The promise of automated cars is that they could eliminate human-error accidents and potentially enable more efficient use of roadways. That sounds, at first blush, like self-driving cars could also mean traffic reduction and lower commute times.
But researchers aren't so sure.
Hesham Rakha is an engineering professor at Virginia Tech who studies traffic's flow — or lack thereof.
Part of the fundamental challenge of traffic, he says, is it's extremely easy for human drivers to create traffic. "One little touching of the brake can pull the whole system down," he says.
That's largely because of delayed human reactions. If a driver sees red tail lights and taps his brake, each subsequent driver approaching from behind also has to react, and that delay forces each one to brake harder than the next. Soon, you're at a standstill for seemingly no reason.
The most minor disturbance like that can cost 10 to 20 percent less flow, Rakha says. "If we can prevent that congestion from happening, not only will we let vehicles travel faster, but we will also get more vehicles through our system."
To demonstrate how automation might affect traffic, I met Rakha at a parking structure overlooking the intersection of Interstate 66 and the Dulles Toll Road just outside Washington, D.C., which is a well-known bottleneck that merges four lanes of traffic into two.
Rakha has already collected traffic data from sensors on the road and designed an algorithm to calculate how real traffic might change as more self-driving cars hit the road.
Automation allows real-time traffic information to help cars anticipate what's ahead, then slow down to avoid disruptions, the way the right flow of sand won't overflow a funnel. It also helps that there is no reaction time in an automated car.
Self-driving cars become synchronized. They communicate with one another, signaling disturbances ahead, and adjust to the optimal speed to not create a backup.
In Rakha's simulation, as the number of self-driving cars increases, we can see how the red dots indicating congestion on the screen gradually clears and become green, moving dots.
Geoff Wardle, director of transportation design at the ArtCenter College of Design, says he expects automated cars will lead to more ride- or car-sharing; or, they will park themselves. Either way, he says, "a significant amount of traffic congestion is caused by going around and around the same block looking for a place to park."
But automation is not a commuter's panacea, either. The reason, ironically, comes down to human beings.
"We tend to like to try to cheat the system to gain personal advantage," Wardle says. Basically the problem is this: Drivers might cut off driverless cars, screwing up their flow.
In fact, Rakha's research shows it might take a while to see any improvements in traffic. That's because while the behavior of automated cars is predictable, that of humans is not. So it's not until enough driverless cars are on the road that we might see some easing of congestion.
In the end, Rakha tells me he cannot predict whether automated cars will improve traffic congestion.
"I don't know the answer," he says. "If the road is less congested, more people are going to be attracted to that road, and so basically it will become congested because it's supply and demand."
If traffic does improve, more cars and trucks are likely to use the roads, and we could be back where we started.
DAVID GREENE, HOST:
All right, here's something that will come as no surprise to anyone who drives to work. Maybe you're driving to work right now. Commuting times are just getting longer and longer. The Census Bureau says the average commute takes 26 minutes. That is 20 percent longer than in 1980. NPR's Yuki Noguchi is asking if self-driving cars, which might improve traffic flow, could shorten her commute.
YUKI NOGUCHI, BYLINE: I'm standing here watching rush hour build. I'm overlooking the place where Interstate 66 and the Dulles Toll Road merge just outside Washington, D.C. It's here I meet Hesham Rakha, an engineering professor at Virginia Tech with a lifelong interest in traffic - how it flows and why it sometimes doesn't.
HESHAM RAKHA: Yeah, that's 66. It's 66.
NOGUCHI: It is, he says, extremely easy for human drivers to create traffic jams.
RAKHA: One little touching of the brake can pull the whole system down.
NOGUCHI: A driver taps his brake, subsequent drivers react, each braking harder than the next. Soon, traffic is crawling for no apparent reason.
RAKHA: So if we can prevent that congestion from happening, not only will we let vehicles travel faster, but we'll also get more vehicles through our system.
NOGUCHI: Self-driving cars can synchronize. They communicate with one another, signaling disturbances ahead, avoiding backups. To illustrate how autonomous cars flow differently, Rakha pulls up a traffic simulator on his laptop.
RAKHA: This is what we call a fundamental diagram.
NOGUCHI: The simulator uses recent actual traffic data collected from the highway merge below us.
RAKHA: And the red is the congestion which spills back from this.
NOGUCHI: An algorithm then allows Rakha to simulate how the colored dots flow when some or all of the cars are autonomous. The biggest bugaboo of traffic - accidents caused by human error - go down dramatically. On the screen and we can see how the red dots indicating congestion gradually clear and become green moving dots.
Geoff Wardle teaches transportation design at the ArtCenter College of Design. He says he expects more people will share automated cars, creating less demand for parking, or the cars might park themselves. Either way...
GEOFF WARDLE: A significant amount of traffic congestion is caused by people going round and round the same two or three blocks looking for that place to park.
NOGUCHI: That's the positive side of self-driving cars. Here's the negative. Again, here's Wardle.
WARDLE: Human beings, we tend to like to try and cheat the system to gain personal advantage.
NOGUCHI: In fact, Virginia Tech engineer Rakha's research shows that once self-driving cars make their way onto our roads, it will take a while to see any improvements in traffic. The behavior of self-driving cars is predictable, but human behavior is not. So it's not until there are a lot of driverless cars on the road that we might see some easing of congestion. Put another way, some humans are jerks who try to cut in at the front of the line. Oh, you know who I'm talking about - Rakha certainly does.
RAKHA: And they'll cut across all the lanes and cause everyone delay just because they want to save a couple of seconds. They break down the whole system.
NOGUCHI: When people ask you so are automated cars going to reduce my commute time, what do you tell them?
RAKHA: I don't know the answer. If the road is less congested, more people are going to be attracted to that road and so basically will become congested because it's supply and demand.
NOGUCHI: If traffic does improve, in other words, more cars and trucks are likely to use the road. And for us right now...
Maybe we should get out of here before traffic gets bad.
RAKHA: Yeah, I think it's a good idea.
NOGUCHI: Yuki Noguchi, NPR News, Washington.
(SOUNDBITE OF REKI SONG, "THE ORIGIN") Transcript provided by NPR, Copyright NPR.