The Germans were fortunate in being able to make rapid progress before the rainy season, for Polish mud is notorious.  Even so, some of the roads were almost impassable, and, above, German transport is seen in difficulties on the way to the front in 1940.

The Polish Mud

The Poles hamper the German advance by blowing up bridges on their line of retreat, as shown in this photo.  Unfortunately such measures only afforded temporary relief.

The Poles valiantly slow the Germans

As shown above, the destruction of bridges is one of the first things carried out by a retreating army, for anything that will delay the enemy's advance is of the utmost value.  Above is a bridge over the Kamionka river blown up by the retreating Polish army.