Signs communicate laws all drivers must understand and obey.

The answer is regulatory signs. The term regulatory sign defines a range of signs that are used to specify or emphasize traffic laws, rules or requirements which apply either at all times or at indicated times or places upon a street or highway, the neglect of which may constitute a violation, or signs in general that regulate public behavior in places open to the public. Traffic signs are the best example for this kind of signs. It is intended to instruct road users on what they must or should do (or not do) under a certain set of conditions. Other types may be signs positioned on streets and in parking lots having to do with parking, signs in public parks and on beaches or on or in architectural facilities prohibiting specific types of activities. 

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