A new study out of Portland State University ITS (TRB paper found here in doc format) discusses the efficacy of the System-Wide Adaptive Ramp Metering (SWARM) system in the Portland, Oregon area. SWARM replaces older fixed-rate metering systems that remain static regardless of the prevailing traffic conditions. This means SWARM should be sensitive to both regular fluctuations as well as non-recurrent changes. Wikipedia has a reasonable sketch of ramp metering here. Here is the abstract:
A System-Wide Adaptive Ramp Metering (SWARM) system is being implemented in the Portland metropolitan area, replacing the previous pre-timed ramp-metering system. SWARM has been deployed on six major corridors and operates during the morning and afternoon peak hours. This study entails a “before” and “after” evaluation of the benefit of the new SWARM system as compared to the pre-timed system using the existing data, surveillance and communications infrastructure. In particular, the objective of this study is to quantify the system-wide benefits in terms of savings in delay, emissions and fuel consumption, and safety improvements on and off the freeway due to the implementation of the SWARM system. A pilot study was conducted for two weeks on a 7-mile freeway corridor in an attempt to develop a strategic design for the future regional-level study. This paper discusses the selection process of the study corridor, experimental design, and the results that were obtained from the pilot study.
Caltrans has also implemented such a system (pdf format) in the Los Angeles area in 2000 and earlier (in pdf, see pages 12-13 for an overview of the Irvine SWARM).