This guide is for general information only and is not legal advice. Laws change — verify current rules with your state DOT or a licensed attorney before relying on this for any legal matter. Read full disclaimer.
The rule
South Dakota requires lamp on the front emitting a white light visible from a distance of at least 500 feet to the front. For the rear, red reflector on the rear of a type approved by the department of public safety, visible from 50 to 300 feet to the rear when directly in front of lawful upper-beam headlamps. a red rear lamp visible from 500 feet to the rear may be used in addition to the reflector. The rule applies whenever it is dark. See SDCL § 32-20A-6 (Lamps and reflectors required on bicycle at night).
The cheapest way to stay legal in South Dakota is a USB-rechargeable front white light and a rear red light or reflector that meets the spec above. A $30 light set covers it.