Spatial Interaction in Supply and Demand

Spatial interaction is the flow of products, people, services, or information among places, in response to localized supply and demand. It is a transportation supply and demand relationship that is often expressed over a geographical space. Spatial interactions usually include a variety of movements such as travel, migration, transmission of information, journeys to work or shopping, retailing activities, or freight distribution. Edward Ullman, perhaps the leading transportation geographer of the twentieth century, more formally addressed interaction as complementarity (a deficit of a good or product in one place and a ​surplus in another), transferability (possibility of transport of the good or product at a cost that the market will bear), and lack of intervening opportunities (where a similar good or product that is not available at a closer distance). Complementarity The first factor necessary for interaction to take place is complementarity. In order for the trade to take place, there has to be a surplus of the desired product in one area and a shortage of demand for that same product in another area. The greater the distance, between trip origin and trip destination, the less likelihood of a trip occurring and the lower the frequency of trips. An example of complementarity would be that you live in San Francisco, California and want to go to Disneyland for a vacation, which is located in Anaheim near Los Angeles, California. In this example, the product is Disneyland, a destination theme park, where San Francisco has two regional theme parks, but no destination theme park. Transferability The second factor necessary for interaction to take place is transferability. In some cases, it is simply not feasible to transport certain goods (or people) a great distance because the transportation costs are too high in comparison to the price of the product. In all other cases where the transportation costs are not out of line with price, we say that the product is transferable or that transferability exists. Using our Disneyland trip example, we need to know how many people are going, and the amount of time we have to do the trip (both travel time and time at the destination). If only one person is traveling to Disneyland and they need to travel in the same day, then flying may be the most realistic option of transferability at approximately $250 round-trip; however, it is the most expensive option on a per person basis. If a small number of people are traveling, and three days are available for the trip (two days for travel and one day at the park), then driving down in a personal car, a rental car or taking the train may be a realistic option. A car rental would be approximately $100 for a three-day rental (with for to six people in the car) not including fuel, or approximately $120 round-trip per person taking the train (i.e., either Amtraks Coast Starlight or the San Joaquin routes). If one is traveling with a large group of people (assuming 50 people or so), then it may make sense to charter a bus, which would cost approximately $2,500 or about $50 per person. As one can see, transferability can be accomplished by one of several different modes of transportation depending on the number of people, distance, the average cost to transport each person, and the time available for travel. Lack of Intervening Opportunities The third factor necessary for interaction to take place in the absence or lack of intervening opportunities. There may be a situation where complementarity exists between an area with a high demand for a product and several areas with a supply of that same product in excess of local demand. In this particular case, the first area would be unlikely to trade with all three suppliers, but would instead trade with the supplier that was closest or least costly. In our example of the trip to Disneyland, Is there any other destination theme park identical to Disneyland, providing an intervening opportunity between San Francisco and Los Angeles? The obvious answer would be no. However, if the question was, Is there any other regional theme park between San Francisco and Los Angeles that could be a potential intervening opportunity, then the answer would be yes, since Great America (Santa Clara, California), Magic Mountain (Santa Clarita, California), and Knotts Berry Farm (Buena Park, California) are all regional theme parks located between San Francisco and Anaheim. As you can see from this example, there are numerous factors that could affect complementarity, transferability, and lack of intervening opportunities. There are many other examples of these concepts in our daily lives, when it comes to planning your next vacation, watching the freight trains roll through your town or neighborhood, seeing the trucks on the highway, or when you ship a package overseas.