Based off pictures found at the following links: [1];[2]. A 1927 Specification Arizona State Route Shield, traced from a picture of a real shield and sized to match File:US 66 Arizona 1926.svg. The Swastika on the shield is historically accurate and represents a Native American design and is not to be associated with Adolf Hitler or the Nazis.
[3]
Matthew Anderson (based on an original .PNG format picture found at the AARoads shield gallery.)