Cleaning out the mailbag: Icy Bridge signs

In the great Commonwealth of Virginia, we have signs with little blue decals (not pictured) that supposedly change color when the bridges freeze.  Not being satisified with a run-of-the-mill answer, I contacted the Dept. of Transportation to explore the mechanism that turns the symbols blue.

Dear Sir or Ma'am: I've always been curious about those yellow diamond bridge road signs. They have reflectors that are supposed to turn blue during freezing conditions to warn of potential ice. How do they work? I have never seen them activate - do they turn blue due to chemical changes in the reflectors? Does a thermometer trigger a strobe in the reflector? Etc. Thank you for clearing up this long-standing mystery!

Please see below about your “mystery.” VDOT does not use this type of sign; however, I have seen some in a few cities. You may want to ask the City Traffic Engineer in the city you are seeing the signs about the device."About fifteen years ago there was a particular company that tried to market such a device. The trouble was, the trigger for changing the color was not only affected by temperature, but to humidity as well. Thus, it would change color at different temperatures on different days based on the humidity. Also, if the sign were in the sun and there were shadows on the bridge, the bridge could reach a potential for freezing long before the color changed in the buttons. To my knowledge, this device never made it past the trials."

Sleep well, gentle reader.