Elena S. Prassas

FORMULATION OF REAL-TIME CONTROL POLICY FOR OVERSATURATED ARTERIALS

The formulation of a real-time traffic control policy designed expressly for oversaturated arterials is presented, and the operating protocol is described. Its objectives are to (a) maximize system throughput, (b) fully use storage capacity, and (c) provide equitable service. This control policy, known as RT/IMPOST (real-time/internal metering policy to optimize signal timing), is designed to control queue growth on every saturated approach by suitably metering traffic to maintain stable queues. Consistent with this approach, bounds on queue lengths and signal offsets are determined. A mixed-integer linear program (MILP) tableau is formulated to yield optimal values of signal offsets and queue length for each approach. A nonlinear (quadratic) programming formulation adjusts the arterial green-phase durations of each signal cycle so that the actual arterial queue lengths on each saturated approach will continually closely approximate the optimal queue lengths computed by the MILP formulation. The policy principles are as follows: (a) the signal phase durations “meter” traffic at intersections servicing oversaturated approaches to control and stabilize queue lengths and to provide equitable service to competing traffic streams; and (b) the signal coordination (i.e., offsets) controls the interaction between incoming platoons and standing queues in a way that fully uses the available storage capacity, keeps intersections clear of queue spillback, and maximizes throughput.

The Highway Capacity Manual: A Conceptual and Research History

Used as a standard in the planning, design, analysis and operation of highway traffic facilities in the United States since 1950, the Highway Capacity Manual has also been widely used abroad and has inspired other countries to develop similar manuals. The manual has developed the twin concepts of level of service and capacity, and it has presented methodologies that allow highway traffic facilities to be designed on a common basis, as well as allowed for the analysis of operational quality under different traffic demand scenarios. Pedestrian, bicycle and transit issues are also addressed in this manual. There are two volumes, the first of which (Volume 1) is this book. It focuses on the development of basic principles and their application to uninterrupted flow facilities, such as multilane highways, two-lane highways, and freeways. Also discussed in detail are merging, weaving and diverging segments on multilane highways and freeways. In Volume 2 the focus is on interrupted flow facilities, such as unsignalized and signalized intersections, arterials, and urban streets.

EFFECTS OF ACCESS FEATURES AND INTERACTION AMONG DRIVEWAYS AS INVESTIGATED BY SIMULATION

The number, placement, and design features of driveways are important issues in access management. Simulation was used extensively so that the key parameters and variables could be controlled efficiently and so that underlying effects could be studied systematically. Some results were not surprising: deceleration lanes are important, as are acceleration lanes; the presence of driveways has some adverse effect on arterial through-vehicle speed; and the effect is greater as the number of driveways increases. But the emphasis was placed also on the effects on driveway traffic, in terms of driveway per-vehicle delay and queuing. For multiple driveways, the effect on the first (most upstream) driveway of the presence of the others was dramatic. Delay and maximum queue size increased significantly, indicating a substantial reduction in driveway capacity. This reduction was estimated at 30 to 50 percent by an analytic investigation. The effect of adding one downstream driveway was equivalent to increasing the arterial volume by 25 percent in the moderate-to-heavy flow range. Finally, the multiple driveway work revealed an oddity in that the downstream driveway performed better than it would have if it were alone. The authors attribute this to the decreased arterial environment and the “sheltering” effect of the turbulence from the upstream driveway.

Capacity Study for Protected-Permitted Phasing from Shared Lanes

Protected-permitted phasing from shared-use lanes (i.e., no left-turn bays) exists in certain situations where space and other constraints make this measure expedient in the face of growing traffic demand. An alternative method for handling this special case is proposed as an enhancement to the existing treatment in the 1997 update to the Highway Capacity Manual. The method proposed is logical and consistent. The more practical issue of whether it is also more accurate is addressed in a series of simulation cases. Simulation was used to provide the reference conditions because of field costs, number of variables to be controlled, and the fact that a well-established model exists. The results show that the existing method systematically overestimates lane group capacity by 10 to 25 percent for this special case, whereas the proposed alternative estimates the lane group capacity by ±10 percent or less.

Multiregime Adaptive Signal Control for Congested Urban Roadway Networks

A hierarchical adaptive signal control was developed and implemented in New York City to manage congestion in a complex urban roadway environment. Control strategies, including strategically regulating traffic demand and balancing the queue–storage ratio at critical intersections, work in concert to systematically alleviate congestion and improve mobility. The high usage of electronic toll collection tags in this area allows large amounts of per trip travel time data to be collected (nearly 1 million per trip travel time records daily) and used in real time for effective control. Congestion levels are mapped to different control regimes. Various demand-regulating strategies are applied at the peripheral roadways of the target control zone. These strategies proactively employ signal offsets and splits to exert a tapering and rebalancing effect on the traffic. Demand regulation results in a better use of available network storage spaces while preserving the capacity of the target control zone. Inside the target control area, a dynamic queue-balancing strategy is implemented at selected critical intersections to prevent propagation of spillovers with stabilized or diminished queues. The initial implementation covered 110 intersections in the highly congested central business district of midtown Manhattan New York City. Results to date are summarized.

