My name is Jonathan and I'm editor of the Chicago Dispatcher. I looked into your question and this is what I found.
I spoke with PACE and they told me you can cash in your vouchers at 547 W. Jackson. They said you can do this Monday through Friday, 10 a.m. to 11 a.m., or 2 p.m. to 3 p.m. Just bring your vouchers to the front desk between those times and they can help you.
Also, I spoke with another independent driver who said he believed the smaller affiliations will cash vouchers from non-members. Negus Solomon at 24 Seven Taxi told me he thought his affiliation could help you, but you'd need to bring in an insurance form. Perhaps you could call him or one of the other smaller affiliations to see if they can help, if you want to go that route.
