Weak street

The street from A to B which, if entered, there is no way back to A.

Weak crossing

A crossing is weak if at least one weak street begins there.

Strong crossing

All streets begins here are not weak.

A connection from crossing A to crossing B

A sequence of streets a truck passes from A to B.

A strong connection

• passes only through strong crossings
• passes through minimum possible number of streets

A target for a crossing X

Another crossing Y is a target for X if there is mutal strong connection between X and Y.

Variability of strong crossing X

The number of targets of X

The cost

Let crossing Y be a target for X. The cost of Y is a number which is equal to twice the length of the strong connection from X to Y plus the length of the strong connection from Y to X.

The cost of a strong crossing X

Is equal to the sum of the costs of all targets for X.

A prospective crossing

Is a strong crossing which variability is maximum and its cost is minimum among all strong crossings with maximum variability.