Staggering at a traffic light controls the _______________ of vehicles allowed to proceed with each green light.

Staggering at a traffic light is a method used to control the number of vehicles that can go through the intersection with each green light. This technique helps to prevent congestion and ensures that vehicles can move safely and efficiently. By staggered timing, traffic lights give priority to specific lanes or directions in a phased manner, allowing a certain number of vehicles from different directions to proceed while managing the overall flow of traffic.

The other options focus on different aspects of traffic management. Speed pertains to how fast vehicles can move, which is not directly affected by staggering. Direction involves the path vehicles take but isn't controlled by a staggered light arrangement. Size might refer to vehicle dimensions, which are not a concern with light timing. Thus, focusing on the number of vehicles accounts for the purpose of staggering at traffic lights for effective traffic control.