When my MIL in another city was stubborn about walking across a very busy highway to get to a nearby grocery store (she used a walker -- it was dangerous), we were able to find a taxi company that let us purchase gift certificates. Since the money was already spent, she was willing to use them so they wouldn't "go to waste."

You can also send an Uber for someone but I can see how that becomes a burden

I put a taxi company number into both my parents phones. I try to remind them often that it is there. Can you post a business card for a taxi company next to their landline phone? I'm not sure you can prepay, but maybe you can set up an account and be billed at the end of the month.

I am dreading the day we have to take away licenses, and I am so disappointed we don't have real self-driving cars yet.

so ... Op, may not have many choices

Op, you may want to establish a monthly account with a txt company. We use to use Barwood Cab in Bethesda. Your Mom might get use to a specific driver(s) and request them

Is there something like uber but for old people who aren't good at technology (can't do smart phones but can do land lines), have limited english, and it's prepaid? Wants to use for things like going to grocery store and rec center.