Source: Texas Transport Institute.
A set of factors have converged to explain the growth in the usage of the automobile in the United States and for most contexts where motorization is occurring, as measured in vehicle-miles:
- Demographic growth is a straightforward factor as the more people, the more potential drivers and thus of vehicles.
- The increase in trip length as well as the number of trips often relates to changes in the urban spatial structure. Particularly, suburbanization involves longer trips.
- A decrease in vehicle occupancy is commonly linked with limited opportunities for carpooling and trip chaining. This leads to a large number of trips taking place as single occupancy.
- In view of limited alternatives or the low convenience of public transit, many will switch to driving. Switching to driving is also the outcome of rising standards of living, where a larger segment of the population reaches an income level where an automobile becomes affordable.
Similar factors apply in other countries, but the proportions would be different. For instance, in developing economies the increase in population and income would be among the most significant factors explaining the growth in vehicle-miles.