As she was looking around for ideas, Miller said, many young mothers spoke to her of being harassed or made to feel unwelcome when they breast-fed in public. Miller had raised her first son in northern California and said she had never heard such complaints there.
There were enough stories to keep Miller interested in the subject, and she did a survey of mothers, through online groups and women she met.
"I wouldn't say every woman in Pittsburgh has been asked to cover up - that would be totally overblowing it," Miller said. "But there were these stories I would hear that seemed almost like urban legends."
The stories were noteworthy because Pennsylvania has a law guaranteeing women the right to breast-feed in public without harassment, she said.
Miller said it's an odd disconnect because doctors and public health officials encourage breast-feeding. According to the U.S. surgeon general, "One of the most highly effective preventive measures a mother can take to protect the health of her infant and herself is to breast-feed."
Responses from the survey and statistics supported that Pennsylvania was different. According to the Centers for Disease Control and Prevention, 75 percent of women nationally report having breast-fed a child at some time. In Pennsylvania, the average is 63 percent, and in California, 87 percent.
"Jill is especially good at picking up on nuances in American society that we might not pay attention to," said Eric Shiner, director of the Warhol Museum.
Miller, who also teaches in the Carnegie Mellon University School of Art, got help with the project from Tara McElfresh, another Pittsburgh resident she met at a mothering forum.
"I also had moms come in and tell me stories about how they had been harassed," said McElfresh, who became manager of the Milk Truck project.
Miller estimates the project cost about $16,000, most of it from Kickstarter, an online funding platform for artists, inventors, and explorers.
Daisy Klaber Miksch, who runs a business that offers singing and music classes to children and families, recalled the first time she saw the truck.
"It made me smile," she wrote in an e-mail. "What it said, and says, to me is, 'Breasts are nice. Nursing is nice! Here's a friendly reminder. Take a moment to consider changing your negative reaction to a mom who's breast-feeding her kid. Lighten the mood!' "
Some people complained, especially after local newspapers and TV stations did stories on the Milk Truck last year.
One man sent an e-mail saying he could "donate money to your silly truck" or continue to give to the local food bank to help feed hungry children. He chose the food bank.
First displayed at the museum in September, the Milk Truck traveled around town, too, attracting attention with its light blue, red, and pink stripes and dots, pink hubcaps, and the phrase "Feed your baby everywhere" in big letters on the back doors.
The Warhol exhibit has closed, but Miller and McElfresh see new possibilities.
"We're now talking about having like a national tour," Miller said. "It would be like a rock band on a tour bus - but we are the tour bus."