According to the gravity model, the number of trips between two zones is inversely proportional to which of the following?

The gravity model, commonly used in transportation and urban planning, suggests that the number of trips between two zones is influenced by the characteristics of those zones, resembling the gravitational attraction between two bodies. According to this model, the number of trips is inversely proportional to the distance between the zones. This principle stems from the idea that as the distance between two locations increases, the likelihood of travel between them decreases. Essentially, the greater the distance, the less attractive a destination becomes for trips, whether for commuting, shopping, or other purposes. In this way, distance functions as a deterrent to travel.

This relationship emphasizes that shorter distances typically lead to a higher number of interactions, as individuals are more likely to make trips that do not require significant travel time or distance. Therefore, the closer two zones are to each other, the more trips they tend to generate between them.

In contrast, the other factors listed, such as the number of vehicles, population of the zones, and traffic volume, affect trip generation but do not have an inverse proportionality with respect to the number of trips in the same manner as distance does according to the gravity model. The population may increase the potential number of trips but does not serve as an inverse factor; similarly, traffic