20 days isn't really enough to judge. If you didn't own a bike at the start of those 20 days, could you really get a bike and all the clothes, safety gear, etc. you need and get used to biking before the end of the experiment? If you're using public transit, can you really learn the routes and schedules for the places you need to go in just 20 days?

Also, assuming these people all owned cars, they were still essentially paying for their cars the whole time. They might not have paid for gas, and the wear and tear would have been very slightly less, but any car loan they had still had to be repaid on schedule. If they rented a monthly parking pass or something, that would have to be paid. Not only that, but when you don't own a car, you tend to make different decisions on where to live, and sometimes where to work too. So, they're living in a place that's car friendly (and maybe not public transit friendly).

I would bet that if you took someone who didn't own a car and intentionally lived next to a major transit hub and asked them to get around by car for 20 days, they wouldn't like it either. They wouldn't have a place to park at home, rush hour traffic would probably be extremely stressful for someone who didn't do it every day, and so-on.

What this really needs is something like what you get in one of those "wife swap" TV shows. Someone goes to live in a completely different place with people who live very different lives. Instead of living in the sprawling suburbs and getting around everywhere by car, you now live downtown in a high-rise right near a great public transit location. In addition, calculate how much someone would save without a car, and give that to them as a cash payment every day/week so they understand that positive side of owning a car as well.