1994 Changes to Chapter 9 of the 1985 Highway Capacity Manual
Dennis W. Strong,
The Highway Capacity Manual (HCM)1 is published by the Transportation Research Board (TRB) under the direction of the Committee on Highway Capacity and Quality of Service (#A3A10). The latest version of this document was published in 1985 in looseleaf form to allow for more frequent updates than have occurred previously. Chapter 9 of the HCM provides detailed methodologies and procedures for the analysis of capacity, delay and level of service for signalized intersections. This chapter is possibly both the most complicated and the most used chapter of the Manual, which has led to numerous critical reviews and suggestions for changes of the Chapter2. Considerable research funding has also been directed at signalized intersections. All of these considerations have led to the 1994 publication of a major update to Chapter 9, along with seven other chapters of the HCM. The subject of this paper is a discussion of the major changes which have occurred in this new version of Chapter 9.
A vast number of significant changes have been made to Chapter 9 of the HCM in this 1994 update. These changes are a result of the efforts of the Signals Subcommittee of Committee A3A10 to review the many research reports and other suggested changes which have been made over the years, and to assemble these changes into a cohesive update of Chapter 9 which would advance the state of the art for capacity analysis of signalized intersections. These changes may be categorized into three major categories 1) new methodologies and procedures which constitute a major change to the way an analysis is conducted, 2) modifications to current methodologies and procedures which result in minor but important changes to existing methodologies, and 3) clarifications which provide clearer instructions and/or extensions to existing methodologies to remove ambiguities which may have existed in the existing procedures. Table 1 below lists the major changes to Chapter 9 according to these major categories. Each of these major changes are described in detail in the paragraphs which follow.
Permitted Left Turn Factor
Defacto Left Turn Check
ProtectedPermitted Left Turn
|Table 1 - Summary of Changes by Category|
NEW METHODOLOGIES AND PROCEDURES
New methodologies and procedures are those changes which constitute a major change to the way an analysis is conducted. Five major changes to these methodologies have been made in the areas of progression factors, left turn analyses and planning methods, as described below.
A major concern regarding progression factors (PF) in Chapter 9 is that the PFs in Table 913 are based on limited field data, and due to the wide range of PF values (0.40 to 1.85), selection of a range of reasonable PFs from the table often results in a change of the level of service. To address these concerns, a major research study was commissioned by TRB which was completed in 19893. The objective of this NCHRP Project #328C was to evaluate the effects of changes in the quality of traffic signal progression on stopped delay. The general approach was to combine the findings of theoretical modeling with those from field studies and controlled simulation to obtain a comprehensive evaluation of these effects. A pilot and four field studies were conducted to collect the data necessary to calibrate the analytical progressiondelay models.
A significant improvement these models now provide is a continuous relationship between the variables rather than the discrete thresholds currently defined, in addition to their inherently better results due to more significant field evaluation. A significant change in the application of PFs is that they are now only applied to the uniform delay term of the
HCM's delay equation. Another significant change is the general use of the proportion of total arrivals which arrive on green, used in place of the platoon ratio, to estimate the PF. A new calibration factor is used for the second term, as well.
Another major improvement in Table 913 is a more logical coordination of the effects of progression with the effects of actuation, as well as the ability to apply PFs to any lane group, including left turns. The revised Table 913 has also been closely coordinated with similar changes which have been made to Chapter 11 (Urban and Suburban Arterials) of the HCM.
Permitted Left Turn Factor
Another major concern with Chapter 9 was with the permitted left turn model which is used, one that frequently produces overly optimistic results. To address these concerns, an FHWA contract was initiated4 with an objective to evaluate and improve the HCM model for estimating the impact of sharedpermissive left turn movements on lane group saturation flow rate. The study, which was completed in 1989, was based on a nationwide data collection effort at 25 intersections in four regional areas and recommended simplified regression models which differed substantially from the analytical models in the HCM. After much debate within the Signal Subcommittee and the A3A10 Committee at large, a new "hybrid" model was proposed5 which retained many of the attractive elements of the HCM's original analytical model while improving the overall results of the model by using some of the field calibrated regression results of the original FHWA study. This resultant model was accepted by the A3A10 committee.
As noted above, these results were limited to permissive left turns made from lanes shared with through movements, but the needs of Chapter 9 required a consistent method for permissive left turns made from exclusive turn lanes. To address this need, a Left Turn Analysis Task Group of the Signal Subcommittee was assembled to extend the logic of the hybrid model to include this situation, as well as other specialized cases. The result of this substantial effort was an entirely new permissive left turn methodology which is expected to deliver more realistic results in general, and in particular, for some of the more extreme left turn situations. This final model also has implications which reach further than just the basic left turn analysis, as described below.
