Well that sounds very annoying.
What a massive design oversight.

 

I am experiencing a similar issue. Most of the time the setpoint resets without my input to the maximum posted limit. The first time it happened was cruise set at 75 mph in 70 zone. Pass below a bridge and it reset to 35 which I think used the brakes to slow. I don't recall seeing a limit sign. I was driving the car from PA back to KS after taking delivery so very new to car. I assumed at the time it was a GPS input to the cruise and it read the limit on the overpass. Later that same day same setpoint and limit and it dropped it to 45 but no overpass. Now I think it read a sign on the frontage road that was close to the highway. Since that time I have not experienced those 30-40 mph drops in the setpoint, but I find it very annoying and sometimes unsafe if it were to do this in traffic. I can't seem to find anything on the internet or Youtube that matches my situation so I joined this forum today. I mentioned it to a MB dealer parts manager when I purchased something and he had not heard anyone describe this situation before. I hope someone can tell me how to turn this feature (or is it a bug?) off.

 

I have the same problem and it's dangerous. Ive had mine go from 75 down to 35 very quickly - I thought I would get rear-ended it slowed down so quickly. I havent found any way to turn the darn thing off and have talked with my dealer about this. I've had a ton of MB over the years with some kind of cruise but this new one has problems. I also don't like driver assist - Lexus and Genesis do a much better job. I keep the driver assist turned off and drive myself since MB wont allow hands off the steering very long before it starts barking.

 

I don't have that sophisticated of a cruise control system, but I imagine there should be a way to turn off speed limit sign monitoring. Check your owner's manual. If it is possible, it will tell you how.

My problem with Distronic on my vehicle is, although I've seen it on other vehicles and in a YouTube video, my GLC's radar sensor is not heated, so when I'm driving in wet snow, it covers the sensor and completely disables the cruise control. The only "fix" is to stop and clean off the sensor, then it takes about 10 minutes or so for the system to re-initialize, only for it to turn off again because the snow packs onto the sensor. Frustrating. Simply frustrating. 😡

 

You are correct that it could be disabled. I finally found a YouTube video produced by MB that told me how to disable the cruise control features I found both annoying and in the case of lowering the setpoint by 50% and applying the brakes dangerous. Still trying to read the 550 page Owner's manual. I am sure what I needed is somewhere in there.

 

This might help: