Traffic is moving fast and freely. You glance down at your phone—just for a second—and then a warning tone alerts you to slam on the brakes. When you look up, you see the rear of the car you nearly plowed into.

That’s the best-case scenario for new vehicle communication technologies that the U.S. Department of Transportation and eight automakers tested in Alameda, California, last week during the last six clinics designed to discover how ordinary drivers from across the United States react to cars that can talk to each other via Wi-Fi and warn drivers of impending collisions.

The DOT is studying whether such systems, known as vehicle-to-vehicle communication, or V2V, can reduce the number of accidents, save lives, and improve traffic flow. Depending on the test results, the agency could begin requiring that automakers install V2V systems in all new cars sometime this decade.

The clinic was held at Alameda Point, a decommissioned naval air station across the bay from San Francisco, where 120 men and women, 20 to 70 years old, got behind the wheels of new cars outfitted with the latest in vehicle-to-vehicle communication and safety gear.

Drivers were accompanied by an observer as they drove in an empty parking lot through a course of traffic cones, where they encountered other cars driven by DOT personnel looking to create the kinds of situations that often lead to traffic accidents—driving in another vehicle’s blind spot, for example, or braking suddenly. An experimental system in each car delivered alerts and warnings by means of alarms, visual effects, and even vibrations in the seat to warn drivers that another car was nearby. The cars also communicated with one another using wireless radios. Ten times each second, every car broadcast 11 data points, including its GPS location, rate of acceleration, brake status, steering-wheel angle, and speed.

Testers watched as the drivers—none of whom had been exposed to V2V before—reacted to bells, beeps, and flashing lights. Later, they interviewed them to find out which features seemed useful.

“These questions try to address the acceptability of the technology,” says Nady Boules, director of General Motors’ Electrical & Controls Integration Lab. “We would like to evaluate different geographical regions and driver characteristics. For example, the clinic in California targeted people who ‘think green’ and like technology in cars. The one in Texas targeted those who would typically use pickup trucks and SUVs.”

While cars will have to broadcast V2V information in a standardized way, automakers will be free to develop their own safety applications and warning indicators. There’s an art to designing these systems, and each of the eight participating manufacturers has done it differently. For example, if you’re preparing to cross an intersection and an approaching car is about to T-bone you from your left, the Acura TL shows an amber warning message above the speedometer. By contrast, a system developed by FOR warns you with a red light that flicks from left to right across your windshield and, to make sure you get the message, also vibrates your seat from left to right.

“We have found the most effective warning strategies are both semantic/acoustic and haptic—typically, a combination of both,” says Brian Lathrop, team leader for human-machine interface at Volkswagen of America’s Electronic Research Laboratory. He says drivers react faster to a vibration or a recorded voice saying “Brake!” than to a light on the dashboard. That may be partly because drivers’ eyes are already taking in a lot of information, while the load on the auditory system is lighter.

Indeed, during the tests, all the systems used sound to signal the most critical alerts, such as warnings of an impending collision. Less important information—say, about a car approaching in the next lane—was typically displayed on the dashboard, windshield, or side-view mirrors. A Kia Sonata tested in the clinic alerted a driver “do not change lanes” by showing a white icon of a nearby car on the navigation display. When a collision was imminent, the icon flashed red and was accompanied by rapid beeping.

With the consumer tests now complete, the project’s next phase is a yearlong field test in Ann Arbor, Michigan, where around 3,000 cars, trucks, and buses will be outfitted with V2V safety systems.

The DOT plans to decide whether to require V2V systems in 2013—and it wants plenty of data to back up its decision. If the agency does mandate that automakers begin installing V2V systems in all new vehicles, the first cars with the technology would arrive in showrooms around 2018. Some automakers are hoping the government will step in, since the more cars are equipped with vehicle-to-vehicle communications, the more effective such systems will be.

“If only a few cars around you have this technology, it’s not going to help much in improving safety,” says GM’s Boules. “A mandate would definitely expedite the rollout.”