This is normal for someone who’s planting his perineum on a giant vibrator. As the vehicle’s vibrations travel to your seat, they stimulate nerves at the base of your penis. This activates your parasympathetic nervous system, directing blood to your penis, says Dr Thomas J. Walsh, 
a urologist at the University of Washington. At the same time, something else is stimulated: your imagination. “When you spark this nervous 
system, you may also activate pleasure sensors that can back-feed and trigger sexual arousal,” Walsh says. If you reach your destination and need to avoid an awkward exit, he suggests giving your inner thigh a firm pinch. Pain triggers nerves that will make your erection evaporate.