The signs were hand-painted with tempera.
A metafilter answer gave me the clue--those signs are banner signs.
At that point, I could use google efficiently.
Here's someone who still paints the signs.
Ask Metafilter is the website to go to if you have a question which can't be easily answered with Google. You can answer questions for free. It takes $5 and a week's wait to start posting questions. And if you want random information going past your eyes, poking around the questions and answers is at least as good a time-killer as Digg or Reddit.
Clearview (the improved-for-legibility road sign font) isn't cheaply available, but hobbyists have recreated it.