Engineers identify three basic network configurations. Minimally connected networks are called spinal systems, networks with high a large number of linkages are called delta systems, and a grid system is in a state of transition between spinal and delta networks.

Spinal networks are characterized by minimal connectivity. In order to be a network, every node (point or place) must be connected to at least one other node, and it must be possible for flow to occur between any two nodes in the network, but only via a solitary pathway. In a spinal network, the alpha index (a ration measure of the number of connections in a network relative to the maximum number possible) will always be 0; (v-1) –v+1 ÷ 2v-5 = 0 ÷ 2v-5 = 0; and, the gamma index (the ratio of edges in a network relative to the number of edges possible) will always be v-1 ÷ 3(v-2).

A delta network is the exact opposite of a spinal network because it approaches a maximum state of connectivity. Given that the number of edges (e) (edges represent linkages or lines) in a delta network is always 2v-3; a delta network’s gamma index can be expressed as: γ = e ÷ 3(v-2) = 2v-3 ÷ 3(v-20). Moreover, a delta system’s alpha index will always be: α = (2v-3)-v+1 ÷ 2v-5 = v-2 ÷ 2v-5. A grid network is transitional. It is better connected than a spinal network, but not as well connected as a delta network.

NOTE: **the use of lines, points, nodes, and edges is covered later in this lecture.**

#### Check Your Understanding

Short Answer

What is meant by the term *connectivity*?

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Places served by efficient transportation networks demonstrate a high level of connectivity. They also demonstrate a positive level of accessibility.

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##### Short Answer

What are the three types of transportation networks?

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Spinal, grid and delta.