If you’re comfortably sitting at a light with your butt on the saddle and your foot on the ground, on a standard geometry road bike, without leaning the bike way over, it’s highly likely your seat is too low.
The fact that the OP’s standard practice is to put a foot down at a light, while seated, and he’s experiencing knee pain suggests a reasonable probability that his seat height is off.
it’s not uncommon for less experienced or knowledgeable cyclists to adjust their seat height precisely for the purpose of putting a foot on the ground while seated, as opposed to for the proper leg extension while pedaling.
If I were the OP, I would check my seat height, or have it checked by someone who knows bike fit.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.