If I am correct, the branching factor is the maximum number of successors of any node.When I am applying bidirectional search to a transition graph like this one belowIf 11 is the goal state and I start going backwards, is 10 considered as successor of 5? Even if it do not leads me further to my start state 1?  
Is the 'direction' considered, when determining the branching factor in bidirectional search?
search;branching factors
If I am correct, the branching factor is the maximum number of successors of any nodeYou are correct, they should also be the immediate ones:If 11 is the goal state and I start going backwards, is 10 considered as successor of 5? Even if it do not leads me further to my start state 1?No, there is also a bit of misunderstanding of bidirectional search: In bidirectional search you run 2 simultaneous searches, one forward from the initial state, and another one backwards from the goal(hoping they meet in the middle and save you steps), if actions are reversible ( going from node to node), the successor nodes become predecessors in one search and vice versa, and your goal becomes your initial state, in your case:Reference/sourceArtificial Intelligence: A Modern Approach, by S. Russell and P. Norvig.