OP, my previous Lexus, MBZ and BMW that each had a "radar cruise control" equivalent to TACC, each operated similar to the way my MS does -- once the speed limit was set, it stayed that way until the driver changed it, or the vehicle was turned off and back on when that vehicle's default went back into effect. I only have my MS today, so I've fairly well adapted to the way it operates except when firmware tweaks are sometimes made. OTOH when I had multiple brand cars at the same time in my garage, the nuances of similar evolving higher-tech capabilities like these were sometimes frustrating at best, especially trying to remember what that particular vehicle would or wouldn't do in emergency situations.
I suppose one could say (or expect) that a Tesla which has TACC and AP could be smarter in regards to MPH as you suggest, however, from a UI and safety perspective, the first questions that came to my mind as I read your initial post was "What if the car can't determine for whatever reason the speed limit for the existing road? Should the speed then just stay as the driver previously set it and the driver assumes the vehicle has changed it (back down or up?) like it would in more normal conditions? Who's problem would that then be, assuming the owner isn't paying attention to very small images/text on the TACC display?"
As much as it pains me to say it from a technology and geekie perspective, having consistency in how vehicles perform driver-assist functions, especially in these early days of newer capabilities and autonomous driving. Every driver, not just enthusiasts (like us) that try to keep up with evolving detail and documented caveats, needs to know what to expect the vehicle will do 100% of the time. It's hard enough to get all drivers to read an owner's manual before they jump behind the wheel, and we know drivers of even MS are not always reading (or comprehending) one-time pop-ups that require driver confirmation to enable specialized functions. Designing or dumbing-down for these (few) is bad for us that would love to push the envelope with more automation, but I believe mfgrs have a tough balancing act as new frontiers are explored, beta code becomes available, out-lying logic is refined, and vehicles are put into the hands of the masses -- many of which are not or don't want to be technically inclined.
Until we have an AI that can take primary driving responsibility away from the driver as our true AutoPilot, IMHO consistency and having a vehicle maintain a driver setting or override like maximum TACC Speed Limit, is best for most drivers today. It's relatively easy to explain, and with less exceptions, easy to remember. Even if it's documented somewhere, it's when the vehicle acts in a way different than the driver expects, when confusion and more accidents happen. Some of the recent MS/MX accidents are examples where the driver thought their vehicle would do something other than it could and did in exception conditions. We each have our POV which is great, but all that to say, I believe it is best for Tesla to not have TACC change the default max speed based on posted or database MPH as it's operating, but it should always reset to the default MPH+offset when the vehicle is turned-off and back on.
