Since the introduction of the 1985

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1 IComparing the 1985 HCM and the ICU Methodologies BY JOHN F GOULD Since the introduction of the 1985 High way Capacity Manual, there has been much discussion concerning its application. This is especially true in Southern California, where most government agencies and consultants have not yet agreed on a methodology for calculating signalized intersection capacity and level of service (LOS). Current methodologies include the 1985 Highway Capacity Manual (HCM), intersection capacity utilization computation (ICU), critical lane analysis, and, in some cases, Circular This paper has been written to present arguments to a recently established committee within the Riverside-San Bernardino ITE Section. This committee was formed to address differences in the current methodologies used within the area. This paper compares the HCM and the ICU methodologies, which are the most widely used and the most controversial. Each methodology is briefly explained, and the deficiencies and advantages of each are identified. The methodologies are then compared and discussed, and the resulting differences in the determination of LOS are examined. Intersection Utilization Capacity The ICU methodology is unique to the state of California and resulted from frustrations encountered using the 1965 HCM. Robert W. Crommelin s article, Employing Intersection Capacity Utilization Values to Estimate Overall Level of Service, m Trajjic Engineering (July 1974), described the ICU methodology in some detail. ~ This methodology requires the calculation of the intersection volume/capacity (V/C) ratio. The LOS is directly related to the intersection V/ C, with LOS A being 0.60 or less, LOS B being between 0.61 and 0.70, LOS C being between 0.71 and 0.80, LOS D being between 0.81 and 0.90, and LOS E being between 0.91 and LOS F is a V/C that exceeds The intersection V/C, or ICU, is the summation of critical lane group flow ratios with an adjustment added to account for cycle length, number of phases, and clearance interval. The equation for this adjustment is as follows:s YA = Where Summation of Critical V/S (CYny)-1 YA = Clearance Adjustment, V/S = Saturation Flow Ratio, C = Cycle Length, n = Number of Phases, and y = Clearance Interval. The VIS is usually referred to as VIC in the ICU methodology for the benefit of lay readers. It was believed that nonprofessionals would better understand capacity than saturation flow. Crommelin identified several weaknesses in this method, which he felt needed further refinement and research: The method assumes optimal signal operation and timing, which too often is not the case. In addition, the calculation method infers that longer cycle lengths with fewer clearance periods would lead to lower ICU and improved operations, which is definite] y not the case. Adjustments for time lost to the inefficiencies of signal coordination systems, maximum pedestrian crossing times and other factors must also be made. s He concluded, however, that the method may be a valuable tool in evaluating present and future traffic conditions. It is important to note that this methodology is often not followed as it was originally published. Current use varies from one government agency or consultant to another different saturation flow values and varying methods of adjusting for the clearance intervals are used. Many traffic engineering professionals in Southern California currently use a saturation flow of 1600 and a constant of 0.10 for the clearance adjustment. Others, however, may use a saturation flow of 1700, and constants for the adjustment may vary depending on the area. The use of constants for the clearance adjustment ignores the dynamics regarding the cycle length and number of signal phases this may have significant effects upon the ICU results. A peak-hour factor (PHF) is also suggested in the ICU methodology; however, this suggestion is usually ignored in current ICU application. The deficiencies identified in Crommelin s article have yet to be addressed in this methodology, and the nonstandardized application of the ICU may present problems regarding differences in opinion. I have identified other defi- ITE JOURNAL. AUGUST

