Modifications to Traditional External Trip Models

Traditionally, internal-external trips were modeled by using regression-based trip generation equations and gravity model trip distribution. Applying these models at the cordon in the traditional way produces a number of problems. A typical solution is the use of external traffic analysis zones and highway networks that are used to model the external traffic similarly to internal traffic. Presented are modifications to each component of the traditional cordon line external model that will remedy these problems for those areas that do not wish to expand their data collection burden to areas outside their planning boundaries. In this model, trip distribution is a traditional cordon applied gravity model modified by using a composite impedance based on the product of travel time and an exponential function of the angular impedance. Angular impedance is defined as the angle between the vector representing the general direction of travel at the cordon and the vector defined by the cordon crossing point and a given internal zone centroid. The trip generation model is modified by the use of accessibility modified independent variables in the regression formulation. This model uses either traditional socioeconomic variables or internally generated production and attractions as the root variable. These root variables are then multiplied by an accessibility term similar to the denominator of the gravity model formula where internal-external trip productions at the cordon are used as the size variables. Application of these modifications to a small metropolitan planning organization in Ohio resulted in dramatic improvements to the model.