For shields, why not just use the shields from Wikimedia Commons? They're designed using real fonts and standards, so they look just like the real thing the vast majority of the time (any error comes from the DOT not following their own published standards).
Because they don't generate the shield with the number in it. I generally grab that and then do some resizing and editing when clarifying planning maps and such. But if you have a link, I'll look at it for the future. I used to use the shield generator that AAroads had; alas, that has gone away.
Sure they do, they're just pre-generated by a bot (or a human editor) and saved to their server. They have a shield for pretty much every extant highway and many decommissioned ones. You just have to download them, and you can have the server resize them to whatever size you want before you download.
How to get shields from Wikimedia Commons:
1) Go to the Wikipedia page for the target route (e.g. "California State Route 1") and click the shield in the infobox at the top right of the page. For California in particular, you can go to the state-detail page (e.g. "Interstate 5 in California" or "U.S. Route 101 in California") to get Caltrans-spec signs.
2) You will be taken to a page with a name like "File:California 1.svg". (Once you've done this for one highway of a type, you can bookmark this page and change the number in the URL to save some time; all highways in the same system generally have a consistent file name scheme.)
3) This page will have links to PNGs in multiple resolutions below the large displayed image, and you can choose whichever you like; right-click the link and save to disk.
4) If you don't see the exact resolution you want, click on one of the options (which one doesn't matter). In the URL bar, change the pixel width (e.g. "230px" to whatever width you like. Height will be calculated automatically. Download to disk and enjoy.
If, for some reason, Wikipedia doesn't have a shield that you need pre-generated (which is rare, especially in the more populous states and states with smaller highway systems), you can make your own. You can download an
SVG template from Commons, which has an editable placeholder text object, already set in the Roadgeek fonts. Download the source SVG by right-clicking and saving the "original file" link on the file page, open it in
Inkscape, change the text, and export to PNG. If this is a real highway that is missing (i.e. it's not a fantasy number), you can also
request a shield be made.
Some toll road shields are not available due to copyright reasons, but those probably wouldn't be in something like Shields Up either.