2 ciencies in the method as it is published in Crommelin s article and in the current application of the methodology. The ICU does not adequately address signal phasing, including permissive left turns, protected left turns with lower volumes and short cycle lengths, concurrent (nonoverlapping) phasing, and shared leftthrough lanes. The ICUmethod calculates aflowratio that can relate to the distribution green time for a critical left-turn movement; however, no consideration is given regarding gaps in the opposing traffic flow when the movement is permissive. Therefore, an assumption of a left-turn phase must be made, which would require most intersections to operate with a minimum of four phases, resulting in an increase in the adjustment for the clearance interval. An analyst using the ICU method may, on occasion, reduce the left-turn volumes to adjust for leftturn movements on the clearance interval. This adjustment assumes that two left-turn movements can be made per cycle without a protected turn phase. This reduction, however, is dependent upon the cycle length. Protected left turns with lower volumes may result in low flow ratios, which may amount to only a few seconds of green time when distributed relative to the available cycle length. Signal timing, however, should consider a minimum of 5 or 6 seconds per green interval. For example, a left-turn volume of 100 vehicles may require a minimum cycle of 135 to 140 seconds in order to achieve a green interval of greater than 5 seconds, depending on saturation flow and clearance time. If a lesser cycle length is used, an adjustment in the flow ratios is necessary to account for a minimum green interval. When overlaps are absent, a situation common among pretimed signals or limited actuated signals, a different approach in determining the critical movements becomes necessary. The intersection V/C or ICU should not be the sum of opposing critical flow ratios, but should be the sum of the critical flow ratios during each given phase. This difference, therefore, may result in the critical left-turn and the critical through movements being from the same approach. It is possible to analyze an intersection with shared left-through lane groups, and the ICU value may indicate an acceptable LOS. The addition of left-turn lanes, however, may theoretically worsen the LOS in some cases. This is because the critical V/S of a left-through lane group would not be evaluated as a separate left turn and an opposing through lane. The determination of an intersection s capacity should account for the shared lane, which may have a much lower saturation flow rate because of conflicting movements from the opposing approach Highway Capacity Manual Many changes in the HCM have occurred since it was first issued in The definition of LOS and the procedures used to calculate it have changed significantly. For signalized intersections, two methodologies have been developed: the planning and the operational methods. The planning method is a quick procedure that requires only the use of traffic volumes and intersection geometry. Its purpose is to develop an approximate intersection geometry; however, it only addresses general capacity and not an intersection s signalized operation or LOS. This method is intended to be used when an intersection s operation or details of geometry and traffic flows cannot be fully defined. The planning method uses a critical lane analysis to evaluate an intersection s proposed geometries, essentially by totaling the critical lane volumes. The planning method has a worksheet for assigning lane volumes, a process that requires the determination of passenger car equivalents. The HCM states that this concept is particularly important in shared-lane situations. The sum of the critical movements is related to under capacity (a critical sum less than 1200), near capacity (a critical sum between 1200 and 1400), and over capacity (a critical sum greater than 1400). The result of near capacity is often misunderstood. Many traffic professionals mistake near capacity to be an LOS D, which is not the case. The intersection s LOS is not addressed in this analysis. The HCM defines near capacity as a condition that may be under capacity or over capacity a possible V/C ratio more or less than Evaluating the capacity and LOS of an intersection with a sum of critical lane volumes between 1200 and 1400 requires more specific details regarding its operation in order to conclude whether it is acceptable. The HCM signalized intersection analysis separates capacity measures and LOS criteria in the operational procedure. The operational method is used when an intersection is relatively well defined and requires a more detailed analysis to measure its level of operation. This method of signalized intersection analysis is very different from all earlier methods. The definition of LOS has changed significantly. Most other methods relate LOS to capacity; however, the HCM relates LOS to delay, thereby providing greater information relative to an intersection s operation. The operational analysis is a very detailed and complex procedure for calculating capacity and determining LOS. Both the intersection s capacity and LOS require evaluation. LOS is related to delay as follows: A delay of 5 seconds or less is an LOS A ; 5 to 15 seconds, LOS B ; 15 to 25 seconds, LOS C ; 25 to 40 seconds, LOS D ; 40 to 60 seconds, LOS E ; and greater than 60 seconds, LOS F. The HCM is structured in a manner such that five modules are used in the calculation of capacity and LOS. These modules or worksheets are labeled input, volume adjustment, saturation flow adjustment, capacity analysis, and level of service. The input worksheet requires the collection of the data used