed specifically to manage traffic flow and don't provide traditional pedestrian crossings except at designated pedestrian areas that would be marked. Therefore, intersections are the crucial locations specified in the law for unmarked crosswalks, making this the correct answer.