I have been at this point before.  The doctors at first used Cardura and slowly increased the dosage.  Then Flomax was added.  I am better so I only take Flomax, but I take it twice a day now.   Why does it happen when riding in a car?  Vibrations stir up the nerves that have been damaged.  Sitting in a vibrating chair for a long enough period of time will do this as well. The nerves are very frayed and the constant vibration sitrs up those nerves to no end.   I finally won my battle after taking medication for a while.  I also took Ditropan for a while.  It only made it worse.  After I quit taking the Ditropan, the doctor also had be go pee every two hours and then pee again 10 minutes later.   The reason for this was that I was not emptying the bladder due to the Ditropan.  The bladder was retaining more urine than most people can hold.  When I wnet to pee I was just "topping off the bladder" so to speak.  But when I got in a car the vibrations stirred up those nerves and even though I had just peeed, all that urine that shouldn't be there came out eitehr in little squirts and sometimes a water fall.   Now that I only retain four oz. of urine, I have to go twice as often as most people, The bladder over time returned to the proper size.   I came very close to haivng to use a cathetor the rest of my life.  Excellent doctors who knew out to treat my problem saved me from that.   I haven't had an accident in several years, though I go from not needing to go to needing to go very bad and then can't hold it, if I don't remember to go regularly.