in the subsequent analysis. The most important inputs include traffic volumes, intersection geometry, signal timing and phasing, arrival type, conflicting pedestrians per hour, and PHFs. The analyst provides this information for each approach to the intersection. The type of signal control (pretimed, semiactuated, or fully actuated) is also required. The arrival type (Types 1 to 5) and signal type, as described in the HCM, are later used in the LOS worksheet to derive the progression factor, which describes the efficiency or inefficiency of system and/or signal controllers. The PHF is important in order to address peaking characteristics of an intersection. The HCM uses a default PHF of This factor should be used when a PHF cannot be determined from the available traffic volumes. 36. ITE JOURNAL. AUGUST 1990

3 Pedestrian traffic becomes a very critical condition, especially within a central business district (CBD) environment. The provision of a minimum green interval is often dependent on the pedestrian volumes and the width of the approach. The required green time may have a substantial impact upon an intersection s delay and the resulting LOS. The provision of additional green time activated by a pedestrian push button may also have an effect. The volume adjustment worksheet calculates the peak 15-minute flow rate for each approach and determines the lane group volumes. Lane utilization factors adjust lane group flows to account for inequality in lane distribution. These factors range from 1.00 for a single lane within a lane group to 1.10for three lanes within a lane group. Lane utilization factors are typically used for through movements and not for left-turn or exclusive right-turn movements. These factors become 1.00 as lane group V/C ratios approach 1.00 or capacity. Therefore, it should be recognized that if lane utilization factors greater than 1.00 are used, the intersection V/C may actually be less than that reflected in the capacity worksheet. In addition to these adjustments, the proportion of left- or right-turn movements within a shared lane group is calculated for their use in the saturation flow adjustment worksheet. The saturation ilow adjustment worksheet requires substantial use of the HCM for the needed factors for number of lanes, lane widths, heavy vehicles, grades, parking, bus blockage, area type, right turns, and left turns. Many of these factors are not significant alone, but collectively, they may reduce a lane group s saturation flow rate substantially. These factors are multiplied by the ideal saturation flow rate of The most important of these adjustments are the right- and left-turn factors. These adjustments become critical in any analysis of shared lane groups or permissive turn movements. The actual saturation flow rate may be substantially less than the ideal rate depending on the lane geometries and conflicting movements. The calculation of the intersection V/ C examines the overall intersection, as well as the identified lane groups. This information may be very helpful in the analysis of either an intersection with existing traffic volumes or future traffic volumes, if one is conducting a traffic impact study. The capacity analysis worksheet determines flow ratios using both the adjusted volume and saturation flows. The intersection V/C or critical X is determined by summing the critical flow ratios during a given phase when concurrent phasing is provided or by summing the critical opposing flow ratios when appropriate overlaps are provided. The critical X is obtained from the following equation: x= Where x= Vls= c= L= (Sum of the Critical V/S) C-L Intersection V/C Ratio, Flow Ratio, Cycle Length, and Start-up Loss Time (default, L = 3 sec.). x C If the sum of the critical flow ratios exceeds 0.90, the probability that the intersection V/C will exceed 1.00 becomes great, requiring improved geometry, phasing, or revised lane utilization adjustments. However, the sum of the critical flow ratios less than 0.90 may, in most cases, provide a V/C of less than The HCM recommends that the intersection V/C be kept less than 1.00 in order to ensure proper intersection geometry and phasing. The capacity worksheet also examines the lane group capacity (v/c) requiring signal timing data. The analysis uses the green time/cycle length and the saturation flow to determine the lane group capacity. The ratio of adjusted traffic volumes to the lane group capacity can then be determined. It is suggested in Roger P. Roess article, Development of Analysis Procedures for Signalized Intersections in the 1985 Highway Capacity Manual; in the Transportation Research Record 1112,4 that high lane group v/c ratios may indicate an increased signal timing efficiency. However, high v/c ratios may result in individual cycle failures during the 15- minute peak; therefore, some judgment should be applied. Low lane group v/c ratios, however, are not encouraged because they imply inefficient signal timing. The lane group v/c ratios are