UMTRI-2015-3
Potential Impact of Self-Driving Vehicles on Household Vehicle Demand and Usage
The trip-scheduling arithmetic behind the claim that automation reduces household car ownership.
- Authors
- Brandon Schoettle · Michael Sivak
- Published
- February 2015
- Pages
- 18
- Format
Paper
The argument
If a vehicle can reposition itself between trips without a driver, a household no longer needs one vehicle per simultaneous journey — it needs one per overlapping journey. The paper turns that observation into arithmetic, using national travel survey data on when household trips actually occur and how often they genuinely conflict in time.
What the data showed
A substantial share of household trips do not overlap at all. Where they do overlap, the conflicts cluster in predictable commuting windows. The authors model the reduction in fleet size available to a household willing to accept some rescheduling, and find it meaningful — enough to change vehicle demand materially if applied broadly, while stopping well short of the single-vehicle-per-household claims circulating at the time.
The constraint the model exposes
The finding depends entirely on trip timing, and trip timing is socially determined. School runs and standard working hours concentrate demand into the same two windows for most households in a given area, which is precisely when self-repositioning is least able to help. The paper is therefore as much about the rigidity of daily schedules as about vehicle technology.
How it has aged
The arithmetic holds; the framing has been overtaken. The paper assumes automation arrives as a feature of privately owned vehicles, and reasons about how many a household would then keep. What happened instead is that driverless capability arrived first as a metered service confined to a handful of metropolitan areas, which turns household fleet sizing into a question about service coverage and price rather than about ownership. The trip-conflict analysis remains the right tool; the ownership question it was pointed at is not yet the live one.
How the paper has been used
This is the analytical backbone of a large body of subsequent argument about urban car ownership, congestion and parking demand. Its trip-conflict method — establish when household journeys actually overlap, then compute the fleet size that overlap requires — has been reused repeatedly, often without attribution and sometimes without the constraints the original was careful to state.
What it got right
The arithmetic, and the limitation the arithmetic exposes. A substantial share of household trips do not overlap at all, so self-repositioning genuinely could reduce household fleets. But the conflicts that remain cluster in the commuting and school-run windows, which are precisely the periods when a shared vehicle is least able to help. The paper is therefore as much a finding about the rigidity of daily schedules as about vehicle technology, and that framing has aged extremely well.
What it did not anticipate
Its central assumption is that automation arrives as a feature of vehicles households own. What shipped first was a metered service, operated by fleets, confined to a handful of metropolitan areas. That inverts the question: instead of asking how many vehicles a household keeps, the live question is what service coverage and price would make keeping none viable. The trip-conflict analysis remains the right tool for answering it — the vehicle simply belongs to somebody else, which changes the economics considerably and the scheduling constraints not at all.
Citation
Brandon Schoettle and Michael Sivak. Potential Impact of Self-Driving Vehicles on Household Vehicle Demand and Usage. University of Michigan Transportation Research Institute, report UMTRI-2015-3, February 2015.
The PDF is served at /wp-content/uploads/2015/02/UMTRI-2015-3.pdf, the path it has occupied since publication. Other papers in the same series and the wider public literature are indexed in the report library.
Reference
About this paper
Who wrote UMTRI-2015-3?
Is this the original PDF?
/wp-content/uploads/2015/02/UMTRI-2015-3.pdf, the path it has occupied since publication in February 2015. Those exact addresses are cited in published academic and trade work, so moving them would break citations that have been resolving for a decade.