Shall, Should & May
As used in the MUTCD, the words “shall,” “should,” and “may” have the following standard meanings:
A mandatory condition. Requirements having “shall” stipulations are mandatory. No discretion in following them is allowed. Items marked as “shall” are typically included as a STANDARD in the MUTCD.
An advisory condition. Where “should” is used, the suggestion is recommended, and normally is to be followed, but is not mandatory. Deviation from such provision is permissible if, and to the extent that, there is justifiable cause to do so. The reasons for any deviation should be documented and filed for future reference. Items marked as “should” are typically included as GUIDANCE in the MUTCD.
A permissive condition. No requirement for design or application is intended. Items marked as “may” are typically included as an OPTION in the MUTCD.