Okay, that I know something about.
Map out the toilets or possible toilets along your route - service stations, cafes etc as well as public toilets. Like the Queen, take every possible opportunity to go to the toilet but don't strain.
The Australian Crohns and Colitis Association issues a card that says:
PLEASE HELP - CAN'T WAIT
Due to a medical condition, our member requires the urgent use of toilet facilities.
Thank you for your consideration.
Do you have anything like that available? It has enabled me to use toilets that are not usually not open to the public.
Not very elegant but invest in some plastic pants and underneath them wear incontinence pants. No-one but you will know that you are wearing them but they will help to build your confidence and reassure you that you can cope, as well as dealing with accidents.
Put something waterproof and preferably disposable on your car seat - old towel (I used to buy, for almost nothing, the ones that they sell as dog towels from the op/charity shop), then waterproof sheet (disposable blue hospital pads are good), then another old towel on top to sit on. If you have an accident, just roll the whole lot up and put in a large plastic bag, tie up and dispose of in a rubbish bin.
Use the next available handicapped toilet to really clean yourself up.
Carry lots of towels, plastic bags, wet ones, changes of clothing - better to have far too much that you don't use than not enough.
As we all know, stress has a very negative effect on our bowels. Being prepared won't stop you having an accident but might reduce the risk of it.