To keep traffic moving, a _________________ is set on some primary highways and expressways.

The correct choice is a minimum speed limit. A minimum speed limit is established to ensure that vehicles do not travel too slowly, which can disrupt the flow of traffic, particularly on primary highways and expressways where higher speeds are generally expected. By preventing slower vehicles from operating below this limit, the minimum speed limit helps maintain a smooth and consistent traffic flow, thereby reducing the likelihood of congestion and improving overall safety on the roadway.

In contrast, a maximum speed limit sets an upper boundary for how fast vehicles may travel, but it does not address issues related to very low speeds that can impede traffic. Traffic signals control intersections rather than manage traffic speed on highways, while lane closures typically concern road work or accidents, which can create delays rather than keep traffic moving efficiently. Therefore, the establishment of a minimum speed limit is crucial for facilitating optimal traffic movement on major roadways.