Why would a poster be a poor choice of genre for "How to Prepare for a Road Trip"?

Because there's too much information to fit on a poster, and people can't take it with them

Posters are usually not as large and if this type of a poster should have a lot of text that can't fit unless the font is reduced to make it barely readable. In addition, the people need the information at their homes so they can look at it again and check if they've prepared correctly and they can't take the poster home.