How self-driving cars will ease traffic congestion

If you would like further insight on what philosopher Jean Paul Sartre meant when he wrote: "Hell is other people," try flying over Highway 401 at rush hour.

Soaring above the grey stripe of Canada's busiest highway, you will see how the intricacies of human behaviour confound highway planners and traffic engineers.

For a while, all goes well: cars flow efficiently through the express and collector lanes like electrons through a massive cable. Then a driver drops his cell phone and swerves into the adjoining lane, sparking a chain reaction that stops traffic two kilometres behind. Minutes later, a minivan driver, distracted by his two-year-old daughter, sideswipes the concrete median and grinds to a stop in a shower of sparks, creating an hour-long delay when thousands of passing cars slow to take in this minor spectacle.

To study traffic congestion is to study human nature, in all of its grand, frustrating imperfection. We tailgate, we speed, and we change lanes without reason. We daydream. We wander. We clog the highway. Yes, we are the problem. And the cure is on its way: the robot car.

Like it or not, the driverless car is the Holy Grail of traffic flow – researchers believe self-driving cars will reduce congestion by as much as 50 per cent.

"The limit isn't the road, it's the driver," says Ron Dembo, CEO of Zerofootprint, a company that specializes in using data to analyze big-picture problems. "There is a physical limitation to what a road can handle, but we're not even close to that point. Doing away with the human driver would have a huge impact."

Exactly when the robot car will take over is unclear, but there's no doubt that the day is coming. The association of Electrical and Electronics Engineers (IEEE) has estimated that by 2040, up to 75 per cent of all vehicles will be autonomous. Some of the technology that will make this possible is already available – driver assist features like radar cruise control, lane assist and blind-spot warning are common. Many major automotive manufacturers (including GM, Ford, Mercedes, Volkswagen, Audi, Nissan, Toyota, BMW, and Volvo) are testing driverless car systems.

One of the best-known autonomous car projects is Google's – the software giant has been operating a small fleet of robot cars on test tracks and public roads for several years (with a human driver along as backup). Although driving enthusiasts hate the idea of giving up control, experts say autonomous cars will yield dramatic improvements in safety and traffic flow.

"If every driver behaved the same, and made consistent, rational decisions, congestion would be dramatically reduced," says Jeff Muttart of Connecticut's Crash Safety Research Centre. "But that's not how humans are. An autonomous car would do better."

Unlike human drivers, a robot car can be programmed to run at a precise speed, maintain an exact distance from the cars ahead, and minimize lane changes. This dramatically increases the number of cars that a given road can accommodate. As researchers like Muttart have discovered, there are wide variations in the capabilities of human drivers, and the performance of a small number can affect thousands of others.

"What causes the problems is aberrant behaviour," says Muttart, who worked as a police officer before turning to a career as an academic and safety researcher. "There's always one driver who goes faster or slower than the rest. When you have varying speeds, or drivers who change lanes too often, risk and congestion go up. And there's a ripple effect – when people in one lane slow down, it causes other lanes to slow down too."

One of the most problematic behaviours human drivers exhibit is inconsistent following distance. Tailgaters often create chain-reaction delays when they are forced to suddenly brake when the car they're following slows down. And drivers who leave excessive room in front of them create an opportunity for others to merge into the space, setting the scene for a chain-reaction delay when someone brakes, forcing following drivers to respond – known as Convoy Effect, this results in a ripple of slowing cars that can stretch back several kilometres. (And the problem is compounded if any of them hit each other.) In theory, this shouldn't happen. But when human drivers are involved, theory means almost nothing: "As long as we follow behind at a distance greater than our response time, theoretically there should never be a crash," says Muttart. "But the system is affected by varying response times and variation in following distance – other drivers change the math."

Dembo says congestion will improve as self-driving cars are introduced, but that the most dramatic change will occur when autonomous vehicles take over completely, eliminating the variables of human drivers.

An all-robot car fleet will allow the introduction of networked control systems that optimize traffic flow by commanding cars to take optimum routes, and letting each car know what other vehicles are doing. (This type of networked traffic system is anything but farfetched, and has already been developed for aviation – the Next Generation Air Transportation System (NexGen) is starting to be phased in across the U.S.) Autonomous cars will also eliminate the impact of "outlier" drivers who drive far above or below the average speed of traffic. As researchers like Muttart have learned, the majority of drivers operate within a narrow speed band – on the highway, almost every car is within 13 km/h of the other cars in the traffic stream.

But the outliers may be 50 km/h or more above or below this average, and have a disproportionate effect on congestion. "For maximum traffic flow, everyone needs to be at the same speed, and maintain consistent separation," says Muttart. "That's the ideal. But it's hard for humans to do that."

Google has studied the impact of poor drivers on road congestion by using what's known as Agent-Based Simulation – computers model traffic on a road system, and determine how flow is affected when a percentage of the drivers engage in behaviours like tailgating, speeding and rapid lane switching. As the research has shown, these drivers have a significant effect, and their impact increases dramatically when traffic volume rises.

I once spent several hours flying above eastern reaches of Highway 401 in my ultralight plane, studying the way traffic ebbed and flowed. Seen from above, the impact of individual drivers was made graphically clear: a slow-moving truck refused to move out of the left lane, creating a long tail of traffic; a speeding Mazda caused a series of slowdowns when other drivers abruptly shifted lanes to make way.

That's when I thought of Sartre's dispiriting quote about the nature of humanity – hell really is other people, especially if they're driving a car on the same busy road as you are.

"The autonomous car will come," says Dembo. "It's inevitable. You're talking about a huge improvement."

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