In general, shield rendering is based on route relations and not on the ref=* tegs on ways. If a way is not a member of any route relations (or none of its relations are understood by the rendering), the map will display a colored lozenge with the way's ref value in the same style as the standard rendering.
The rendering expects data tagged according to the documentation on the wiki and further refined based on discussions on the talk-us mailing list. In particular, two things are worth knowing:
Details about individual networks follow.
Interstates should be tagged with network US:I. US Highways should be tagged with network US:US. Business and downtown interstates should be tagged according to whether they're signed as a loop or a spur, e.g. US:I:Business:Loop or US:I:Downtown:Spur. A few US highways have more than one modifier, like US Business Truck 17. The network modifiers for those may be added in any order; either US:US:Business:Truck or US:US:Truck:Business will work.
State highways should be tagged with US: followed by the two-letter abbreviation for the state, so California's state highways would use the network US:CA. Special cases are listed below.
County highway coverage is still incomplete and there's no documented consensus about their tagging. Most states are using the state abbreviation followed by the full name of the state, so Bergen County in New Jersey uses network US:NJ:Bergen. Special cases are listed below.
Some states have unnumbered routes with their own shields, such as the New Jersey Turnpike. The rendering finds them by looking for their name and (usually) a network corresponding to the state they're in (in case the name is not globally unique). Thus, the New Jersey Turnpike is expected to have network=US:NJ, name=New Jersey Turnpike, and no ref.
Washington DC's one signed road is expected to have a network of US:DC
Historic Route 66 is expected to have network=US:US, name=Historic Route 66, and no ref.
Florida's toll routes are supported with a network of US:FL:Toll. This includes Florida's Turnpike (which should have its name set and should not have a ref).
Georgia has many variant routes with the variant placed directly on the shield (as opposed to a banner above the shield as most other states use). They should be tagged the same as other variant routes, so Georgia State Route 5 Business would use a network of US:GA:Business.
Some counties are supported, though the numbering ranges for each county are only estimates. Information about county numbering would be welcomed. In particular, information about the counties that use letters and numbers would be greatly appreciated.
The Indiana Toll Road is supported.
The Kansas Turnpike is supported.
Kentucky's parkway system is supported under the US:KY:Parkway network. The AA Highway should use the US:KY network with a ref of AA.
Maryland's scenic byways are supported, currently under the US:MD network. Maryland's white-on-green shields are supported for state business routes, but not for US business routes.
Some counties are supported. Most are not, yet. Information about signs and numbering is welcomed.
Some counties are supported. Number ranges have been estimated from data present in OSM.
Missouri's single- or double-lettered supplemental routes are supported under the US:MO:Supplemental network.
Nebraska's recreation routes are supported under the US:NE:Recreation network.
New Jersey's parkways are supported under the US:NJ network.
The 500-series county routes are supported under the US:NJ:CR network. All other county routes are supported under the usual US:NJ:<county name> networks. Feedback on the Bergen County shields would be appreciated.
New York's parkways are supported under the US:NY network. They can either have just their name with no ref tag or use the initials on the sign as the ref (in which case the name tag doesn't matter).
Some counties are supported.
The Ohio Turnpike is supported under the US:OH network.
Oklahoma's turnpikes are supported under the US:OK:Turnpike network.
Pennsylvania's turnpikes are supported under the US:PA:Turnpike network. This includes the Pennsylvania Turnpike.
Pittsburgh's belt system is supported with a network of US:PA:Belt and the colors in the ref tag.
All routes with the US:TN network are rendered as primary routes and US:TN:Secondary routes are rendered as secondary routes. This may be less than ideal, since some routes can go back and forth between primary and secondary signage. Suggestions for improvement are welcomed.
Texas's many types of routes are supported in what is hoped is a straightforward manner. Here are the various network values currently understood by the rendering:
Primary roads are supported under the US:VA network. Secondary roads are expected under the US:VA:Secondary network.
County roads are expected to use the county name in the network, even though it doesn't appear on the signs. Fractional routes should use a forward slash in the ref tag, e.g. 220/7.
Not all counties have shields generated for them yet.
Provincial roads are generally supported with networks of CA: followed by the two-letter abbreviation for the province. Right now, there's no separate network supported for the Trans-Canada Highway; it's expected under the networks of the provinces it passes through.
Right now, only Alberta, British Columbia, Manitoba, and Quebec are supported.
Federal Highways are supported with the network MX:MX.
State roads are not supported, as the project maintainers don't know enough about them. More information is welcome.