NHTSA complaint #11732594

Owner complaint summary

The issue is regarding the “Traffic Sign Assist” (Owner’s Manual page 221) feature of the Adaptive Cruise Control System. Vehicle will be made available if requested. The function of the "Traffic Sign Assist" is to monitor roadside "Speed Limit" signs and the cruise control will increase or decrease speed to stay within the permissible legal speed limit. The system worked except occasionally it would change the indicated permissible speed limit on the instrument display when there is no corresponding roadside speed limit sign and the permissible speed limit for that stretch of roadway is unchanged. When this happens, suddenly and abruptly, the vehicle display would indicate a "phantom" Higher or Lower Speed Limit sign and rapidly and abruptly accelerate or decelerate. Again there were no roadside signs posted indicating a higher or lower speed limit! The risk is potential for rear end collisions from or to other traffic due to the abrupt acceleration or deceleration. when we returned home from a 4556 mile road trip, we took the vehicle to the Mercedes dealer for routine service and mentioned this to the Service Advisor. He indicated that when this defect is reported their solution is to deactivate the system. I don't feel this is adequate since another driver could reactivate the system through the settings. It has not been inspected by anyone except the dealer. There were no warning lamps, messages or other symptoms before the display changed and the vehicle abruptly accelerated or decelerated.