Layers and Layers
- Site: Right of Way & Alignment (Horizontal and Vertical)
- Street: Structure (Bridges, Pavement, Tracks, Runways etc.)
- Signs and markings
- Purpose of travel (Service)
Transportation systems are designed and constructed as interdependent layered networks. The physical systems are layered as shown in the Figure, in an analogy with the Open Systems Interconnection (OSI) model of communication layers that define the internet. The hierarchy of services include short- and long-distance transportation services (e.g., buses, freight) that make use of different and overlapping parts of the street network.
The question of resilience is relevant to most of these layers. Transportation will be interrupted to a greater or lesser extent if the physical alignment is blocked (e.g., flood, snow, hazmat leak), if the road or bridge structure fails (e.g., potholes, sinkholes, heaving, a bridge collapse), if traffic control devices (signs, markings, signals) fail or deteriorate (e.g., an electrical outage, weathering), if vehicles fail to work (e.g., gasoline shortages, electrical outages in the case of electric vehicles, or system-wide vehicle recalls), if the driver is unable to drive the vehicle (e.g., strikes or illness), or if a service is interrupted (e.g., airplanes are grounded, trains or buses are canceled, traveler information systems are disrupted). Overuse of transportation facilities (congestion) may lead to a type of traffic failure that results when travel times increase beyond an acceptable level (or the area reachable in a given time shrinks). This type of failure can have consequences ranging from annoying to critical.
Within these layered networks are a hierarchy of roads and services. The hierarchy of roads is an emergent phenomenon that has become enmeshed in road and highway design – some roads are more significant (faster, more intensively used) than others. Similarly, transit systems are often designed with feeder routes connecting with regional routes, and freight systems have collector / distributor networks (local delivery networks) that connect to a long distance system. This hierarchical arrangement is also common in other networked utility systems (electrical, natural gas, water, etc.).
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