Delivering packages with drones can reduce carbon dioxide emissions in certain circumstances as compared to truck deliveries, a study from University of Washington transportation engineers finds.

Researchers say that drones tend to have carbon dioxide emissions advantages over trucks when the drones don’t have to fly very far to their destinations or when a delivery route has few recipients.

Trucks — which can offer environmental benefits by carrying everything from clothes to appliances to furniture in a single trip — become a more climate-friendly alternative when a delivery route has many stops or is farther away from a central warehouse.

For small, light packages drones compete especially well. But the carbon benefits erode as the weight of a package increases, because these unmanned aerial vehicles have to use additional energy to stay aloft with a heavy load.

“Flight is so much more energy-intensive — getting yourself airborne takes a huge amount of effort. So I initially thought there was no way drones could compete with trucks on carbon dioxide emissions,” says senior author Anne Goodchild, a UW associate professor of civil and environmental engineering. “In the end, I was amazed at how energy-efficient drones are in some contexts. Trucks compete better on heavier loads, but for really light packages, drones are awesome.”

The analysis compares carbon dioxide emissions and vehicle miles traveled from drone and truck deliveries in 10 different scenarios in Los Angeles. The model incorporated 330 different service zones, with the number of recipients varying from 50 to 500 in each zone.

The researchers relied on models for estimating truck data that were previously used in comparing the environmental footprint of grocery delivery services to personal shopping trips. The analysis also assumed that drones could carry only one package at a time and would return to a depot after each delivery — requiring far more back-and-forth and vehicle miles traveled than for an equivalent truck route.

The researchers estimated how much energy generation the drone deliveries would require, based on consumption for 10 different hypothetical drones. Carbon dioxide emissions were calculated using an average fuel mix for the state of California.