Traffic Sign - What does it mean?

I lived in San Miguel de Allende where ALL of the intersections are based on 'uno y uno' - but there are No signs indicating this. Everyone who lives in SMA knows this rule and without it there would be traffic chaos. It makes it easy to spot a 'foreigner', but they pick it up quite quickly, or expensively! It works very well there, and the standard of driving is far more polite and efficient - even the taxi drivers!