|designation that distinguishes a special route from its parent route, identified by an auxiliary plate above or below the route marker|
|Used on these elements|
|Status: de facto|
|Tools for this tag|
This key is used to indicate the designation that distinguishes a special route from its parent route. The modifier is typically identified by an auxiliary plate above or below a route marker. In an MUTCD-compliant country, such as the United States, a special route is identified by the combination of its network=*, ref=*, and modifier=*.
- Distinct network=* tag for a route that has a shield distinct from its parent route, for example:
- modifier=* for a route that is signposted using the parent route's unchanged shield plus an auxiliary plate, for example:
Unfortunately, any data consumer that doesn't understand modifier=* would end up incorrectly handling some routes, such as by losing the distinction between a truck route and its parent route. This phenomenon is known as a "troll tag".
Over the years, modifier=* has remained in use but has become less important, as mappers have tended to always add to network=*, even when there is only an auxiliary plate and no distinction in the route shield. No renderer currently uses modifier=* to affect the displayed shield.
modifier=left and modifier=right have also been used many times in an import of road_marking=solid_stop_lines in several U.S. cities. The meaning of these tags is unclear, but it is probably a nonstandard way to indicate the side of the road.