Freeway Ramp Metering Research Articles

A dynamic system-optimum control model for commuting traffic corridors

The need to implement effective traffic control in commuting corridors has long been recognized by transportation professionals. However, most existing focused either on optimizing freeway ramp metering rates or providing coordinated surface street signals, without taking account of the vital interaction between these two subsystems. As a result, it is not unusual that an effective control strategy for freeway operation may cause significant detrimental effects to the adjacent surface streets. On the other hand, the access to freeway ramps is often impeded by the formation of congestion or bottlenecks on surface streets due to the increasing peak-period traffic demand and ineffective signal operation. This paper presents a dynamic system-optimal control model (DSOCM) for commuting corridors which consist of both freeway and surface street segments. The proposed DSOCM considers the complex interactions among the freeway, surface street and diversion flows, and allows the system operators to compute the optimal time-dependent ramp metering rate and signal setting over the selected time horizon. Depending on the input reliability, DSOCM need not be executed at every control interval as long as the differences between the projected and actual traffic conditions are within the acceptable range. An effective and coordinated control operation for integrated traffic systems can then be achieved.

Technological responses to urban traffic congestion

This article discusses technological responses to urban traffic congestion. Some of these responses are: freeway management systems; traffic surveillance; freeway ramp metering; parking management systems; and IVHS.

Freeway ramp control using fuzzy set theory for inexact reasoning

This paper presents a fuzzy controller for freeway ramp metering, which uses rules of the form: IF “freeway condition” THEN “control action.” The controller has been designed to consider varied levels of congestion, a downstream control area, changing occupancy levels, upstream flows, and a distributed detector array in its rule base. Through fuzzy implication, the inference of each rule is used to the degree to which the condition is true. Using a dynamic simulation model of conditions0fj at the San Francisco-Oakland Bay Bridge, the action of the fuzzy controller is compared to the existing “crisp” control scheme, and an idealized controller. Tests under a variety of scenarios with different incident locations and capacity reductions show that the fuzzy controller is able to extract 40 to 100% of the possible savings in passenger-hours. In general, the fuzzy algorithm displays smooth and rapid response to incidents, and significantly reduces the minute-miles of congestion.

On decentralized estimation and control with application to freeway ramp metering

This paper presents an approach to the suboptimal design of time invariant, infinite time horizon, L-Q-G regulator in which decision making and state estimation are decentralized into local subsystems. For a prespecified information pattern (set of admissible intercommunications) the problem of finding a best linear control law and state estimator is converted to a parameter optimization which can be solved off line. A modified Davidon-Fletcher-Powell algorithm is used to obtain numerical solutions to the static optimization. A design is illustrated by means of a 12 state freeway corridor ramp metering problem; behavior of centralized and decentralized designs in a nonlinear macroscopic simulation of the freeway are provided.

On decentralized estimation and control with application to freeway ramp metering

On decentralized estimation and control with application to freeway ramp metering

Allocation of freeway ramp metering volumes to optimize corridor performance

A simple macroscopic freeway corridor model is formulated for the purpose of developing optimal allocations of freeway on-ramp metering volumes. The model represents a freeway and a single parallel equivalent surface street, both unidirectional, which are interconnected by freeway on-ramps. Since a choice of routes is included in the problem formulation, the fairly standard methodology of traffic is employed to predict how drivers will react to the on-ramp metering. An explicit assignment algorithm which includes ramp queueing is developed. Both the corridor and assignment models are restricted to stationary patterns. Two specific corridor performance criteria, freeway travel rate and total travel-time rate, are formulated in terms of the model variables. Then the problem of choosing ramp metering volumes to optimize a performance criterion is posed. A computationally efficient suboptimal dynamic programming scheme is developed and illustrated by an example.

Adjustment of freeway ramp metering rates to balance entrance ramp queues

IN A FREEWAY RAMP METERING SYSTEM THE VARIABLES WHICH CAN BE CONTROLLED ARE THE ENTRANCE RAMP RATES. IN ORDER TO AVOID CONGESTION ON THE FREEWAY THE RAMP RATES SHOULD BE KEPT BELOW CRITICAL LEVELS. TO CONTROL QUEUE BUILDUP, IT IS NECESSARY TO CONTROL THE DIFFERENCE BETWEEN DEMANDS AND RAMP RATES. SINCE DEMANDS ARE UNCONTROLLABLE, THIS IMPLIES THAT RAMP RATES SHOULD BE KEPT AS HIGH AS POSSIBLE TO LIMIT QUEUES. IT IS SHOWN THAT THESE CONFLICTING OBJECTIVES IN THE SELECTION OF RAMP RATES CAN BEST BE COMPROMISED BY SOLVING A QUADRATIC PROGRAMMING PROGRAM. THE RAMP RATES ARE SELECTED TO MINIMIZE THE SUM OF THE SQUARES OF THE DIFFERENCES BETWEEN THE RAMP RATES AND THE CORRESPONDING EQUIVALENT DEMANDS. AN EQUIVALENT DEMAND IS DEFINED AS THE ACTUAL DEMAND PLUS THE RATIO OF THE NUMBER OF QUEUED VEHICLES DIVIDED BY THE TIME BETWEEN ADJUSTMENTS OF THE RAMP RATES. THIS MINIMIZATION IS PERFORMED SUBJECT TO THE CONSTRAINT THAT THE RAMP RATES ARE KEPT LOW ENOUGH THAT NO FREEWAY CONGESTION OCCURS. /AUTHOR/