Mercedes-Benz A-Class Operator's Manual page 482

480 Display messages and warning/indicator lamps
Driving systems
Warning/indicator lamp
Possible causes/consequences and
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The red distance warning lamp lights up while the vehicle is in motion.
The distance to the vehicle in front is too small for the speed selected.
If there is an additional warning tone, you are approaching an obstacle at too high a speed.
Distance warning lamp
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Active Brake Assist .
Be prepared to brake immediately.
Increase the distance.
M M Solutions