Which sign indicates that one should enter the interstate system?

The sign that indicates one should enter the interstate system is the Interstate Route sign. These signs are specifically designed to guide drivers onto the interstate highways, which are major roads designed for high-speed travel that connect various regions and states. The Interstate Route sign features a distinctive shield shape with red, white, and blue coloring, making it easily recognizable on the road.

State Route signs and County Route signs do not indicate entry onto the interstate system; rather, they guide drivers along local and state roads. The Off-Interstate Business Route sign is meant for directing traffic to businesses that are located near an interstate but not directly on it. Understanding the purpose of these signs helps in navigating the roadway system effectively and ensures that drivers can safely enter the interstate when needed.