very important in the LOS worksheet. This worksheet calculates the lane group, average approach, and average intersection delays, which can then can be related to LOS. Both uniform and random delays are determined using equations developed for the HCM. These delays are then adjusted to account for the arrival type and type of signal (pretimed, semiactuated, or fully actuated). This adjustment is related to the lane group v/c ratio in order to account for the specific traffic demand. The approach and intersection delays are weighted averages resulting from the lane group delays. It is important to understand that lane group v/c ratios exceeding 1.20 should not be used to determine average delays or LOS, because of the limitations of the delay algorithms. Caution should also be used when calculating delays using lane group v/c ratios exceeding Research made clear that delay was related to several variables: signal progression, cycle length, green times, V/C ratios. The V/C ratio is considered least important for ratios up to approximately This research resulted in the new definition of LOS. The HCM has several weaknesses, including the time required to perform calculations manually and the analysis of protected plus permissive left-turn phasing. Also, questions have been raised concerning the adequacy of the progression factors. The calculation of the intersection V/C ratio also does not address lower left-turn volumes and typical cycle lengths; however, the calculation of lane group VICratios and LOS does address this potential situation. Current research and re-evaluation of the HCM signalized intersection methodologies are underway, and these deficiencies and questions may be resolved. Comparison Discussion The ICU and the HCM methodologies have some similarities; however, differences in the results derived from them are significant. The most important similarity between the methodologies is the calculation of the intersection capacity. Both utilize the summation of critical flow ratios to determine the intersection V/C ratio. The equations used for calculating an ICU and an intersection V/ C, or critical X, are identical, with the exception that the HCM uses the startup loss time rather than the time lost to the clearance intervals (L= ny). The following simple proof illustrates this rela- ITE JOURNAL. AUGUST

4 tionship (substitute L for rry and x for the summation of critical flow ratios): Icu = x + {x/[(c/l)-l]}. If ICU = Critical,%, x+ {x/[(c/l)-l]} = (xc)/(c-l). [x(c/l)-x +x]/[(c/l)-l] = (xc)/(c-l). [x(c/l)]/[(c/l)-l] = (xc)/(c-l). (xc)/(c-l) = (xc)/(c-l). Therefore, ICU = Critical X. The calculation of the intersection V/C and an ICU, however, may still vary between the methodologies because of the saturation flows used and the HCM S use of phasing as a factor in determining critical flow ratios. These differences may or may not be significant, depending on the intersection geometry and whether phasing overlaps are available. The HCM better defines the saturation flow, whereas the saturation flow used in the ICU method is often an arbitrary number ranging between 1500 and 1800 vehicles per hour. The calculation of the saturation flows provides an effective procedure to analyze shared lanes and permissive movements, which is currently deficient in the ICU methodology. Single-lane approaches are also better analyzed by the HCM because conflicting movements that may lower the saturation flow rate are considered. The ICU does not consider the conflicting movements of opposing single-lane approaches, which may significantly reduce the ICU value, resulting in a much higher LOS. The greatest difference between the methods is in the definition of LOS. The HCM utilizes delay, whereas the ICU utilizes capacity. The Transportation Research Board, in evaluating the definition of LOS, acknowledged that the basis of delay used in the HCM was an improvement over the previous method of describing intersection operations. The research indicated that V/C ratios less than 0.90 were minor relative to the effects of progression, signal timing, and cycle lengths. Long delays, indicating poor intersection operations, could often occur with low V/C ratios. Intersection VIC ratios between 0.80 and 0.90 can occur with a minimum delay cycle length. However, this range of V/C ratios using the ICU definition would indicate an LOS D. The actual intersection operation may in fact be better than this indicates. The emphasis on signal progression, timing, and cycle length addresses many of the weaknesses in the ICU methodology as identified by Crommelin in The ICU methodology has a broad range describing LOS A (V/C of 0.60 or less). The HCM has a very narrow range for a LOS A (a delay of five seconds or less). My experience is that an ICU analysis results in an LOS A much more often than an analysis using the HCM. With the installation of a warranted signal, the operation of the intersection is not typically an LOS A without significant intersection geometries or