Author: Dr. Jean-Paul Rodrigue
A spatial interaction is a realized flow of passengers or freight between an origin and a destination. It is a transport demand / supply relationship expressed over a geographical space.
1. Conditions for Spatial Flows
Estimating flows between locations is a methodology of relevance to transportation. These flows, known as spatial interactions, enable to evaluate the demand (existing or potential) for transport services. They cover forms of mobility such as journeys to work, migrations, tourism, the usage of public facilities, the transmission of information or capital, the market areas of retailing activities, international trade, and freight distribution. Mobility can be physical (passengers or freight) or intangible (information), and each form of mobility is subject to a form of friction.
Economic activities are generating (supply) and attracting (demand) movements. The simple fact that a movement occurs between an origin and a destination underlines that the costs incurred by a spatial interaction are lower than the benefits derived from such an interaction. As such, a commuter is willing to drive one hour because this interaction is linked to an income, while international trade concepts, such as comparative advantages, underline the benefits of specialization and the ensuing generation of trade flows between distant locations.
Three interdependent conditions are necessary for a spatial interaction to occur:
- Complementarity. There must be a supply and a demand between the interacting locations. A residential zone is complementary to an employment zone because the first is supplying workers while the second is supplying jobs. The same can be said concerning the complementarity between a store and its customers and between an industry and its suppliers (movements of freight). An economic system is based on a large array of complementary activities.
- Intervening opportunity (lack of). Refers to a location that may offer a better alternative as a point of origin or as a point of destination. For instance, in order to have an interaction of a customer to a store, there must not be a closer store that offers a similar array of goods. Otherwise, the customer will likely patronize the closer store and the initial interaction will not take place.
- Transferability. Mobility must be supported by transport infrastructures, implying that the origin and the destination must be linked. Costs to overcome distance must not be higher than the benefits of the related interaction, even if there are complementarity and no alternative opportunity.
Spatial interaction models seek to explain existing spatial flows. As such it is possible to measure flows and predict the consequences of changes in the conditions generating them. When such attributes are known, it is possible to better allocate transport resources such as conveyances, infrastructure, and terminals.
2. Origin / Destination Matrices
Each spatial interaction, as an analogy for a set of movements, is composed of a discrete origin/destination pair. Each pair can itself be represented as a cell in a matrix where rows are related to the locations (centroids) of origin, while columns are related to locations (centroids) of destination. Such a matrix is commonly known as an origin/destination matrix, or a spatial interaction matrix.
In the O/D matrix, the sum of a row (Ti) represents the total outputs of a location (flows originating from), while the sum of a column (Tj) represents the total inputs (flows bound to) of a location. The summation of inputs is always equaling to the summation of outputs. Otherwise, there are movements that are coming from or going to outside the considered system. The sum of inputs or outputs gives the total flows taking place within the system (T). It is also possible to have O/D matrices according to the age group, income, gender, etc. Under such circumstances, they are labeled sub-matrices since they account for only a share of the total flows. If the sample is small and disaggregated it is possible to use a simple list of interactions instead of a matrix. Still, an origin/destination matrix can be constructed out of this list.
In many cases where spatial interactions information is relied on for planning and allocation purposes, origin/destination matrices are not available or are incomplete. Overcoming this lack of data commonly requires surveys. With economic development, the addition of new activities and transport infrastructures, spatial interactions tend to change very rapidly as flows adapt to a new spatial structure. The problem is that an origin/destination survey is very expensive in terms of efforts, time and costs. In a complex spatial system such as a region, O/D matrices tend to be quite large. For instance, the consideration of 100 origins and 100 destinations would imply 10,000 separate O/D pairs for which information has to be provided.
In addition, the data gathered by spatial interaction surveys are likely to become obsolete as economic and spatial conditions change. It is therefore important to find a way to estimate as precisely as possible spatial interactions, particularly when empirical data is lacking or is incomplete. Further, the emergence of ‘big data’ enabled to collect large amounts of personal mobility information that is possible to convert into flows between spatial units. In such a context the purpose of a spatial interaction model is to complement and even replace empirical observations through reliable estimates of flows between locations.
3. Spatial Interaction Models
Spatial interaction models are usually the first two steps in the standard four step transportation / land use model as they estimate the spatial generation and distribution of trips. The basic assumption concerning many spatial interaction models is that flows are a function of the attributes of the locations of origin, the attributes of the locations of destination and the friction of distance between the concerning origins and the destinations. The general formulation of a spatial interaction model is as follows:
- Tij : Interaction between location i (origin) and location j (destination). Its units of measurement are varied and can involve the number of passengers, tons of freight, traffic volume, etc. It also relates to a time period such as interactions by the hour, day, month, or year.
