Final answer:
The eight-sided sign with white letters that signifies danger on the road is a Stop sign. It is universally recognized and requires drivers to come to a complete stop.
Step-by-step explanation:
The only eight-sided sign on the roadway with white letters that always indicates there's a danger is a Stop sign. This sign is designed to alert drivers to stop completely at the point of the sign to prevent accidents and ensure traffic flows safely and orderly. Unlike other signs that may be multi-sided or convey different types of warnings, the stop sign's distinctive shape and color make it instantly recognizable and universally understood as requiring a full stop.