The cost of false positives at freeway speeds is part of the reason: > When a car is moving at low speeds, slamming on the brakes isn't a big risk. A car trave

The cost of false positives at freeway speeds is part of the reason: > When a car is moving at low speeds, slamming on the brakes isn't a big risk. A car traveling at 20mph can afford to wait until an object is quite close before slamming on the brakes, making unnecessary stops unlikely. Short stopping distances also mean that a car slamming on the brakes at 20mph is unlikely to get rear-ended. But the calculation changes for a car traveling at 70mph. In this case, preventing a crash requires slamming on the brakes while the car is still far away from a potential obstacle. That makes it more likely that the car will misunderstand the situation—for example, wrongly interpreting an object that's merely near the road as being in the road. Sudden braking at high speed can startle the driver, leading to erratic driving behavior. And it also creates a danger that the car behind won't stop in time, leading to a rear-end collision. When training new human drivers, they're taught to to steer around obstacles at freeway speeds, instead of braking for them. This is partly because it can take too long to brake. The other half is these are often separate modules calling each other. > And like adaptive cruise control, automatic emergency braking is often implemented as a separate system from the lane-keeping module. Most AEB systems lack the kind of sophisticated situational awareness a fully self-driving system would have. That means it may not be able to tell if an object 100 meters ahead is in the current travel lane or the next lane over—and whether it's a temporarily stopped car, a pedestrian, or a bag of garbage. The auto-braking system could be a basic distance sensor calling the drive-by-wire API with "FULL STOP". This would definitely be non-ideal for freeway situations and speeds. "If you're at lower speeds, at 30mph, and it detects a stationary object, these systems will generally respond and slow the car down and bring it to a stop," Abuelsamid told us. "When closing speed is above about 50mph, if it sees a stationary car, it's going to ignore that." Indeed, automated braking that can only apply 100% braking is not ideal at freeway speeds. Collision avoidance at that speed must depend more on steering than stopping, but systems in cars aren't integrated in a way that would allow them to create this level of awareness.