Descriptors are attributes that describe characteristics of the network or its elements. Unlike costs, descriptors are not apportioned. This means that the value does not depend on the length of the edge element. For example, the number of lanes is an example of a descriptor on a street network. Speed limit of streets is another descriptor attribute for a street network. Although it is not a cost attribute and cannot be used as an impedance, it can be used in conjunction with distance to create a cost attribute (for example, drive time) that can be used as an impedance