Defacto Left Turn Check
A problem often encountered with the defacto left turn check for sharedpermissive left turns was that the defacto check did not indicate an exclusive lane analysis while the actual left turn analysis did, or vice versa. This was due to completely independent techniques which were used for the defacto check and the actual analysis. This situation has been corrected by a technique employed in the new permissive left turn analysis described above. During the course of the analysis, an intermediate calculation is made which determines the proportion of left turners in the leftmost through lane of the lane group. If this proportion is 100 percent, then the leftmost lane is declared a defacto left turn lane and the analysis is restarted with this new lane group definition. Using this process, the dilemma of the current manual is avoided and consistent results are always obtained. This comes at the cost of a few more computations than before, but it is anticipated that this will be performed by computer software in most cases, with little or no impact on the user.
ProtectedPermitted Left Turn
One of the most pervasive problems of Chapter 9 is that of analyzing protectedpermitted left turns. It is this author's experience that this is the most common difficulty people have about Chapter 9 analyses and the one that generates the most discussion about the "correct" way to conduct an analysis. The reason for this is that it is fundamentally impossible to perform a "correct" analysis with the tools presented in the 1985 HCM, compounded by a number of errors in the example of the HCM which describes how to do the analysis. This problem has been discussed ad nauseam, with maybe the most complete discussion presented by Bonneson and McCoy6. The basic problem is that the delay equation used in Chapter 9 assumes a single green interval during which a constant saturation flow rate is available. In the case of protectedpermitted left turns, this is clearly not the case, having two distinct green periods during which substantially different saturation flows are available. Much of the discussion about this problem has centered on how to make the most reasonable calculation using the 1985 HCM equations. The modifications made in the 1994 update allow a whole new approach which recognizes the dual phase nature of the problem.
The Left Turn Analysis Task Group refined the techniques presented by Bonneson and McCoy using the concepts described by Hagen7. Two key principles were added, these being that the analysis should represent the actual timings provided by the analyst, not an attempt to determine the best timings, and that the method should provide a distinction between protectedpermitted and permittedprotected phasings. The resulting methodology thus provides a completely new, clear and concise procedure for calculating capacity, V/C and delay for protectedpermitted and permittedprotected left turn treatments, all in a manner consistent with the newly adopted permitted left turn methodology described earlier.
A point of consistent frustration in using Chapter 9 procedures is with the planning method which is presented. It has been felt by many that the method is too vague, and that the implicit levels of service which are determined are too often in conflict with a more rigorous operations analysis which may be done subsequently. As a result, the current planning method is all but unused, with analysts deferring to other criticallanebased methods such as TRB Circular 212 or locally generated methods. With these problems in mind, a completely new planning method has been adopted in Chapter 9 which is closely patterned after one which has been developed for use in the state of Florida. This method produces intersection status much like the old method, but adds a new status called "At Capacity". It also bases the status on a new variable called the critical V/C ratio which is an indicator of overall sufficiency of the intersection geometrics, and is quite similar to variables used in other methods such as the ICU. This method is essentially based on the use of default values for many of the variables of the operations analysis, with the specific intent of generating results which are more consistent with the actual operations analysis.
An additional feature of
the new method is that a reasonable and effective signal timing
plan can be produced, including phasing, which can then be used
as the basis of a complete operational analysis which will
produce the normal levelofservice results. Although this
estimated timing plan is by no means an optimization of the
intersection's operation, it is felt that for planning purposes
it will be good enough to provide the basis for generating a
delaybased, level of service result from the planning analysis,
a result which has been in great demand since the release of the
Modifications are those changes to current methodologies and procedures which result in minor but important changes to existing methodologies. Two major modifications have been made in the areas of ideal saturation flow rates and heavy vehicle equivalency, as described below.
Default Ideal Saturation Flow Rate
Many surveys which have been conducted since the release of the 1985 HCM have indicated that the ideal saturation flow rate for a signalized analysis should be higher than the 1800 vphgpl indicated in the HCM. Although this has always been a variable which could be changed by a user based on local conditions, the default ideal saturation flow rate in the new Chapter 9 has been changed from 1800 to 1900 vphgpl to reflect these surveys. This is also consistent with similar changes which have been made in other chapters of the HCM, and is believed to generate more reasonable results when local data is not available. Local surveys of both ideal and adjusted saturation flow rates are still recommended whenever possible.
Heavy Vehicle Equivalency Factor
The basic heavy vehicle
equivalency factor used in the current Chapter 9 is 1.5, meaning
that each heavy vehicle is equivalent to 1.5 passenger car units
in terms of saturation flow rates. Surveys which have been
conducted in recent years have indicated that this value is lower
than observed, so the basic heavy vehicle relationships in
chapter 9 have been changed to reflect a new equivalency factor
of 2.0. This is also consistent with changes made to other
chapters of the HCM.
