I received several questions about this post from the other day.
So I googled it. There were many pages of men (and women} driving without pants Most of them were wearing underwear.
Some were evading the law, one at least (a woman) was winning a bet, one thought it was more comfortable to drive in his briefs and several had no explanation.
They even included a quote from Moe, the bartender on The Simpsons to the effect of "you think you're so high and mighty because you've never been stopped for driving without your pants".
I think one municipality made it legal to drive in your underwear but not shoeless.
Aren't you glad you asked?