Roads/Highways

Structure of the Transition Zone Behind Freeway Queues

Munoz, Juan Carlos
Daganzo, Carlos F.
2003

Observations of freeway traffic flow are usually quite scattered about an underlying curve when plotted versus density or occupancy. Although increasing the sampling intervals can reduce the scatter, whenever an experiment encompasses a rush hour with transitions in and out of congestion, some outlying data stubbornly remain beneath the “equilibrium” curve. The existence of these nonequilibrium points is a poorly understood phenomenon that appears to contradict the simple kinematic wave (KW) model of traffic flow. This paper provides a tentative explanation of the phenomenon, based on...

Lane-Changing in Traffic Streams

Laval, Jorge A.
Daganzo, Carlos F.
2006

It is postulated that lane-changing vehicles create voids in traffic streams, and that these voids reduce flow. This mechanism is described with a model that tracks lane changers precisely, as particles endowed with realistic mechanical properties. The model has four easy-to-measure parameters and reproduces without re-calibration two bottleneck phenomena previously thought to be unrelated: (i) the drop in the discharge rate of freeway bottlenecks when congestion begins, and (ii) the relation between the speed of a moving bottleneck and its capacity.

Empirical Reassessment of Traffic Operations: Freeway Bottlenecks and the Case for HOV Lanes

Cassidy, Michael J.
Daganzo, Carlos F.
Jang, Kitae
Chung, Koohong
2006

An earlier empirical study of San Francisco Bay Area freeways concluded that HOV lanes unfavorably affect freeway traffic by creating congestion. That study attributed the observed congestion to HOV lanes and tentatively recommended their elimination over the full lengths of the freeways it examined; and even from all Bay Area freeways. It recognized, however, that its analysis is fragmentary and recommended further work to solidify its conclusions. This is logical since the study lacks a spatiotemporal analysis to pinpoint where and how congestion first forms (at bottlenecks).The present...

Impacts of Lane Changes at Merge Bottlenecks: A Theory and Strategies to Maximize Capacity

Laval, Jorge
Cassidy, Michael
Daganzo, Carlos
Schadschneider, Andreas
Pöschel, Thorsten
2007

Recent empirical observations at freeway merge bottlenecks have revealed (i) a drop in the bottleneck discharge rate when queues form upstream, (ii) an increase in lane-changing maneuvers simultaneous with this “capacity drop”, and (iii) a reversal of the drop when the ramp is metered.

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...

Spillovers, Merging Traffic and the Morning Commute

Lago, Alejandro
Daganzo, Carlos F.
2007

Theoretical studies of the morning commute for mono-centric cities have ignored that drivers choose their home departure times knowing that they must compete with other drivers for road space, which becomes scarcer with proximity to the center. This paper examines two important aspects of this competition: queue spillovers caused by insufficient road space, and merging interactions caused by the convergence of trips. For maximum transparency the paper focuses on an idealized two-origin, single-destination network with limited storage space because this system exhibits all the essential...

Effects of HOV Lanes on Freeway Bottlenecks

Menendez, Monica
Daganzo, Carlos F.
2007

High occupancy vehicle (HOV) lanes are restricted-use freeway lanes reserved for vehicles with more than a predetermined number of occupants. This paper examines the physics of HOV lanes placed on median lanes, with open access everywhere. HOV lanes can affect the capacity of freeway bottlenecks through both an under-utilization effect and a disruption effect. An under-utilized HOV lane passing through a bottleneck obviously discharges less flow than possible. But lane changes in and out of the HOV lane can also disrupt the flow on the adjacent general purpose (GP) lanes, and reduce their...

Effects of High Occupancy Vehicle Lanes on Freeway Congestion

Daganzo, Carlos F.
Cassidy, Michael J.
2008

Previous research on the effect of HOV (high occupancy vehicle) lanes on bottleneck flows is extended here to entire freeways using both theory and empirical evidence. The paper shows that if the flows of both high- and low-occupancy vehicles remain invariant before and after a freeway lane is converted to HOV use, then the freeway’s overall traffic density upstream of its bottlenecks is reduced – albeit less than expected – if the HOV lane is underutilized. As a result, HOV lanes can extend queues over longer distances. These expansions can be problematic if the queues’ expanded portions...

Smoothing Effect on Freeway Bottlenecks: Experimental Verification and Theoretical Implications

Cassidy, Michael J.
Jang, Kitae
Daganzo, Carlos F.
2009

Real data show that reserving a lane for carpools on congested freeways induces a smoothing effect that is characterized by significantly higher bottleneck discharge flows (capacities) in adjacent lanes. The effect arises because disruptive vehicle lane changing diminishes in the presence of a carpool lane. The effect is reproducible across days and freeway sites: it was observed, without exception, in all cases tested; and queueing analysis shows that the effect greatly reduces the times spent traveling on a freeway. The effect is so significant, in fact, that even a severely...

The Smoothing Effect of Carpool Lanes on Freeway Bottlenecks

Cassidy, Michael J.
Jang, Kitae
Daganzo, Carlos F.
2010

Real data show that reserving a lane for carpools on congested freeways induces a smoothing effect that is characterized by significantly higher bottleneck discharge flows (capacities) in adjacent lanes. The effect is reproducible across days and freeway sites: it was observed, without exception, in all cases tested. Predicted by an earlier theory, the effect arises because disruptive vehicle lane changing diminishes in the presence of a carpool lane. We therefore conjecture that smoothing can also be induced by other means that would reduce lane changing. The benefits can be large....