In what country is it required, by law, to wear an piece of reflective material that is at least 15cm x 15cm when walking the streets at night?

Safety reflector
Estonia's traffic laws require everyone to wear a luminous reflector ('helkur') on the right side of outer clothing at night or in conditions of poor visibility, particularly in winter.

A safety reflector is a small device that can be attached to any article of clothing. It is most commonly made from plastic or textile and is widely sold in different stores, kiosks and supermarkets in Estonia.

In Finland, Estonia, Latvia and Lithuania, pedestrians are required by law to wear safety reflectors when walking during dark conditions.