Warranting Traffic Signals on the Basis of Proximity of Railroad Grade Crossings

There were two concurrent paths in this research. Path 1 estimated the probability, pvehicle/train, that a vehicle will be in the train dynamic envelope given that a train arrives. This was done by an extensive simulation effort that included some data collection on relevant gap acceptance patterns. Path 2 identified the exposure factors, selected a risk metric, and established a target or threshold for the risk, which is expressed as an upper bound on pvehicle/train. As a result of the investigation, the warrant is based on the principle that the expected fatality rate at the crossing should not exceed rates found at unsignalized intersections. The two paths converged by expressing the constraint pvehicle/train ≤ ___ (where ___ is in terms of traffic terms in common use—e.g., volumes, composition, track location distances). This constraint led to the warrant, which was then validated with data acquired or collected for the purpose. The draft warrant was prepared as a result of the undertaking, was endorsed by the National Committee on Uniform Traffic Control Devices, and is currently in the formal review process, culminating in final edits and a decision on inclusion in the next edition of the Manual on Uniform Traffic Control Devices.

Overview of Uninterrupted Flow Methodologies of the Highway Capacity Manual

“Uninterrupted flow” describes a type of facility; it is not a description of the quality of flow on a given facility or segment. Specifically, uninterrupted flow exists on any facility where there are no causes of interruption external to the traffic stream. Thus, a freeway is an uninterrupted flow facility, even when it is operating under breakdown conditions: in such cases, the causes of the interruption(s) to flow are interactions among vehicles that are internal to the traffic stream.

Modified Critical Movement Analysis Approach for Signalized Intersections

In the preparation of the fourth edition of the textbook Traffic Engineering, a modified critical movement analysis methodology for signalized intersections was developed. The methodology was developed for consistency with signal timing approaches and to allow a simplified way to do a quick, manual analysis of a signalized intersection. This paper presents this methodology and reports on a comprehensive analysis of its results as compared with those resulting from application of the 2000 Highway Capacity Manual (HCM) methodology as it is implemented in the highway capacity software package. For a wide range of situations, the critical movement analysis provides reasonable estimates of delay and level of service compared with the more complex HCM 2000 approach. This critical movement analysis methodology provides analysts with a more straightforward approach and the ability to either conduct or check an analysis manually, and it can be used as a replacement for the quick estimation method of Chapter 10 of the HCM 2000.

FREEWAY PLANNING METHODOLOGY

A major new chapter of the Highway Capacity Manual (HCM) 2000 is on freeway facilities. It is a detailed operational methodology that combines analyses of basic freeway segments, weaving areas, off-ramp areas, and on-ramp areas. However, the new chapter does not contain guidance or examples for planning or preliminary engineering applications. To meet its numerous needs, Florida Department of Transportation engineers wanted to develop a freeway facility application that extends the HCM for generalized planning and preliminary engineering purposes but is not inconsistent with HCM 2000. Such a methodology was developed, documented, made into an executable software program called FREEPLAN, and is now being implemented throughout the state. The methodology is firmly based on HCM detailed analysis procedures but has assumptions and defaults that allow planners and engineers to use it effectively. At a generalized planning level, the basic construct was to provide tables of design volumes, v, and annual average daily traffic that could be achieved for various levels of service and freeway configurations for the default parameter values. At a preliminary engineering level, specific freeway facility inputs are used to determine v/c ratio, average travel speed, average density, and level-of-service grades. The initial results of applying the Florida freeway planning methodology to actual Florida data were outstanding in both urbanized and rural areas.

Improving the running times in Highway Capacity Manual table 11-4; Related observations on average travel speed

An arterial planning methodology has been in use in Florida for several years and influences decisions in developments of regional impact and other analyses. It is incorporated into the Florida Level of Service Manual. Experience has indicated that analysts using the default values are concerned that the average travel speeds are underestimated by 10 km/h or more. Given the known limits of the data underlying the arterial running time and running speed default estimates in the Highway Capacity Manual (HCM), it was decided to conduct an extensive simulation analysis using a well-established model (NETSIM, now part of CORSIM). The findings are that (a) simulation-derived running speed estimates are at least some 10 km/h higher than the HCM default values, (b) moreover, the running speed estimates generally do not decrease as sharply as the HCM default values as signal density increases, and (c) unlike the HCM default values, the simulation-derived estimates are sensitive to increased demand volume. In addition, the derived values clearly reflect the through vehicles, consistent with the original HCM intent.