If trips that require a car are prohibitively expensive (in money, time or convenience) without owning a car, more people will own a car. Once you own a car, it's often much easier to use it for trips that you would otherwise do without a car.
Reducing the (perceived) need to buy a car, e.g. by making it easy, cheap and reliable to get from A to B using a self driving car service, will reduce the number of people who own a car and thus the number of car trips.
Reducing the (perceived) need to buy a car, e.g. by making it easy, cheap and reliable to get from A to B using a self driving car service, will reduce the number of people who own a car and thus the number of car trips.