Plumbing is an applied discipline where you do stuff within a protocol.
Most public works and building is template driven with standardized components. Highway bridges pretty much look the same if they were built at the same time. There’s probably 10-15 guys in the average state who periodically update the standards. 80% of the rest of the work is drudgery done by overworked, underpaid assistants and non-professional engineers, signed off on by a PE later.
What PEs do accomplish is making it outrageously expensive to perform basic public works that were accomplished successfully by a DPW guy with a paint roller 40 years ago. Painting a crosswalk and installing a traffic light costs nearly $1M, because you need to hire an engineering firm to duplicate a template, and sign off on placement of lines, etc.