Traffic Control Devices, sign number M1-1, which states specifically on page I-1 that: Any traffic control device design or application provision contained
DescriptionI-291 (1961).svg English: 30 in by 25 in (750 mm by 625 mm) Interstate shield, made to the specifications of the 1961 MUTCD (specs from Caltrans)
DescriptionI-291 (1957).svg English: 24 in by 24 in (600 mm by 600 mm) Interstate shield, made to the specifications of the 1961 MUTCD (specs from Caltrans)