How often do you travel around town where you need to carry more than a backpack would hold, but less than what you can jam in a small car? For me, I could probably use a bike about 90% of the time. I don't always, but it's mostly because I'm lazy.
About once a week, maybe more. Visiting multiple stores (where it's already annoying to lug your prior purchases in with you) or just buying something like toilet paper will quickly achieve this qualification.
Nearly all the grocery chains in the UK let you order online and have the goods delivered.
Even if cars were only used by each person a couple of times a week that's still a massive improvement over commuting everyday. (I also think you're exaggerating, toilet paper can easily fit in a backpack.)