It is assumed that you were originally taking the shortest way possible to get to your destination. If that route is blocked, then any other route is longer. Ie. you will have to take a longer route. Most of the time when someone says something like this, they know the area they are in well, so they already have another route that they know is longer in mind. If they don't know the area, they would probably say that they will have to "go around" the road block.