What could be considered the SUPPLY model varies considerably with the application intended. The classic supply model of ECONOMICS is a function that describes unit cost vs. quantity available. The supply fn. can be equated with the DEMAND function to determine the EQUILIBRIUM quantity when the two unit costs are equal.
More general and detailed supply or SERVICE models will be considered in this note. These may describe CAPACITY and other characteristics of service as well as cost.
Some elements of the transport service supply description are:
For Highways (Roads and Streets) the supply description is the details of the network involved, the classes of roadway, the intersections and interchanges, traffic control devices, and other requirements to provide a functioning system of a determinable capacity. The costing will essentially be for the system. A cost breakdown may begin with the separation of Capital Account Costs, and Current Account Costs that include annual operations and maintenance.
The major conceptual difficulty in determining unit transport system supply costs is that major elements of a total transport system are 'fixed' and have a relatively steady state capacity. This characteristic leads to service pricing difficulties. Many of the fixed components of the system are developed at different times and means of financing. It is therefore difficult to estimate a 'current cost' or 'salvage value' of a system and/or its various components. .
It becomes apparent that the above difficulty can only be resolved by adopting a 'revenue requirements' or recovery approach to current costs. Much of the fixed transport plant may have little salvage value and should be considered as 'sunk costs'. This means that decisions should be based on future costs. These are mostly operations and maintenance, (O and M). Since the advent of the automobile and growth of the population the highway system has been expanding. The rate and amount of expansion has decreased so that it may not be as significant in the future as the system tends to mature. However there are many parts that have capacity and other characteristics that are less than desirable so that major rebuilding may incur significant future costs.
Abandonment of parts of the system means salvaging as much as as possible and writing off the remainder. The main salvage is the land.
If the 'revenue requirements' accounting includes the capital recovery costs of the fixed facilities, then a good approximation of commonly understood costs can be estimated.
The system may have too much or too little fixed plant. Part of the challenge of transport system supply management is to maintain a reasonably good balance between the capacity and quality of the total plant available with the market for service.
Transport models represent Markets as 'Centroids' of 'Zones' that describe a local market area. For many models the zones completely cover the modeled region, i.e. zonal boundaries are contiguous. in reality the boundaries often overlap and/or do not provide complete coverage.
Zones are intended to describe the limits of the market which can be considered concentrated at the centroid. The assumption is that zones and catchment areas are coincident. In the case of air service the zone centroid can be taken as the airport, even though it may not be ideally located in its market area or zone.
The zonal traffic is usually assumed to be all concentrated at the zone centroid. In fact the traffic may use an adjacent centroid, especially if the trip ends are located near the boundary. In general trip length tends to be minimized regardless of the zone centroids. Apparent backtracking probably means that the traffic will move through the nearest centroids that are on the most direct or apparently shortest route.
In these cases the zonal boundaries can be considered to overlap and a zonal 'market share' notion can be introduced to make the model more realistic.
The market location can be assumed to be the terminal nearest the zone centroid, and it will serve some or all of the traffic from and to that centroid, but there will be some directional bias when the use of adjacent centroid is advantageous.
Because a centroid is a point there are really no zonal coverage implications or constraints imposed by the models which use this structure. The centroid catchments’ area and market share estimates can overlap, but should not duplicate. Similarly the service provided at centroids can overlap. Overlaps however make a model indeterminate.
If you are using ws TRN you may use the following as an illustration of centroid location:
NLP DRAWM MAP ¦ Draw Map showing airport locations
End to date: 060125; ams