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A research note from the University of Michigan Transportation Research Institute (UMTRI) has poured cold water on the concept that autonomous cars would lead to a reduction in fuel consumption.

The study, from UMTRI researchers Brandon Schoettle and Michael Sivak, weighs up the energy-saving pros and cons of self-driving vehicles. In theory, if cars were driverless then there would be fewer of them on the road, and would conserve fuel through more eco-friendly driving standards; increasing accelerator speed from 55mph (90km/h) to 75mph (120km/h) can raise fuel consumption levels by up to 20%, for instance.

But even taking this into account, the increased number of trips made by the autonomous cars as a result will wipe out all the previous good work.

Schoettle and Sivak came to their conclusion by thumbing through US National Household Travel Survey data, which revealed a lack of “trip overlap” between drivers; the idea that partner one shoots off to work in car one while partner two takes the kids to school in car two before going to work, for instance, is something of a misnomer. Nearly 84% of households on an average day had no trips that overlapped or conflicted.

Basing it on the most extreme scenario, self-driving vehicles could cut average ownership rates of vehicles by 43% - from 2.1 vehicles on average to 1.2 – but could also result in a 75% increase in individual vehicle usage, from 11661 to 20406 annual miles per vehicle.

It’s worth noting the survey methodology has imperfections; the researchers say their results represent an “upper-bound approximation” of the maximum effects of autonomous vehicles and energy consumption, due to several unknowns, such as gaps between trips and vehicle-sharing strategies within households.

There are also other factors to consider, such as the impact going forward of services such as Uber, as well as more eco-friendly fuel resources. Schoettle told Bloomberg alternative fuels, such as electricity or hydrogen, would “probably” be part of future autonomous cars.

The University of Michigan is taking connected cars seriously, building a project called M-City which offers a detailed urban test area for driverless and connected vehicles. You can read more about the UMTRI study here.