In the United States, stop signs have a size of 750 mm across opposite flats of the red octagon, with a 20 mm white border. The white uppercase letters in small caps forming the stop legend are 250 mm tall. Larger signs of 900 mm (35 in) with 300 mm (12 in) legend and 25 mm (7⁄8 in) border are used on multilane expressways. Regulatory provisions exist for extra-large 1,200 mm (47 in) signs with 400 mm (16 in) legend and 30 mm (1 ⁄ in) border for use where sign visibility or reaction distance are limited, and the smallest permissible stop sign size for general usage is 600 mm (24 in) with a 200 mm (7.9 in) legend and 15 mm (5⁄8 in) border. The metric units specified in the US regulatory manuals are rounded approximations of US customary units, not exact conversions. The field, legend, and border are all retroreflective. In the Vienna Convention on Road Signs and Signals of the UN, the instruction on the sign to stop is specified to be either in English as stop or written in the local language. Some countries use both. The sign 's distinctive design was developed and first used in the U.S., and later adopted by other countries and by the U.N. Despite this, the US is not a signatory to this UN Convention.
what shape is a standard stop sign according to the highway code
octagon