The technology automatically applies brakes to prevent or mitigate collisions, rather than waiting for the driver to act. It's the most important safety technology available today that's not already required in cars.     

Such systems should be standard in all new cars, says the National Highway Traffic Safety Administration. But instead of mandating it, the government is trying to work out a voluntary agreement with automakers in hopes of getting it in cars more quickly.

But safety advocates say voluntary agreements aren't enforceable and are likely to contain weaker standards and longer timelines than if the government had issued rules.

There are about 1.7 million rear-end crashes a year in the US, killing more than 200 people, injuring 400,000 others and costing about USD 47 billion annually. More than half of those crashes could be avoided or mitigated by automatic braking or systems that warn drivers of an impending collision, NHTSA estimated.