This is not about peeing but pooping, but I guess it works the same, assume if you are not restricted on a car. A few months ago, I was on the travel bus heading somewhere far, I had this sudden abdominal cramping as if the feces were going to break through my anus within seconds, however the next pick-up bus station with toilet is still far away and i had to suffer for quite a while. Anyway after like 30 minutes, finally the bus stopped for toilet time and i rushed to the toilet with rocket speed. However, when i got inside the toilet, i didnt feel like pooping already, it was pretty weird at that time but of course it didnt last for long, very soon the feces came knocking door again. Now that i think of it, it totally makes sense, our nervous system is divided into central nervous system and autonomic nervous system. Autonomic system itself is then divided into sympathetic nervous system which responsible for fight & flight respond and parasympathetic nervous system which play important roles in food digestion, urination and defecation. So as i ran towards toilet with my full power and adrenaline pumped through my blood, all these stress just activated my sympathetic nervous system and shut down my gastrointestinal activity which is controlled by parasympathetic nervous system. As a result, it stopped the intestinal contraction and relieved my abdominal pain (urge to poop) as well. Ergo, if you somehow got caught in a situation which you have to urinate or poop and you can't, try to deceive your body that you were in stress by running around.Just kidding, don't do that on road, safety first!!! LOL...