I used to live in a city that had about 10 dwellings/acre. It covered 45 sq mi. Just about anything I wanted to buy was within 10 miles. I worked for the local transit authority and knew how to get pretty close to wherever I wanted to go without using a car. I rarely used the bus unless I was going where I had to pay to park. Therein lies a conservation incentive rarely talked about which is what I would call a parking space tax. People drive because there is no charge for parking at Walmart and the malls. The cost of parking along with available public transit is why car use is much lower in NYC. Add a $5 parking charge to every purchase at Walmart and people will take fewer trips but purchase more per trip. Use the money to eliminate bus fares and improve the service and watch gasoline use go down.
I now live at the edge of a small Iowa town 30 miles from the nearest Walmart. It currently costs me about $5 worth of gas to make the round trip a few times per month. The population density around here is 1/100th what it is in the city I used to live in. Public transit is a twice a day stop by a bus that goes between Des Moines and Kansas City. The 20% who do not live in urban areas must continue to use cars. We may find ways to use them less but we must still use them.