Good question. I've put my tongue across a 9v battery a number of times to check them. Fully charged, it's uncomfortable but bearable. A 33% is pretty big, so I assume that 12V probably hurts enough that you wouldn't want to do it for long. I doubt it'd do much damage if it was brief though.
I looked, and it seems like the resistance of the tongue is between 10k and 1M ohms. Assuming the average value of 100k ohms, 12V is current limited to 0.12mA and 1.44 milliwatts. It takes like 10 watts to run the average LED lightbulb, which makes it about 7000x more powerful. Gene would be okay.
My quick assessment for yellow lights is "can I comfortably stop before the stop bar?" If I'm going to have to hit my brakes harder than I normally would for a stop sign, I'm giving it gas. Quickly making that judgement is actually really intuitive most of the time and has worked great for me over the years.