- Vi : Attributes of the location of origin i. Variables often used to express these attributes are socio-economic in nature, such as population, number of jobs available, industrial output or any proxy of the level of economic activity such as gross domestic product.
- Wj : Attributes of the location of destination j. It uses similar socio-economic variables than the previous attribute to underline the reciprocity of the locations.
- Sij : Attributes of separation between the location of origin i and the location of destination j. Also known as transport friction, friction of distance or impedance. Variables often used to express these attributes are distance, transport costs, or travel time.
The attributes of V and W tend to be paired to express complementarity in the best possible way. For instance, measuring commuting flows (work-related movements) between different locations would likely consider a variable such as working-age population as V and total employment as W. From this general formulation, three basic types of interaction models can be constructed:
- Gravity model. Measures interactions between all the possible location pairs.
- Potential model. Measures interactions between one location and every other location.
- Retail model. Measure the boundary of the market areas between two locations competing over the same market based upon the intensity of their respective interactions.
4. The Gravity Model
The gravity model is the most common formulation of the spatial interaction method. It is named as such because it uses a similar formulation than Newton’s law of gravity. Gravity like representations have been applied in a wide variety of contexts, such as migration, commodity flows, traffic flows, commuting, and evaluating boundaries between market areas. Accordingly, the attraction between two objects is proportional to their mass and inversely proportional to their respective distance. Consequently, the general formulation of spatial interactions can be adapted to reflect this basic assumption to form the elementary formulation of the gravity model:
- Pi and Pj : Importance of the location of origin and the location of destination.
- dij : Distance between the location of origin and the location of destination.
- k is a proportionality constant related to the rate of the event. For instance, if the same system of spatial interactions is considered, the value of k will be higher if interactions were considered for a year compared to the value of k for one week.
Thus, spatial interactions between locations i and j are proportional to their respective importance divided by their distance. The gravity model can be extended to include several calibration parameters:
- P, d and k refer to the variables previously discussed.
- β (beta): A parameter of transport friction related to the efficiency of the transport system between two locations. This friction is rarely linear as the further the movement the greater the friction of distance. For instance, two locations serviced by a highway will have a lower beta index than if they were serviced by a regular road.
- λ (lambda) : Potential to generate movements (emissivity). For movements of people, lambda is often related to an overall level of welfare. For instance, it is logical to infer that for retailing flows, a location having higher income levels will generate more movements (customers).
- α (alpha) : Potential to attract movements (attractiveness). Related to the nature of economic activities at the destination. For instance, a center having important commercial activities will attract more movements.
A significant challenge related to the usage of spatial interaction models, notably the gravity model, is related to their calibration. Calibration consists of finding the value of each parameter of the model (constants and exponents) to ensure that the estimated results are similar to the observed flows, that those results can be replicated and that changing the parameters would generate valid results. If it is not the case, the model is of limited use as it predicts or explains little. It is impossible to know if the process of calibration is accurate without comparing estimated results with empirical evidence. Consistent calibration enables the model to be more rigorous and adaptable to other contexts.
In the two formulations of the gravity model that have been introduced, the simple formulation offers good flexibility for calibration since four parameters can be modified. Altering the value of beta, alpha and lambda will influence the estimated spatial interactions. Furthermore, the value of the parameters can change in time due to factors such as technological innovations, new transport infrastructure, and economic development. For instance, improvements in transport efficiency generally have the consequence of reducing the value of the beta exponent (friction of distance). Economic development is likely to influence the values of alpha and lambda, reflecting growth in mobility.
Calibration can also be considered for different O/D matrices according to age, income, gender, type of merchandise and modal choice. A part of the scientific research in transport and regional planning aims at finding accurate parameters for spatial interaction models. This is generally a costly and time-consuming process, but a very useful one. Once a spatial interaction model has been validated for a city or a region, it can then be used for simulation and prediction purposes, such as how many additional flows would be generated if the population increased or if better transport infrastructures (lower friction of distance) were provided.
Outside the gravity model, there are other models that can be used to measure spatial interactions. Destination choice models are considered an extension of the gravity model that is gaining popularity since they provide a more extensive range of factors explaining the assignment of spatial interactions. While the gravity model assumes that flows are generated as a function of attributes of the origin and destination pondered by impedance functions, the destination choice model allows for additional behavioral attributes to mobility, including income, walkability, the availability of parking and psychological barriers. The main goal is to explain flows that the standard gravity model does not capture well.
- The Notion of Accessibility
- Urban Land Use and Transportation
- Urban Mobility
- Transportation / Land Use Modeling
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