____ at the major ports has led planners to develop satellite ports near them.

If there is congestion in a place, the place is extremely crowded and blocked with traffic or people. The problem of congestion is thus essentially a problem of space. It could be got rid of through addition of space (by developing satellite ports). Blockade is slightly different. It is the action that is taken to prevent goods or people from entering or leaving it. The problem here is political - blockades are the creation of strikers or political groups. And the problem could persist even at the newly-developed satellite ports.