That is weird. You'd think calibration and such would be done with the first 5 minutes after starting the car. It's also possible the composition of that snow on the emblem changed during the drive? It was snow (so there's air pockets) when you first started and maybe it melted and formed ice which then triggered the warning? You know what? I've never tried to test it in the snow. Mine's a cruder version than the one i the Gen 4 so if it was triggered for real in icy conditions, I probably would hit the car in front.
I parked my car on a large sheet of ice and then floored the accelerator. The wheels did not spin out of control. They completely gripped the ice and the car slowly accelerated. I was impressed.
We drove through the Black Hills of South Dakota over the weekend during a snow advisory. It was difficult to see lanes at times, but road handling was just fine.
FWIW our third gen rear wiper seems pretty token. The spoiler split, separate lower window and small area swept by the wiper all make for a wiper hardly worthwhile.
If the radar sensor gets coated, then a message will keep flashing on your MFD, over, and over and over and over and over and over and over and over and over and over, telling you to clean the sensor. It is currently the most annoying "feature" imaginable. Also, if you have the parking assist system turned on, I would advise turning it off for the winter. The sensor coat over very easily and quickly and you will get warning messages until you either clean them, or turn the parking assist system off.
Not bothering to sweep off the back window is far too common. My guess is they are lazy. The window-heater worked fine on that trip for us last weekend for keeping it clear.
No, the problem is that snow accumulates on the back window while driving. It's hard to sweep off the back window while you are in the driver's seat unless you have a rear window wiper. This was a problem on my '88 MX6 (no wiper) and on my Prime (no wiper). It was not a problem on my '91 Ford Explorer (rear wiper) or 2004 Prius (rear wiper). It's actually unusual for us around here to have snow that doesn't accumulate on the back window while driving. For that to be the case, it has to be very cold (thus the window is frozen and the snow is dry and light) and it's usually right around freezing temperature when it's snowing because the cloud cover prevents radiative cooling. It's usually only cold around here when the sky is clear and the ground can cool to space. I realize that's not the case in Minnesota.
In snow that accumulates on the car while driving, absolutely. I've been driving in unsafe conditions in my Prime because of this exact problem. I started with clear windows. The sun went down and it started snowing. I turned on the back and front window defrosters, the car was over 70 degrees inside, and snow soon completely covered both my rear window and my side mirrors. I had no way to see what was behind me, and I was driving at night, in traffic, in snow. That is definitely unsafe.
I might be wrong, but I think none of current Toyota sedan lineups have rear wiper. None of sedans we had in the past 30 years had rear wiper, but never felt any more unsafe driving it during snow than driving car with rear wiper. Of course, my PRIME is often stuffed with trash all the way to the roof so that I have no visible rear window even in sunny days.