The MUTCD specifies standardized sign dimensions for application on public roadways and presents sign size tables in the manual based on type of road (i.e., conventional road single lane, conventional road multi-lane, expressway, and freeway). The tables also present Minimum and Oversized sign sizes. The sizes shown in the Oversized columns in the various sign size tables should be used for special applications where speed, volume, or other factors result in conditions where increased emphasis, improved recognition, or increased legibility is needed. A qualified engineer should make this determination.
Location of the sign relative to the motorist, the design speed at which the motorist will be approaching the sign, the volume of traffic, and the content and urgency of the sign's message all contribute to the determination of the sign's message and physical size.