Clarifications are those changes which provide clearer instructions and/or extensions to current methodologies to remove ambiguities which may have existed in the current procedures. Nine major areas where such changes have been made are identified here, although many other changes of a minor nature have also been made. These notable changes are in lane utilization factors, right turn on red, improved tabulations and formulas, critical movement definition, analysis when V/C exceeds 1.0, default values, saturation flow rate surveys, editorial changes and interpretations/errata, as described below.
Lane Utilization Factors
The definition of and intended use of lane utilization factors (LUF) has been a vague commodity since the initial release of the 1985 HCM. Two particular problems were whether LUFs should be used for lane groups which included any turning vehicles, and whether they should be used at all for a typical analysis. While investigating this problem, it was determined that Case 8 (dual turn lanes) of the left and right Turn factor tables were actually accounting for lane utilization, but that the effect was being applied to saturation flow, not demand volume as with through lanes. As a result, each of these Case 8 conditions have been removed from the turn factor tables and now it is recommended that all lane groups with more than one lane should make use of the LUFs. It is also important to note that the current version of the Chapter implies that LUFs should only be used when the conditions of the "worst" lanes of each lane group are to be determined. The language now clearly indicates that LUFs should be used under virtually all circumstances.
In addition, experience has shown that the use of surveyed LUFs can generate results which are much more consistent with field observations, so a methodology has been included in the new Chapter which shows how to survey and calculate LUFs for use in the analysis.
Right Turn on Red
Treatment of rightturnonred (RTOR) vehicles has always been an unclear entity in the 1985 HCM, even after the errata of 1986 was published. Although it is clear that some treatment of these vehicles is necessary, it was still not clear what is appropriate. The new Chapter addresses this with a recommendation which is somewhat more limiting than the current Chapter, suggesting that RTOR vehicles be removed from the analysis at the start, but only when the actual number of RTOR vehicles has been surveyed. One exception has been allowed, that being when the right turn is from an exclusive turn lane and is "shadowed" by an exclusive left turn phase on the crossing street. This recommendation all but eliminates the use of RTOR in any type of planning analysis where RTOR cannot be counted.
Improved Tabulations and Formulas
An area of Chapter 9 which has been frustrating to many analysts is in the table lookup procedures and the subsequent need for interpolation which is required for any kind of a precise analysis. Although this problem is often relegated to computer software, further frustration can set in when one program uses one table lookup/interpolation strategy and another uses a different strategy, leading to unexplainable different results. The problem is exacerbated when extrapolation is required, where some programs will not allow extrapolation while others will use differing extrapolation logic. The final problem in this arena is where tabulated values do not agree with the equations which might be provided, and where the precision of the tabulated values is not enough to prevent aggravating rounding problems.
To address all of these problems, several new strategies were implemented in all numerical tables of the new Chapter. First, each table presents the equation which was used to produce the table, thereby effectively eliminating the need to interpolate in any case, since each equation is a continuous function. Second, the limits of many of the tables were better defined and expanded to encompass all possible input values, thereby effectively eliminating the need for any extrapolation. In the few cases where this was not possible, explicit instructions were provided for extrapolation, the most notable case being in the lane width adjustment factor. Thirdly, the precision of all tabulated values was increased to one decimal place beyond that which would normally be used so that any rounding errors which are created in the analysis are at a precision where the error does not create any analytical problems. Lastly, every attempt has been made to verify the accuracy of the tabulated values so that they are in exact agreement with the published equations. In this same arena, a major simplification of the right turn adjustment table has attempted to clarify the relationships of the variables and equations with each of the cases.
Critical Movement Definition
The discussion of how to determine critical movements and the related lost times for the critical path and critical V/C calculation has been substantially expanded to cover the less obvious and more complex multiphase cases. This is intended to eliminate the great amount of confusion which has surrounded this area of Chapter 9 over the last nine years. Particular attention is now paid to how and where movement lost times should be applied in the analysis, particularly for protectedpermitted and lead or lag cases. This improvement will hopefully curtail many of the problems users have had in understanding the analysis of lost time and critical V/C.
Analysis When V/C Exceeds 1.0
An area of the Chapter which has always been lacking is for the analysis of conditions under which V/C exceeds 1.0. With the new Chapter, this will still be the case, but a number of clarifications have been made in the text to better describe the conditions which cannot be analyzed accurately, as well as what options the user has. The most significant of these is with the somewhat arbitrary limitation of 1.2 for V/C. In reality, the V/C cannot exceed 1/PHF (the inverse of the peak hour factor), or a condition where the hourly volume exceeds the hourly capacity will result, a condition which is clearly beyond the ability of the Chapter to address with any accuracy. Other related clarifications in the text have been made.
