Congestion duing road construction Traffic congestion is when vehicles travel at slower speeds because there are more vehicles than the road can handle. This makes trip times longer, and increases queueing. This is also known as a traffic jam. Congestion may result from a decrease in capacity, for example accidents on the road or roads being closed. Bad road layouts can also restrict capacity. Increased traffic, for example by many cars leaving a sports stadium at the same time, can also cause congestion.