Search Content

What would it take to build our way out of congestion in the Twin Cities? As part of this research project, researchers identified a method to answer that question and found a minimal set of highway capacity expansions that would accommodate future travel demand and guarantee mobility. The problem of identifying a set of capacity expansions that are in some sense optimal, while accounting for traveler reaction, is known as a network design problem. A literature review reveals numerous formulations and solution algorithms over the last three decades, but the problem of implementing these for large-scale networks has remained a challenge. This project presents a solution procedure that incorporates the capacity expansion as a modified step in the Method Successive Averages, providing an efficient algorithm capable of solving realistic problems of real-world complexity. Application of this method addresses the network design problem for the freeway system of the Twin Cities by providing a lower bound on the extent to which physical expansion of highway capacity can be used to accommodate future growth. The solution estimates that adding 1,844 lane-kilometers, or 1,146 lane-miles, would be needed to accommodate the demand predicted for the year 2020.

This report presents an approach to assess the effect of vehicle traffic volumes and speeds on pedestrian safety. It shows that the probability of standardized pedestrian conflict resulting in a collision can be computed given data on the distribution of vehicle speeds and headways on a residential street. Researchers applied this method to data collected on a sample of 25 residential streets in the Twin Cities and found that collision rates varied between 4 and 64 collisions per 1,000 pedestrian conflicts, depending primarily on the street's traffic volume. Using a model that relates the impact speed of a vehicle to the severity of pedestrian injury, they computed the probabilities of a severe collision. Sensitive to both traffic volume and traffic speed, the severe collision rate varied between 1 and 25 collisions per 1,000 conflicts. Using the same data, researchers also computed the crash reduction factor, used to assess the potential safety effect of a 25 miles per hour speed limit on the sample of residential streets. The estimated crash reductions ranged between .2% and 45%, depending primarily on the degree to which the vehicle speeds currently exceeded 25 miles per hour. Researchers also showed how this computation assists with the reconstruction of actual vehicle/pedestrian collisions.

"What would it take to build our way out of congestion in the Twin Cities?" was the question posed by researchers five years ago. This previous study solved a roads-only network design problem (NDP) for the Twin Cities of Minnesota. Building on that work, another network design problem is examined for the Twin Cities metropolitan area of 3 million, to examine the tradeoff between demand side reductions and the limited access capacity expansion necessary to achieve desired levels of service. The problem is simplified by pre-determining a mode split, which allows for incorporating decreasing demand directly as an input rather than in the model formulation. The problem is solved using Sequential Linear Expansion (SLIE), a modified method of successive averages (MSA). Computation time for the large network is decreased to a reasonable length using another modification, the MSA with decreasing re-initialization (MSADR). A typical personal computer can solve this large-sized problem within 24 hours. For forecasted travel demand for 2030, it was found that if the number of trips were reduced by 20%, lanemiles needed to achieve LOS D decreases by up to 43%.