Connected cruise control uses vehicle-to-vehicle communication to let automated vehicles respond to multiple cars at a time in an effort to save energy and improve safety. University of Michigan researchers have demonstrated its effectiveness on public roads, even when just one automated vehicle is moving among human-driven cars.

Vehicle-to-vehicle communication, or V2V, refers to the ability of cars to wirelessly share data including their speed and position in real time. Connected cruise control can adjust a vehicle's speed based on information obtained through V2V. It's different from adaptive cruise control in that it tracks more vehicles than just the car in front of it.

The tests on public roads have shown how connected cruise control and V2V between automated and conventional cars performs in a common traffic scenario – a chain-reaction braking and re-accelerating caused by one car at the head of several others. An automated vehicle utilizing connected cruise control was able to brake with 60 percent less of the G-force required by a car with a human driver.

And that smoother transition from braking to accelerating improved energy efficiency by as much as 19 percent for the automated vehicle equipped with V2V. It also surpassed the performance of other automated vehicles operating without V2V.

"Automated cars utilizing V2V data will not only perform better, but they can also foster a friendlier environment where few safety hazards sneak into traffic and higher efficiency is possible for all cars on the road," said Gabor Orosz, a U-M associate professor of mechanical engineering who led the research.

Automated cars are coming, but they will face many challenges when sharing the roads with human-driven vehicles. On-board sensors cannot see around corners or see through buses and trucks. If a car suddenly appears within the sensors' view, the automated car has little time to respond and may need to brake hard to avoid a potential collision – just as a human driver would do.

Similarly, if a vehicle a few cars ahead triggers a cascade of braking, on-board sensors only tell the automated car to respond when the car immediately ahead hits the brakes. Not seeing beyond the direct line of sight means lots of surprises to deal with in driving.

The research group carried out a series of experiments on public roads in Southeast Michigan, where the automated vehicle received motion information from up to six human-driven vehicles ahead.

In the experiments, Orosz's group recorded scenarios where braking got increasingly more severe while cascading along a chain of human-driven vehicles. When the speed decreased from 55 mph to almost zero and then reached 55 again, some humans decelerated heavily up to 0.8 G, sending anything not buckled down flying toward the windshield. However, the V2V-based automated driving algorithm maintained a steadier speed profile, gliding through the ripples of rapidly changing traffic. The deceleration of the automated vehicle was kept less than 0.3 G, not spilling a drop from a full cup of coffee.