This paper proposes a security region of transportation networks

which is defined as the set of flow states (operating points) satisfying the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$N-1$ security in transportation networks. The boundary of the set is closed

inside all the operating points are <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$N-1$ secure; oppositely

outside all the operating points are insecure. First

<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$N-1$ security of the transportation networks is defined

which means that when a road is blocked

vehicles on other roads within the networks can also reach their destination through different paths without being trapped on the road. Then

the security region of transportation networks is modeled. The algorithm of a boundary calculation is studied and the boundary equations of security distances are proposed. Finally

the proposed security of the transportation networks is presented in different examples. This paper demonstrates the following: 1) the security region can be accurately predicted by the simulation of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$N-1$; 2) the boundary distance can show the necessary degree of security; 3) security can be improved by a preventive control scheme. The simulation on TransCAD is used to verify the correctness of the results.