A detour is a longer, less direct way to get where you're going. A traffic accident sometimes means you have to take a detour to get home.
When signs direct drivers to take a detour, it's usually because of road work or an emergency that's blocking traffic. You can also detour, or take an unexpected route, while you're running errands, stopping at your favorite bakery for a cookie before you continue on. The French détour comes from the verb destorner, "turn aside."