Final answer:
The center lane on an expressway is used for passing or for vehicles making left turns, improving traffic flow and safety.
Step-by-step explanation:
In general, the center lane or lanes on an expressway are used for passing or for vehicles that are making left turns. These lanes are usually marked with broken lines to indicate that vehicles can enter and exit freely. The purpose of having designated passing and turning lanes is to improve traffic flow and safety on the expressway.