First of all, be sure you know how YOU respond to Imodium, since some experience little effect, while others get completely constipated from a single pill! So, taking a survey won't help you there.
If you already are accustomed to it, then anytime within a few hours before you leave is fine. It begins to have an effect within 30 minutes or so, peaks around 4-5 hours, and still has an effect 24 hours later.
I usually take extra while traveling, mostly to be sure I don't have any panicky episodes (hasn't happened, but I tend to worry about it more than I should- I guess a life-time of UC...). Also, it is more aboutso eating foods of a new region without too much concern about it affecting my gut function too much.
For long car rides, my husband usually needs to stop to pee before I need to empty my pouch. Plus, it is good to get out and stretch every few hours. So, we usually stop at the rest stops and for meals.
Jan