A white rectangular sign with black lettering, such as a speed limit sign, is what type of sign?

White rectangular signs with black (and sometimes red) lettering are regulatory signs — they state traffic laws and regulations that drivers must obey, such as speed limits, lane-use rules, turn restrictions, and one-way designations. Disobeying a regulatory sign is a traffic violation. This contrasts with yellow warning signs (cautioning about hazards) and green guide signs (providing directions). The speed limit sign is the most familiar regulatory sign. Recognizing that white-and-black rectangular signs convey mandatory rules — not mere suggestions or warnings — is a key part of reading the road-sign system correctly.