That's what the blue 'max deceleration' border is necessary. At least it makes it easier to see it from further away. Maximum deceleration values of a car are

That's what the blue 'max deceleration' border is necessary. At least it makes it easier to see it from further away. Maximum deceleration values of a car are about up to 1G for normal cars (found the data on an unofficial forum, so it may not be right) Suppose a car in front of you brakes slowly, but for some reason you press harder the pedal, say to a deceleration of -5ms^-2 (1/2 G). Suppose a town with speed limit of 50km/h, and you and all cars drive at max speed. Suppose an initial car separation of 20m. If you do the calculations, you'll see that it will take less than 3 seconds for the back car to crash your car end. 3 secs sounds like a lot in reaction time, but the driver may be distracted, which may make him notice your braking a bit later, but not too late to apply appropriate braking and causing a car on his end.