Unless there is anything physically wrong with your bladder and urethra, I think that you may have paruresis--called shy bladder, bashful bladder, or stage fright. There are a lot of people near by. You may not want anyone to hear you pee. (Probably they won't because of the outside noise fromthe bus or plane.)There is the motion of the bus or plane. When you try to urinate and have to steady yourself, you may be tightening your sphincter muscles (male) or simply the sphincter (woman). Xanax or other medications for anxiety may help you relax. Check with your doctor.