Roger P. Roess

Level of Service: 2010 and Beyond

The concept of level of service (LOS) was introduced in the 1965 edition of the Highway Capacity Manual (HCM). It provided for the familiar letter-grade system for characterizing the quality of operations on a variety of traffic facilities from intersections to freeways. The LOS concept will remain primarily unchanged in the next edition of the HCM, expected in 2010. It will, however, introduce material directly related to user perceptions for the first time, and discussions surrounding LOS have raised interesting issues that may result in more extensive changes in the future. This paper attempts to address some of these issues in the context of the history of the LOS concept and its use in the planning, design, and analysis of traffic facilities. Among the major issues that should be thoroughly examined in the future is whether the concept is needed with the rapidly advancing state-of-the-art, which produces many quantifiable measures of service quality. The application of LOS to corridors, networks, and multimodal systems needs to be addressed, as it will differ from previous applications to points and uniform segments. Incorporating the results of research concerning user perceptions into the LOS framework has also raised interesting issues as the 2010 HCM has been developed. With the forthcoming 2010 HCM as a starting point, this paper explores current issues and makes suggestions as to how to address them while moving toward the as yet undefined edition that will follow the 2010 HCM.

Capacity of Freeway Weaving Segments

The analysis methodology for freeway weaving segments specified in the current Highway Capacity Manual (HCM 2000) provides a cumbersome set of multipage tables for the determination of weaving segment capacity. As part of NCHRP Project 3-75, Analysis of Freeway Weaving Sections, a more straightforward approach to estimating weaving segment capacity has been developed. This paper presents the methodology and discusses its background and development. The methodology continues to define capacity as in the HCM 2000: capacity occurs primarily when a density of 43 passenger cars per mile per lane is reached. Various other controls in HCM 2000 are replaced with revised limits on the number of weaving vehicles that can be accommodated in a weaving segment. The methodology substitutes a regression-based equation for the cumbersome tables of the HCM 2000.

Level of Service Analysis of Freeway Weaving Segments

This paper reports on the product of NCHRP Project 3-75, which resulted in the development and calibration of new models for freeway weaving segments, including a draft chapter for the forthcoming Highway Capacity Manual (HCM). The most critical element of the current procedures in the HCM 2000 is the prediction of the average speeds of weaving and nonweaving vehicles in the weaving segment. These speeds are converted to an overall density for the segment that determines the prevailing or expected level of service. The proposed method does away with the strict assignment of weaving geometries to one of three configuration categories and also the need to determine whether the operation of the weaving segment is constrained or unconstrained. That approach led to the need to calibrate 12 equations to predict speeds. In research, this greatly increased the size of the desirable database for calibration. The new approach relies on the prediction of lane-changing activity within the weaving segment to quantify the impact of configuration and type of operation on resulting speeds and densities.

Weaving Area Analysis in Year 2000 Highway Capacity Manual

The development of weaving area analysis procedures for the next edition of the Highway Capacity Manual, which is to be published in 2000, is described. Weaving has proved to be one of the most complex types of operation to analyze consistently. Although many studies and investigations have examined weaving areas, no single comprehensive research document exists that has resulted in an analysis procedure that has withstood the test of time and application, despite many attempts to do so. As the publication of the Highway Capacity Manual for the Year 2000 (HCM 2000) nears, weaving analysis procedures continue to be influenced by a variety of historic studies and documents and by several databases. None of these is statistically adequate given the number of variables involved. Thus, procedures continue to require the application of the professional judgment of the members of the Committee on Highway Capacity and Quality of Service of the Transportation Research Board. Documentation for the analyses and judgments that support the analysis procedure as it is recommended to appear in the HCM 2000 is given in this paper.

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.

HIGHWAY CAPACITY ANALYSIS AFTER "HIGHWAY CAPACITY MANUAL" 2000

The publication of the “fourth” full edition of the Highway Capacity Manual (HCM) in the fall of 2000 represented a major step forward in state-of-the-art highway capacity and quality-of-service analyses. Even as this major step is taken, however, old issues reemerge and new ones arise as to the core concepts involved, the directions that such analyses should take in the future, and the needs of HCM users. Now that HCM has begun to address system and multimodal measures, the nature and meaning of capacity and level-of-service concepts need to be reexamined. The role of simulation must be more clearly defined, as must the limitations of more conventional highway capacity analyses. As databases improve, the question of statistical accuracy and stochastic variations in standard measures may seem more important, even though the inherent variability in traffic flow characteristics remains essentially unchanged. The need for software to implement ever more complex methodologies raises additional issues. As the Committee on Highway Capacity and Quality of Service and its members consider these and other important issues, an attempt to outline the major issues and alternatives that should be examined is made. In addition, some suggestions as to potential paths to follow are provided.

Comparative Analysis of FRESIM and 1994 Highway Capacity Manual Models for Ramp Junctions with an Independent Data Base

The results of a comparative analysis of the 1994 Highway Capacity Manual (94HCM) ramp junction models and the FRESIM simulator model with an independent data base are described. The analysis was done as part of NCHRP Project 3-37 (Phase II), whose objective was to identify and investigate common ranges of application, consistency of internal logic, consistency of results, and potential modification to both the 94HCM and the FRESIM models. The results indicate that FRESIM may be a reasonable tool for predicting operational parameters within the area of ramp junctions for isolated ramps, although they also indicate that FRESIM may not be sensitive enough to some geometric parameters to be used confidently in its current form.

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.

Speed–Flow Curves for Freeways in Highway Capacity Manual 2010

Since the 1965 edition of the Highway Capacity Manual (HCM), fundamental curves representing the relationship between speed and volume or flow rate have formed the basis of design and analysis methodologies for basic freeway segments and for uninterrupted flow segments of multilane highways. As the 2010 edition of the HCM was being developed, a new set of freeway speed–flow curves was recommended. Somewhat late in the development process, the recommended curves became controversial. This paper presents the recommended curves and the research behind them and the controversies that affected the outcome. A final set of curves for HCM 2010 was recommended on the basis of extensive comparative statistical analyses of a substantial database and on several conditions that the TRB Highway Capacity and Quality of Service Committee voted to impose on the curves characteristics. Recommendations for the future are made.

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.