Well, the first thing you can do is to HODL and do not shake/move your body. I hope the roads do not have bumps. :) Maybe change your position a little bit. Loosen up your belt or open up your pants' button to give some space to your bladder. Switch off the A/C for sure and open the windows to get some fresh air, if that is not too cold.
If you can't HODL @kevinwong then Pee in the bottle. There is no shame in saving up your life. Take care sire. :-) Also, if you can find a toilet, there is no better resort to this situation.