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MIT study: Driverless truck platoons will save fuel and money, especially if they tailgate

MIT study: Driverless truck platoons will save fuel and money, especially if they tailgate

Autonomously driven trucks can save the cost of drivers. Assembling into platoons (what used to be called convoys) that depart at regular intervals raises scheduling efficiency. Truck platoons that are closely bunched for aerodynamic efficiency can save up to 20% on fuel costs. This is all according to new research from Massachusetts Institute of Technology.

The more trucks in the convoy, the better the efficiency — because the first and last don’t garner the full effects of drafting. MIT found it’s most efficient to have a flock of trucks leave the marshaling yard at fixed times, or dispatch once a minimum number of trucks have shown up. The research is about mathematical models, not about how civilian drivers on the interstate like the idea of a wall of trucks in the right lane.

Source: Freightliner

Source: Freightliner

Learn from flocks of birds, or drafting bike racers

Traveling in close proximity boosts fuel efficiency. “Ride-sharing and truck platooning, and even flocking birds and [fighter jet] formation flight, are similar problems from a systems point of view,” says MIT professor Sertac Karaman, also with bicyclists or race drivers drafting in a pack. He adds, “People who study these systems only look at efficiency metrics like delay and throughput. We look at those same metrics, versus sustainability such as cost, energy, and environmental impact. This line of research might really turn transportation on its head.”

MIT was also concerned with the costs of vehicles waiting around before heading out, and similarly the cost and annoyance of passengers in ride sharing services or passenger buses waiting to depart. In a session this week in San Francisco at the International Workshop on the Algorithmic Foundations of Robotics, Karaman and colleagues compared regular interval departures with a feedback policy, where vehicles gather and then depart only once a minimum number of vehicles are gathered.

World Premiere Freightliner Inspiration Truck

Simple policies work best

What the MIT research team found was this: “The simplest policies incurred the least delays while saving the most fuel.” That is, deploying platoons of trucks at fixed departure times was more sustainable and efficient than staggered interval departures. At the same time, waiting for the same number of trucks to convoy was more efficient than varying the size of the convoy.

Between the two, fixed departure time versus feedback scenarios, the feedback policies saved about 5% more in fuel. “You’d think a more complicated scheme would save more energy and time,” Karaman said. “But … in the long run, it’s the simpler policies that help you.”

Truck convoy mpg

Pack the trucks closely for aerodynamic efficiency

Karaman is working with trucking companies in Brazil to set up models for efficiency transportation. The not-yet-final model suggests trucks follow closely at 3-4 meters (10-13 feet) to maximize aerodynamic savings. Since the model for human driven cars is 3 seconds between vehicles, or 264 feet at 60 mph, and even the most skilled human can’t maintain a 10-foot gap for sustained periods, this means the vehicles will be autonomous.

According to MIT, previous research shows: “Scientists have previously calculated that if several trucks were to drive just a few meters apart, one behind the other, those in the middle should experience less drag, saving fuel by as much as 20%, while the last truck should save 15% — or slightly less, due to air currents that drag behind. Assuming this and the lead truck gaining 10%, a five-truck convoy would save 17% on fuel compared to single trucks, eight would see an 18% gain, and only at 15 trucks would efficiency reach 19%.” In other words, there’s a diminishing return after convoys of about 10 trucks.

Not studied: How others on the highway see convoys

Studies can’t study everything. Here, there’s nothing about the reaction of other motorists who aren’t part of the convoy. It’s likely some — a lot, maybe — other drivers will have qualms initially about sharing the road with self-driving vehicles. Forget for a moment that the safer vehicles on the road may be the ones driven autonomously. They’ll have routines for self-driving, and also for how to get off the road safely if there’s a malfunction or flat tire. It may be that self-driving trucks would start with a driver in each truck (who sits back once it’s on the interstate), shifting over time to a lead driver only, and then doing away with the lead driver entirely.

If the trucks are loosely spaced, so there’s room for human-driven cars to ride between the trucks if desired, that’s one thing. But to have a solid wall of trucks stretching for several hundred yards might be even more disconcerting. (A 10-truck convoy with a fuel-saving spacing of 10 feet would stretch about 750 feet, 0r 0.15 miles.) MIT’s research examples talked about a handful of trucks in a platoon.

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