The average car is an idle resource 92% of the time.
You had a brainwave. Mary's car next door sits in her driveway most of the day, while your family is feeling the need for a second car. Why not rent Mary's car occasionally? Your family avoids a big outlay, and Mary enjoys finding rental payments in her glove compartment.
Great idea! Until Mary's insurance agent says her car isn't covered if it's rented. That kills your scheme.
But not if you live in - where else? - California (Oregon soon). The California Personal Vehicle Sharing law prevents insurance companies reclassifying shared cars as 'for-hire,' and specifically encourages car-sharing companies to help drivers find a rental in the neighborhood (not just from Mary) any time of day or night. Check out Getaround and City CarShare which provide low-cost insurance and, like a dating service, have systems to both connect and protect participants.
It's another example of collaborative consumption and neighborhood culture. Personal Vehicle Sharing is like your plan, but with dozens of Marys. It's like ZipCar, but with cars that belong to the participants.
The average cost of owning and operating a Honda Civic is well north of $5,000 a year and rising rapidly. (A Ford Expedition is $13,000+.) So a convenient rental system - that terrific idea of yours - will save you and your neighbors a lot of money. Look for one coming soon to your neighborhood. If its's not on the horizon, ask your state representative to make it happen.