Road Boner or, as I've always referred to it, Highway Boner, is much the same as Runner's Boner; it's another one of the naturally occurring erections men get. These usually crop up (pun intended) on long road trips and are caused by the constant vibrations of the tires on the road, transferred up to the car seat through the frame. That constant vibration over time stimulates the prostate - and, now that we know this is the Male G-Spot it makes perfect sense that we'd develop a hard-on because of it.
"On his trip across country, Cletus developed such a case of highway boner he had to stroke one out (several times) along the way."
Even as passengers, we are not immune.
Of course, with today's smoother suspensions it doesn't happen quite as much as it used to. The closer to the ground, tighter suspension vehicles, like sports cars, are more likely to raise your bar on this score. Since we love it when our cocks get hard, maybe that explains men's love for the sports car...
...and stroking behind the wheel