The shuttles, which run every 15 minutes during Summerfest evenings, allow commuters to park for free at nine Park & Ride lots around the area.
But when Herr and her family returned to the lot, she said it was one of five vehicles blocked in by illegally parked cars.
Herr said she waited more than two hours for MPD officers to finish ticketing the vehicle blocking her path and having it towed away.
"It's just the principal of it," Herr said. "You can't do this sort of thing."
Her son, who has Asperger's Syndrome, was stuck waiting with her.
"I'm glad there wasn't a serious medical emergency at some point. This could have been a lot worse," Herr said.
She said she's not angry at Summerfest or at MCTS, but rather at the drivers who didn't follow the rules.
A spokesperson for Summerfest issued the following statement:
We work closely with the Wisconsin Department of Transportation, Milwaukee County Transit System, and local law enforcements to serve the community by providing transit options to and from Summerfest.
This was an unfortunate situation and we appreciate the frustration of the patron being unable to exit the Park and Ride lot, due to illegal parking. We encourage all of our guests to abide by the rules posted in the lots so everyone attending the festival can enjoy their experience.