I'm talking like ancient olympics or sporting events before gunpowder was widely used.
Did they use handkerchiefs like in movies? Or something like phosphorus flares?
According to Sidonius (5th century) Roman chariot races used a trumpet:
Tandem murmure bucinae strepentis suspensas tubicen vocans quadrigas effundit celeres in arva currus.
At last the herald with loud blare of trumpet calls forth the impatient teams and launches the fleet chariots into the field.
Edit: To add a bit more detail about the type of trumpet used, it was a long, curved trumpet called a bucina. Check it out!