Traffic Operations and Management

A Capacity-Increasing Mechanism in Freeway Traffic

Kim, Kwangho
Cassidy, Michael J.
2012

A reason is unveiled for the time-varying pattern in discharge flow that is commonly observed at freeway bottlenecks. We hypothesize that four known effects in freeway traffic can interact upstream of a bottleneck in ways that trigger periodic bursts in its discharge flow. Repeated observations of a 3-km freeway stretch support the hypothesis. Controlled experiments show that the capacity-increasing mechanism can be favorably modulated by metering the site’s on-ramps in an unconventional manner. The unconventional strategy repeatedly produced higher average discharge flows and shorter on-...

Inhomogeneous Flow Patterns in Undersaturated Road Networks: Implications for Macroscopic Fundamental Diagram

Doig, Jean C.
Gayah, Vikash V.
Cassidy, Michael J.
2013

Travel conditions on a road network are described by a macroscopic fundamental diagram (MFD) only when traffic is homogeneously distributed over all of the network's links. Otherwise, a network's measured traffic states fall beneath its MFD, and network flow therefore falls below the maximum. The present study found that inhomogeneous link flowed in a network may emerge before the beginning of a peak period, even before persistent queues had formed. This finding is potentially problematic because it is known that inhomogeneities of this kind can persist in a network for long durations....

Mitigating Negative Impacts of Near-Side Bus Stops on Cars

Gu, Weihua
Cassidy, Michael J.
Gayah, Vikash V.
Ouyang, Yanfeng
2013

Bus stops are often placed short distances upstream of signalized intersections. Buses that dwell at one of these so-called near-side stops can impede queued cars upstream from discharging through the intersection during green times. Residual car queues can form at the intersection as a result. The smaller the distance between a stop and its intersection, the greater the problem can be. Models are formulated to address this problem using kinematic wave theory. The models can be used to determine where to place a near-side stop to achieve a target level of residual car queueing. In addition...

Maximizing Bus Discharge Flows from Multi-Berth Stops by Regulating Exit Maneuvers

Gu, Weihua
Cassidy, Michael J.
2013

Upon having loaded and unloaded their passengers, buses are often free to exit a multi-berth bus stop without delay. A bus need not wait to perform this exit maneuver, even if it requires circumventing one or more other buses that are still dwelling in the stop’s downstream berths. Yet, many jurisdictions impose restrictions on bus entry maneuvers into a stop to limit disruptions to cars and other buses. Buses are typically prohibited from entering a stop whenever this would require maneuvering around other buses still dwelling in upstream berths. An entering bus is instead required to...

Spatiotemporal Studies of Traffic Phenomenon on Freeways with Limited-access Special Lanes

Cassidy, Michael J.
Kim, Kwangho
2015

Most special-use freeway lanes in the US, whether reserved for carpools, toll-paying commuters or both, are physically separated from the adjacent regular-use lanes by some form of barrier. Vehicle movements in and out of a special lane of this type are permitted only at select access points along the route. The barrier at each select point might open for a distance of 400 m or so. Limiting access in this way is said to reduce the “turbulence” that might otherwise occur were the special lane not to have a buffer, such that vehicles could instead enter or exit that lane anywhere along its...

Models of Bus Queueing at Curbside Stops

Gu, Weihua
Cassidy, Michael J.
Li, Yuwei
2015

We consider curbside bus stops of the kind that serve multiple bus routes and that are isolated from the effects of traffic signals and other stops. A Markov chain embedded in the bus queueing process is used to develop steady-state queueing models of this stop type, as illustrated by two special cases. The models estimate the maximum number of buses that can arrive at and serve a stop and still satisfy a specified target of average bus delay. These models can be used to determine, for example, a stop’s suitable number of bus berths, given the bus demand and the specified delay target. The...

A Problem of Limited-Access Special Lanes. Part I: Spatiotemporal Studies of Real Freeway Traffic

Cassidy, Michael J.
Kim, Kwangho
Ni, Wei
Gu, Weihua
2015

Most special-use freeway lanes in the US, whether reserved for carpools, toll-paying commuters or both, are physically separated from the adjacent regular-use lanes by some form of barrier. Vehicle movements in and out of a special lane of this type are permitted only at select access points along the route. The barrier at each select point might open for a distance of 400m or so. Limiting access in this way is said to reduce the “turbulence” that might otherwise occur were the special lane not to have a barrier, such that vehicles could instead enter or exit that lane anywhere along its...

A Problem of Limited-Access Special Lanes. Part II: Exploring Remedies via Simulation

Ni, Wei
Cassidy, Michael J.
Kim, Kwangho
Gu, Weihua
2015

Spatiotemporal analyses of freeway sites in Part I have shown that special-lane access points are prone to become bottlenecks. These can degrade traffic flows, sometimes in all lanes. Part II explores select impacts of re-designing the means of entering and exiting a special lane, and of altering the policy governing its use. Parametric tests were conducted using a computer simulation model that was calibrated to one of the sites studied in Part I; one with a buffer-separated carpool lane. Though less reliable than what might have been observed via experiments in real settings, the...

A Low-Cost Alternative for Higher Capacities at Four-way Signalized Intersections

Kozey, Peter
Xuan, Yiguang
Cassidy, Michael J.
2016

Protecting left-turn movements on all four approaches to a signalized intersection conventionally requires a minimum of two extra phases per cycle. Losses in capacity often result. Various intersection designs have been proposed to combat those losses. Perhaps the best known of these designs is the continuous flow intersection. It features specially-configured approach lanes and mid-block pre-signals. These enable opposing left-turn and through-moving vehicles to proceed through the intersection free of conflicts, and without need for additional protected-turn phases. The present paper...

Cordon Control with Spatially-Varying Metering Rates: A Reinforcement Learning Approach

Ni, Wei
Cassidy, Michael J.
2019

The work explores how Reinforcement Learning can be used to re-time traffic signals around cordoned neighborhoods. An RL-based controller is developed by representing traffic states as graph-structured data and customizing corresponding neural network architectures to handle those data. The customizations enable the controller to: (i) model neighborhood-wide traffic based on directed-graph representations; (ii) use the representations to identify patterns in real-time traffic measurements; and (iii) capture those patterns to a spatial representation needed for selecting optimal cordon-...