What do interstate route signs vary based on?

Interstate route signs vary based on the type of roadway and the state because each state has its own standards and color codes for road signs, which must also comply with federal guidelines. The design of interstate signs reflects the specific characteristics of the roadway, indicating whether it is an interstate highway or a different type of road. This ensures that drivers can easily understand the information being presented, regardless of where they are traveling. Various states may use differing color codes or symbols to convey information relevant to that state's traffic regulations, thereby influencing how the interstate routes are marked.

The other options do not fully capture the primary factors that govern the design of interstate route signs. While different states may use various colors to signify different types of roads, the essential variability lies in the type of roadway as well as the local jurisdiction's preference and rules. Speed limits and the shape of the signs are generally standardized across states to maintain consistency and enhance safety, hence they do not provide the variability indicated in the question.