Often you can't tell if the car in front is stopping or just slowing down, so as we have speedometer, waze could notify wazers behind that it's car speed got from say 100km/h to 20km/h rapidly. This way wazers behind have more caution and can start to brake earlier. Should be optional voice notification and only valid for all wazers 100/300 meters behind (affected by your car's speed)