Autonomous vehicles (completely self-driving, level 4) may reduce the number of vehicles a family needs, but may lead to an increase in total miles driven per vehicle, according to a new analysis by researchers at the University of Michigan Transportation Research Institute.
UMTRI researchers Brandon Schoettle and Michael Sivak examined the 2009 National Household Travel Survey (NHTS) data set, which contains detailed information about each trip made by a person within a selected household, including the exact start and stop times of each trip. They found a general lack of “trip overlap” between drivers within a majority of households based on vehicle sharing. In other words, families rarely use more than one vehicle at a time.
The study is based on sharing of completely self-driving vehicles that employ a “return-to-home” mode, acting as a form of shared family or household vehicle.
To illustrate the self-driving vehicle sharing concept, consider the following example. A household has two drivers, A and B. Driver A requires a vehicle for commuting to work by 8:00 a.m. and home again at 5:00 p.m. Driver B normally runs errands during the day while driver A is at work, returning home after each errand. The level 4 self-driving vehicle “return-to-home” mode would involve the vehicle performing the following basic actions:
- Drop driver A off at work by 8:00 a.m.
- Return home to driver B for an errand from 10:00 a.m. to 11:00 a.m.
- Take driver B on a second errand from 2:00 p.m. to 3:00 p.m.
- Return to work location of driver A by 5:00 p.m. for commute home.—Schoettle and Sivak
In the most extreme scenario, self-driving vehicles could cut average ownership rates of vehicles by 43%—from an average of 2.1 vehicles to 1.2 vehicles per household, the researchers say.
However, the shift could result in a 75% increase in individual vehicle usage—from 11,661 to 20,406 annual miles per vehicle (this increase in mileage does not include additional miles that would be generated during each return-to-home trip).
Schoettle and Sivak found that, on an average day, nearly 84% of households had no trips that overlapped or conflicted. Just under 15% of households had two drivers and less than 2% had three drivers with overlapping trips that created a conflict.
The researchers say their results represent strictly an upper-bound approximation of the maximum possible effects of self-driving vehicles on reductions in household vehicle ownership, given several unknowns: sufficient gaps between trips, acceptance and adoption of autonomous vehicles and possible vehicle-sharing strategies within households.
In the hypothetical scenario outlined in this analysis, the potential reduction in vehicle ownership per household would include a correspondingly large increase in vehicle usage, consequently increasing wear-and-tear and required maintenance frequency, while reducing the average vehicle life span (in total years on the road). Given the current average on-road vehicle age of 11.4 years and the assumption that the underlying scrappage rate are largely functions of overall wear-and- tear and total mileage, then a 75% increase in annual mileage per vehicle could reduce this average on-road age to approximately 6.5 years.
… One possible silver lining to decreased vehicle life span (in years) involves the more rapid introduction of new technology into the on-road fleet, and the consequent benefits for road safety. In the hypothetical scenario we describe in this analysis, with an average turnover rate that is 75% faster for these vehicles, the rate at which new self- driving technology would be replaced or updated in the on-road fleet would nearly double (versus current average vehicle age and annual usage rates).Schoettle and Sivak