Investing for Reliability and Security in Transportation Networks

Alternative transportation investment policies can lead to very different network forms in the future. The desirability of a transportation network should be assessed not only by its economic efficiency but also by its reliability and security, because the cost of an incidental capacity loss in a road network can be massive. This research concerns how investment rules shape the hierarchical structure of roads and affect network fragility to natural disasters, congestion, and accidents and vulnerability to targeted attacks. A microscopic network growth model predicts the equilibrium road networks under two alternative policy scenarios: investment based on beneÞtÐcost analysis and investment based on bottleneck removal. A set of Monte Carlo simulation runs, in which a certain percentage of links was removed according to the type of network degradation analyzed, was carried out to evaluate the equilibrium road networks. It was found that a hierarchy existed in road networks for reasons such as economic efficiency but that an overly hierarchical structure had serious reliability problems. Throughout the equilibrating or evolution process, the grid network studied under beneÞtÐcost analysis had better efficiency performance, as well as error and attack tolerance. The paper demonstrates that reliability and security considerations can be integrated into the planning of transportation systems.

Ramp metering and freeway bottleneck capacity

The objective of this study is to determine whether ramp meters increase the capacity of active freeway bottlenecks, and if they do, how. The traffic flow characteristics at twenty-seven active bottlenecks in the Twin Cities have been studied for seven weeks without ramp metering and seven weeks with ramp metering. A series of hypotheses regarding the relationships between ramp metering and the capacity of active bottlenecks are developed and tested against empirical traffic data. It is found that meters increase the bottleneck capacity by postponing and sometimes eliminating bottleneck activations (a 73 percent increase in the duration of the pre-queue transition period), accommodating higher (2 percent) flows during the pre-queue transition period, and increasing queue discharge flow rates after breakdown (3 percent). The two-capacity hypothesis about flow drops after breakdown was also examined and results strongly suggest the percentage flow drops at various bottlenecks follow a normal distribution (mean 5.5 percent, standard deviation 2.3 percent). The implications of these findings on the design of efficient ramp control strategies are discussed, as well as future research directions.transportation, travel behavior, congestion, ramp meters

Economics of Road Network Ownership

This paper seeks to understand the economic impact of centralized and decentralized ownership structures and their corresponding pricing and investment strategies on transportation network performance and social welfare for travelers. In a decentralized network economic system, roads are owned by many agencies or companies that are responsible for pricing and investment strategies. The motivation of this study is two-fold. First, the question of which ownership structure, or industrial organization, is optimal for transportation networks has yet to be resolved. Despite several books devoted to this research issue, quantitative methods that translate ownership-related policy variables into short- and long-run network performance are lacking. Second, the U.S. and many other countries have recently seen a slowly but steadily increasing popularity of road pricing as an alternative to traditional fuel taxes. Not only is the private sector encouraged to finance new roads, this transition in revenue mechanism also makes it possible for lower-level government agencies and smaller jurisdictions to participate in network pricing and investment practice. The issue of optimal ownership is no longer a purely theoretical debate, but bears practical importance. This research adopts an agent-based simulator of network dynamics to explore the implications of centralized and decentralized ownership on mobility and social welfare, as well as potential financial issues and regulatory needs. Components of the simulator: the travel demand model, cost functions, and key variables of pricing and investment strategies, are empirically estimated and validated. Results suggest that road network is a market with imperfect competition. While there is a significant performance lag between the optimal strategy and the current network financing practice in the U.S. (characterized by centralized control, fuel taxes, and budget-balancing investment), a completely decentralized network suffers from issues such as higher-than-optimal tolls and over-investment. For the decentralized ownership structure, appropriate regulation on pricing and investment practices is necessary. Further analysis based on simulation comparisons suggests that with appropriate price regulation, a decentralized road economy consisting of profit-seeking road owners could outperform the existing centralized control, achieve net social benefits close to the theoretical optimum, and distribute a high percentage of welfare gains to travelers. Decentralized control is especially valuable in rapidly changing environments because it promptly responds to travel demand. These results seem to favor the idea of privatizing or decentralizing road ownership on congested networks. Further tests on real-world transportation networks are necessary and should make an interesting future study.Network economics, Modeling network dynamics, Road pricing, Transportation financing, Privatization.

Road Pricing with Autonomous Links

This research examines road pricing on a network of autonomous highway links. By autonomous it is meant that the links are competitive and independent, with the objective of maximizing their own profits without regard for either social welfare or the profits of other links. The principal goal of the research is to understand the implications of adoption of road pricing and privatization on social welfare and the distribution of gains and losses. The specific pricing strategies of autonomous links are evaluated first under the condition of competition for simple networks. An agent-based modeling system is developed which integrates an equilibrated travel demand, route choice, and travel time model with a repeated game of autonomous links setting prices to maximize profit. The levels of profit, welfare consequences, and potential cooperative arrangements undertaken by autonomous links will be evaluated. By studying how such an economic system may behave under various circumstances, the effectiveness of road pricing and road privatization as public policy can be assessed.Network dynamics, road pricing, autonomous links, privatization, agent-based transportation model

Travel Time Variability After A Shock: The Case Of The Twin Cities Ramp Meter Shut Off

Ramp meters in the Twin Cities were turned off for 8 weeks in the Fall of 2000. This paper analyzes travel time variability with and without ramp metering for several representative freeways during the afternoon peak period. Travel time variability is generally reduced with metering. However, it is found that ramp meters are particularly helpful for long trips relative to short trips. The benefits from reducing travel time variability with meters are on the order of 37% of the benefits from reducing average travel time.Freeway Operations, Ramp Meters, Travel Time Variability Before-and-after study; Ramp Metering System; Variable Message Signs (VMS); Freeway Service Patrol; Highway Helper Program.

Pricing, Investment, and Network Equilibrium

Despite rapidly emerging innovative road pricing and investment principles, the development of a long run network dynamics model for necessary policy evaluation is still lagging. This research endeavors to fill this gap and models the impacts of road financing policies throughout the network equilibration process. The manner in which pricing and investment jointly shape network equilibrium is particularly important and explored in this study. The interactions among travel demand, road supply, revenue mechanisms and investment rules are modeled at the link level in a network growth simulator. After assessing several measures of effectiveness, the proposed network growth model is able to evaluate the short- and long-run impacts of a broad spectrum of road pricing and investment policies on large-scale road networks, which can provide valuable information to decision-makers such as the implications of various policy scenarios on social welfare, financial situation of road authorities and potential implementation problems. Some issues hard to address in theoretical analysis can be examined in the agent-based simulation model. As a demonstration, we apply the network growth model to assess marginal and average pricing scenarios on a sample network. Even this relatively simple application provides new insights into issues around road pricing that have not previously been seriously considered. For instance, the results disclose a potential problem of over-investment when the marginal cost pricing scheme is adopted in conjunction with a myopic profit-neutral investment policy.Transportation network equilibrium; Road growth; Pricing; Congestion toll; Investment; Transport policy analysis.