Traffic Theory

Freeway Traffic Oscillations: Observations And Predictions

Mauch, Michael
Cassidy, Michael J.
Taylor, Michael A. P.
2002

Freeway traffic was observed over multiple days and was found to display certain regular features. Oscillations arose only in queues; they had periods of several minutes; and their amplitudes stabilized as they propagated upstream. They propagated at a nearly constant speed of about 22 to 24 kilometers per hour, independent of the location within the queues and the flow measured there; this was observed for a number of locations and for queued flows ranging from about 2,000 to 850 vehicles per hour per lane. The effects of the oscillations were not felt downstream of the bottleneck. Thus,...

Critique of a Freeway On-Ramp Metering Scheme and Broader Related Issues

Cassidy, Michael J.
2002

Offered here is a critique of a simple scheme recently proposed for metering freeway on-ramps. An earlier report of this scheme's potential for reducing commuter delay is shown to be exaggerated. The discussion makes clear that to reduce delay, metering should increase the rates at which commuters exit the freeway. The scheme critiqued here, as well as other well-known metering algorithms, are shown to have deficiencies in this, particularly when the freeway is plagued by a diverge bottleneck with a congested off-ramp. Other more effective schemes for reducing the delay caused by these...

Identifying Density-Flow Relations on Arterial Surface Streets

Ahn, Soyoung
Cassidy, Michael J.
2002

A simple car-following rule was verified by studying vehicles discharging from long queues at signalized intersections. These observations indicated that the time-space trajectory of a jth vehicle discharging on a homogeneous intersection approach was essentially the same as the j−1th vehicle except for a translation in space and time. This is in agreement with a simplified theory proposed by G.F. Newell. The finding indicates that the congested branch of a density-flow curve is linear in form.

Complications at Off-Ramps

Cassidy, Michael
2002

You're driving along the freeway when suddenly everything slows down. A crash? A sudden overload of cars joining the freeway from on-ramps up ahead? Maybe. Sometimes the cause never reveals itself to you—inexplicably, everything just starts moving again. If this happens everyday in the same spot, you may develop a theory or two as to why it happens. Would it occur to you that the congestion might be caused not by too many cars getting on the freeway but b ytoo many cars trying to...

Vehicle Reidentification and Travel Time Measurement on Congested Freeways

Coifman, Benjamin
Cassidy, Michael
2002

The paper presents an algorithm for matching individual vehicles measured at a freeway detector with the vehicles’ corresponding measurements taken earlier at another detector located upstream. Although this algorithm is potentially compatible with many vehicle detector technologies, the paper illustrates the method using existing dual-loop detectors to measure vehicle lengths. This detector technology has seen widespread deployment for velocity measurement. Since the detectors were not developed to measure vehicle length, these measurements can include significant errors. To overcome this...

Validation of Daganzo's Behavioral Theory of Multi-Lane Traffic Flow: Final Report

Banks, James H.
Amin, Mohammad R.
Cassidy, Michael J.
Chung, Koohong
2003

A study was conducted to verify C. F. Daganzo's behavioral theory of multi-lane traffic flow (1, 2). This study was conducted by teams from San Diego State University and the University of California at Berkeley who worked independently on a series of case studies to test predictions derived from the theory. The results of the study suggest that some of the phenomena predicted by Daganzo do occur, but not at all locations, and that the underlying behavioral assumptions are oversimplified. Specifically, the types of flow- density (or flow-occupancy) relationship assumed by Daganzo were...

Test of Theory of Driver Behavior on Homogeneous Freeways

Chung, Koohong
Cassidy, Michael J.
2004

A theory that can explain even some of the more puzzling aspects of freeway traffic flow is tested here. Some of the observations furnished to this end pertain to dynamic features of traffic that have not been previously verified with data. These include findings that (a) freeway traffic can enter a semicongested state marked by a fast-moving queue in the passing lane with unqueued flow in the shoulder lane; (b) the possible devolution of traffic to a fully congested state when vehicle speeds in the passing lane drop below those in the shoulder lane supports the contention that freeway...

Driver Turn-Taking Behavior in Congested Freeway Merges

Cassidy, Michael J.
Ahn, Soyoung
2005

Data from four merge locations in northern California and Toronto, Ontario, Canada, unveil a notable feature of driver turn taking. It was observed that queued vehicles from the on-ramp and freeway traffic streams entered a congested merge in some (nearly) fixed ratio that was independent of the merge outflow. Drivers in competing traffic streams thus entered the merge by adopting some definite turn-taking behavior, and this behavior was not influenced by the severity of the exogenous flow restriction from downstream. The findings validate part of an existing theory of merging traffic and...

Relation Between Traffic Density and Capacity Drop at Three Freeway Bottlenecks

Chung, Koohong
Rudjanakanoknad, Jittichai
Cassidy, Michael J.
2007

Three freeway bottlenecks, each with a distinct geometry, are shown to share a relation between vehicle density and losses in discharge flow. Each bottleneck suffered reductions in discharge once queues formed just upstream. This so-called “capacity drop” was related to the density measured over some extended-length freeway segment near each bottleneck. Pronounced increase in this density always preceded a capacity drop. For each bottleneck, the densities that coincided with capacity drops were reproducible. When normalized by a bottleneck’s number of travel lanes and averaged across...

Deploying Lanes for High Occupancy Vehicles in Urban Areas

Cassidy, Michael J.
Daganzo, Carlos F.
2007

Simulations and field experiments in previous works suggest that a freeway’s general purpose lanes (those not dedicated to high occupancy vehicles) discharge vehicles from bottlenecks at an equal or higher average rate when one of the lanes is devoted to high occupancy vehicles than when it is not. This result was used in these previous works to develop formulae for the total discharge rate of bottlenecks, with and without dedicated lanes, as a function of the percentage of high occupancy vehicles in the traffic stream.This present paper extends these ideas by examining the effect of...