Under uniformity, which designation is explicitly allowed to move traffic in a single direction on the highway?

Answer: A — Designating a highway as a one-way highway and requiring each vehicle on the highway to move in one specific direction.

The main idea is that to move traffic in a single direction on a highway, the rule is to designate it as a one-way highway and require every vehicle to travel in that specific direction. This designation creates a uniform flow of traffic and prevents conflicts from opposing directions, which is exactly what a one-way setup is intended to accomplish. The other choices don’t establish a single-direction flow for all vehicles on the highway. A through highway designation focuses on continuous movement through an area, not necessarily enforcing a single direction everywhere on the highway. School crossing guards and zones deal with pedestrian safety at crossings, not limiting the direction of vehicle travel. Altering a speed limit sets allowable speed but doesn’t mandate a uniform single direction of travel.