This was news years ago. Cities were adding heaters to lights to solve the problem.
Here's one article from 2009: https://boingboing.net/2009/12/17/led-traffic-lights-d.html
https://products.currentbyge.com/transportation-lighting/led...
(which only uses 30W for the heater, so it's still far more efficient than the old-style ~150W incandescent.
If the Minnesota DOT wasn't aware that this could be a problem, they are incompetent.
It's even a problem with little used incandescent lights... on a street where the light is nearly always green, the red light can still ice/snow over and not have time to melt during brief red intervals in cold weather.
For most climates, designing in a temperature sensor to keep the light a little above freezing would pay back in well under a year of power savings.
Update: Did a quick search for LED traffic balls… looks like about 10 watts when on, and I don't see any of them integrating heaters. Market opportunity! Make LED traffic balls but add a tiny micro controller, thermistor, and a heating element, maybe attached to the existing heat sink and you are set. (Yes, attaching a heater to the heatsink sounds counterproductive, but you will be running it when the heatsink is nearly freezing so the LEDs will still have plenty of thermal differential to dump their waste heat.)