A SIMPLE GRAVITY MODEL ILLUSTRATION
Assume that in the trip-generation step, we have determined that the area shown as the origin will generate 1,000 trips to work per day. This will go to the three destinations shown in the accompanying figure. The force of attraction of each destination is proportional to its square feet of floor space divided by the square of the distance. Then the trips will be distributed as shown in the calculations below the figure. If there are actual data on trips, the model can be made to approximate reality by adjusting the exponent of distance to some value other than 2.0 or by adjusting the values of distance.
Since the speed of travel on a road depends on its volume of traffic, such simulations will give planners insight into how potential changes in the road system will affect travel times. Travel in metropolitan areas has morning and evening peaks, corresponding to commuting hours. Traffic flow is very different in peak and nonpeak hours. The model can be run to simulate different times of the day.
In the past it was common to assume a single scenario for metropolitan- area growth, often an extrapolation of present trends, and use the model to explore different transportation alternatives to provide service for that pattern. More recently, it has become common to evaluate different patterns of land use in order to both think about how to service them and also to inform land-use planning for metropolitan-area development. This takes us back to the point made before, that transportation planning and land-use planning, in an ideal world, should be done as part of the same process. According to Keith Bartholomew and Reid Ewing, much of this scenario exploration is done with a view toward achieving a more compact pattern of development and, as a function of that, reduced fuel use.11