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When to Use 36″ and 48″ Temporary Warning Signs

According to the MUTCD:

Transportation engineers may find this answer acceptable since they have a deeper understanding of the MUTCD than most, but not everyone has this knowledge. Luckily, definitions of these terms are provided to help make some sense of what they are trying to convey. Here’s how the MUTCD defines each of those types of roadways:

• "Conventional Road" is a "street or highway other than a low-volume road, expressway, or freeway."
• "Low-Volume Road" is a facility lying outside of developed areas of cities, towns, and communities, which serves fewer than 400 vehicles per day on average. A low-volume road shall not be a freeway, an expressway, an interchange ramp, a freeway service road, a road on a designated State highway system, or a residential street in a neighborhood.
• "Expressway" is a divided highway with partial control of access.
• "Freeway" is a divided highway with full control of access.

Still confused? It's not just you. Generally speaking, if the road is a divided highway with a concrete barrier or median, a 48 × 48″ sign will be required. A 36 × 36″ sign can be used on all other types of roadways.