The table of default values has been expanded to include analysis variables which were previously omitted, as well as new variables which have been added by the new methodologies described earlier. In some cases, the definition of the default values has been clarified so as not to cause confusion created by the earlier table. Separation of the defaults according to the operational versus planning analyses has also been provided.
Saturation Flow Rate Surveys
A critical discussion of and proposed improvements to the saturation flow rate survey techniques provided in Appendix 4 was written which has now been incorporated into Chapter 9. These additions provide a clearer description of what should be surveyed and when certain events in the survey are defined to have occurred. These changes will provide a more consistent base procedure to be used in future saturation flow rate surveys, thus resulting in a more consistent base of survey data to be used in future editions of the HCM.
A great number of editorial changes have been made throughout the text with the intent of clarifying the methodologies of the Chapter. In some cases, additional tutorial material was added to better define the conditions which are being analyzed, but the greatest portion of these changes are directed at a more precise definition of the procedures, with the objective of reduced ambiguity and increased clarity.
Interpretations and Errata
Over the years, a number
of interpretations and errata have been generated by the A3A10
Committee for Chapter 9. In all cases where these publications do
not conflict with other changes to the methodologies of the
Chapter, additional text has been added to the Chapter to clarify
the problems which these interpretations and errata address.
This paper has described many important, significant and useful changes which have been made to Chapter 9 of the Highway Capacity Manual. These changes incorporate new research and fix identified problems with the existing materials of Chapter 9, and as such are intended to advance the state of the art of signalized intersection capacity analysis. These changes are also designed to remove some of the ambiguities which have existed in the Chapter, thereby allowing users and related software to produce more consistent analyses. Software such as the HCS from McTrans for capacity analysis and SIGNAL85/TEAPAC from Strong Concepts for optimization using the new methods is anticipated to be released concurrently with the updated HCM document. Other packages have also indicated their intent to update to the new procedures.
It should be noted that
not all recognized needs in the Chapter have been addressed by
these changes, with many of these being under active research or
in search of funding for related research. Most notably,
considerations of oversaturated conditions and the affects of
actuations on intersection performance are significantly lacking
and currently under study. Other areas of need have yet to be
addressed directly, but are under active discussion within the
A3A10 Committee and Signals Subcommittee. Furthermore, the new
methodologies introduced as part of these changes have the
distinct possibility of raising new issues which further research
may need to address. As such, the Highway Capacity Manual
will continue to be a living document, with the attempt always to
put the best and most current research in front of the end user
for practical application in the "real world".
Suggestions for such future changes will be readily accepted by
the author, or any other members of the A3A10 Committee.
The author would like to
acknowledge the contributions of the A3A10 Committee, and in
particular the members of the Signals Subcommittee for their hard
work and diligence in the preparation of the materials included
in this major update to Chapter 9. As a completely volunteer
organization with no compensation for their valuable time, the
preparation of this update has been an enormous task which could
not have been completed without the dedication and commitment of
these professionals. We would also like to acknowledge the
members of the subcommittees on Unsignalized Intersections
(Chapter 10) and Urban and Suburban Arterials (Chapter 11) for
their efforts to coordinate the changes between these three
highly interrelated chapters.
1. "Highway Capacity Manual," Transportation Research Board, Special Report 209 (1985).
2. Strong, D.W., "RealWorld Use of the 1985 HCM Analysis of Signalized Intersections," Compendium of Technical Papers, Institute of Transportation Engineers, Washington, D.C. (September, 1989).
3. Fambro, D.B., Chang, E.C.P., and Messer, C.J., "Effects of the Quality of Traffic Signal Progression on Delay," National Cooperative Highway Research Program, Project 328C, Texas Transportation Institute, Texas A&M University, College Station, Texas (August, 1989).
4. Roess, R.P., Papayannoulis, V.N., Ulerio, J.M., and Levinson, H.S., "Levels of Service in SharedPermissive LeftTurn Lane Groups at Signalized Intersections," Federal Highway Administration, Contract No. DTFH87C00012, Transportation Training and Research Center, Polytechnic University, Brooklyn, New York (September, 1989).
5. Prassas, E.S. and Roess, R.P., "The LeftTurn Adjustment for Permitted Turns from Shared Lane Groups: Another Look," a paper presented at the midyear meeting of TRB Committee A3A10, Transportation Training and Research Center, Polytechnic University, Brooklyn, New York (July, 1992).
6. Bonneson, J.A. and McCoy, P.T., "Operational Analysis of Exclusive LeftTurn Lanes with Protected/Permitted Phasing," Transportation Research Record 1114, Transportation Research Board, Washington, D.C.
7. Hagen, L.T., "A Comparison of Intersection Analysis Techniques," University of Florida (Fall, 1987).
Reprinted with permission, Institute of Transportation Engineers (ITE), 1994
this page last updated March 11, 2004