It's hard to communicate with a gesture or ???.?????.???.?????.??????nod at a passing car when there's no one at the wheel.

When it comes to driverless cars, this is a bigger, and potentially more life-threatening, issue than it may seem. Road safety relies on all types of communication between people and vehicles, and most importantly on a psychologist-coined term, "communicating intent." 

As pedestrians and fellow drivers we're constantly indicating what we're about to do, whether that's with our words, body positions, or hand gestures. Studies have shownthat up to 84 percent of pedestrians attempt to make eye contact with drivers before crossing in front of them. So now robo-cars need to be programmed to show what they're planning to do — and pedestrians will need alternatives to smiles, glances, and frantic hand waves.

Self-driving cars also need to be able to communicate with paying customers, otherwise it'll be a slow and messy process just matching passengers with a ride. 

Amazon-owned Zoox is well aware of these driverless limitations as it builds a robo-taxi that will never have a human driver in the front seat. There isn’t even a steering wheel. Instead of retraining the car to function without a driver, Zoox is creating a new driving systemfrom the ground up.

The truly driverless vehicle allows Zoox’s lead product experience manager ​​Riccardo Giraldi to explore how future users and outside pedestrians, cyclists, and other (human) drivers will communicate with each other.

“We’re making sure we make an experience that is mindful of everybody,” Giraldi said about the development process.

With today’s human-driven vehicles, we use light, sound (think of all the honking on the road), hand and head gestures, eye contact, and every road rager’s favorite tool, voice, to communicate while in a car. Giraldi wants to build “something better than what we are used to today,” but isn’t completely unfamiliar.

He's experimenting with sound and light cues to notify those around the vehicle what the Zoox vehicle is planning to do. He said the Zoox car is a new entity on the road — it's not a person, bicycle, or the usual car or even the usual self-driving car with a safety driver up front — so it will need to gain everyone's trust. That starts with interacting and communicating without forcing everyone to learn a new language just to cross the street.

He talked about creating a "natural interaction" that's intuitive, "so you don’t have to learn how to interact with it." So instead of learning a new color-coded light system or figuring out where to even look on the car, Zoox wants to subtly convey information through the blinkers or with a simple sound. The colored lights on a Zoox car are similar to turn signals and brake lights, but can produce a diverse range of pulses and color combinations that, in theory, could communicate all sorts of things.

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Other self-driving car companies are thinking more about the interactions between robot cars and their potential customers. Waymo already has fully driverless rides available in the Phoenix metro area. Earlier this year it started using "Car ID" to give people a way to quickly spot the self-driving taxi. Now a digital dash displays whatever the user has programmed to light up.

I could type in my initials in the app and then that'd be displayed on the car – or I could honk the vehicle's horn remotely through the app.

When it comes to interactions with passing pedestrians, a Waymo spokesperson said safety and courtesy are being programmed into the Waymo driving platform in the form of "subtle cues" rather than explicit communication. For example, Waymo cars try to give cyclists a wider berth when passing, or pedestrians more time and space at the crosswalk.

The makers of Cruise's self-driving Chevy Bolt EVs and future fully driverless autonomous shuttle, the Origin, are also concerned about interactions with pedestrians.

Brandon Basso, senior director of motion planning and control on Cruise's robotics team, said in a recent phone call that Cruise’s focus is on that same psychological term "communicating intent." He said, "The primary way we signal to other road users is our actions." 

So a Cruise Bolt might do something like moving over to give room to a bicycle coming from behind. That doesn't require any extra signaling, sound, or visual cues. 

Cruise last month started testing fully driverless rides in San Francisco with employees as the first riders (although not without some controversy among local officials). As seen in some of the initial videos from the rides, an interaction within the mobile phone app is Cruise’s solution to the problem of robot-drivers not being able to spot their fares and let them know they’ve arrived. 

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But for passers-by, Basso recommends treating self-driving taxis as cautiously and warily as any other car on the road, especially since Cruise studies real-life interactions between pedestrians, cyclists, and other drivers to determine how it programs its cars to behave. There's no need to act weird and hesitant around a self-driving car.

For other motorists, "bear in mind the Cruise vehicle is designed to comply with the way in which the roadway is set up." That's Basso's way of defending what human drivers might perceive as overly cautious and slow driving. 

It's also why Cruise has "self-driver in training” bumper stickers on all its cars. The short phrase is the clearest way to communicate what it's like to be around a driverless robo-taxi.