Traffic Operations and Management

Increasing the Capacity of Signalized Intersections with Separate Left Turn Phases

Xuan, Yiguang
Daganzo, Carlos F.
Cassidy, Michael J.
2011

A separate turn phase is often used on the approach leg to an intersections with heavy left turns. This wastes capacity on the approach because some of its lanes cannot discharge during its green phases. The paper shows that the problem can be eliminated by reorganizing traffic on all the lanes upstream of an intersection using a mid-block pre-signal. If drivers behave deterministically, the capacity that can be achieved is the same as if there were no left turns. However, if the reorganization is too drastic, it may be counterintuitive to drivers. This can be remedied by reorganizing...

Dual Influences on Vehicle Speeds in Special-Use Lanes and Policy Implications

Jang, Kitae
Cassidy, Michael J.
2011

Slow speeds in a special-use lane, such as a carpool (HOV) or bus lane, can be due to both high demand for that lane and slow speeds in the adjacent regular-use lane. These dual influences are confirmed from months of data collected from all freeway carpool facilities in the San Francisco Bay Area. Both influences hold for other types of special-use lanes, including bus lanes. New US regulation stipulating that most classes of low-emitting vehicles, or LEVs, be banned from slow-moving carpool lanes. While LEVs invariably constitute only about 1 percent of the freeway traffic demand in the...

Strategies for Mitigating Impacts of Near-Side Bus Stops on Cars

Gu, Weihua
Cassidy, Michael J.
Gayah, Vikash V.
Ouyang, Yanfeng
2012

"Near-side stops" are bus stops located a short distance upstream of a signalized intersection. A bus dwelling at a near-side stop can impede queued cars upstream as they discharge during their green time at the intersection. Added car delays and residual queues can result. All else equal, the closer the stop’s location to the intersection, the greater the potential damage to car traffic. Models for locating these near-side stops to achieve target levels of residual queueing among cars are formulated using kinematic wave theory. This same approach was also used to develop a strategy for...

Dual Influences on Vehicle Speed in Special-use Lanes and Critique of US Regulation

Jang, Kitae
Cassidy, Michael J.
2012

We verify that slow speeds in a special-use lane, such as a carpool or bus lane, can be due to both, high demand for that lane and slow speeds in the adjacent regular-use lane. These dual influences are confirmed from months of data collected from all freeway carpool facilities in the San Francisco Bay Area. Additional data indicate that both influences hold: for other types of special-use lanes, including bus lanes; and for other parts of the world. The findings do not bode well for a new US regulation stipulating that most classes of Low-Emitting Vehicles, or LEVs, are to vacate slow-...

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...

On the Impacts of Bus Stops near Signalized Intersections: Models of Car and Bus Delays

Gu, Weihua
Gayah, Vikash V.
Cassidy, Michael J.
Saade, Nathalie
2014

Models are formulated to predict the added vehicle and person delays that can occur when a bus stop is located a short distance upstream or downstream of a signalized intersection. Included in the set of models are those that predict the expected delays that cars collectively incur when a bus blocks one of multiple lanes while loading and unloading passengers at the stop. Others in this set predict the expected added delays incurred by the bus due to car queues. Each model is consistent with the kinematic wave theory of highway traffic, as is confirmed through a battery of tests. And each...

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...