marginal turning movement volumes. During the development of the 1985 HCM and the process of defining of LOS, the question regarding the boundary between LOS A and LOS B provoked the most discussion. Data proving the practicality of LOS A and delays of 5 seconds or less were provided. The remaining delay thresholds relative to LOS elicited no significant controversial discussion by the Transportation Research Board committee.4 Another advantage of the HCM is that results of the analyses can be verified in the field by measuring an intersection s delay. The ICU method, however, does not have a clear means of verifying an intersection s LOS. LOS descriptions for the ICU method do indicate the number of cycles that a vehicle may wait, implying delay, but do not establish any thresholds as does the HCM method. In addition, LOS A through C, as defined in the ICU method, suggest waits of less than one cycle. The differences between LOS A through C relative to their delay are less precisely defined. These waits can also vary depending on the cycle length. The first comparative analysis between the HCM operational method and the ICU method applied to an intersection located in the city of Riverside, California, and resulted in calculations of very diverse LOS. This comparative analysis was conducted by various members of the committee established in the Riverside/San Bernardino ITE Section. The intersection analyzed was considered to have typical geometry: a separate left-turn lane, a through lane, and a shared through and right-turn lane on each approach. The left-turn volumes were low except for one approach, which had a fairly high volume. The intersection operated with an eight phase actuated controller. The results, summarized in Table 1, illustrate a great difference in LOS. It is interesting to note that the V/C ratios derived from using the 1985 HCM method and the ICU method as described by Crommelin are very close, even though LOS differ greatly. When a member of the Riverside/San Bernardino Section who was very familiar with the intersection was asked his perception of the intersection s LOS, he identified it as LOS D. This member was unaware of the prior comparison. The concern about perceived LOS and the ICU method is illustrated by this one example. This concern help lead to the Transportation Research Board s conclusions that LOS is better based on delay rather than on the intersection s V/C ratios. Other comparison studies conducted during the development of different traffic studies have also resulted in great variations between LOS. However, many more comparisons are needed. Arguments against the HCM method have primarily concentrated on the time required to perform these calculations without the assistance of microcomputers and the question of whether the detailed results in the procedure justify the time commitment when using it to conduct a traffic impact study. The ICU method has an advantage over the HCM method in that its simple procedure allows the analyst to evaluate an intersection quickly. Computer spreadsheets also allow for these calculations to be easily accomplished. If the HCM procedure had to be done manually, it would not be the ideal procedure for the development of traffic impact studies. However, it is important to learn the methodology, even while using the available software currently on the market. The availability of a variety of microcomputer software programs developed for the 1985 HCM has essentially resolved the problem of the time required to perform the calculations manually and therefore should resolve the reluctance that many feel toward using the HCM. The degree of detail generated by the HCM provides important information in the evaluation of an intersection. A trafficimpact study should consider the evaluation of existing intersections and street sections and a project s impact upon them. Signalized intersections un- 38. ITE JOURNAL. AUGUST 1990

5 Table 1. Comparative Analysis AM. Peak PM, Peak Method Used Vlc LOS Vlc LOS HCM 0.61 c 0.70 D Icu 0,58 A 0.70 B Icu 0.57 A 0.59 A lcu calculating conducted with proccdurc described in cited reference 3. bicu calculation m currently conducted by many agencies and consultants (does not account for cycle length). der existing conditions have a defined capacity and LOS. The HCM better evaluates the conditions, resulting in a more accurate determination of capacity and LOS. Existing project analyses should also use this method in order to better evaluate a site s specific impact upon any intersection. Analyses for future year scenarios should use the same method in order for the results to be truly comparable. A traffic study should act as a preliminary design report, in addition to addressing a project s specific impact. The preliminary design should address the possibility of shared lanes, lane widths, signal phasing, and evaluation of signal timing relative to any limitations regarding minimum time required for a phase or phases. The HCM can help identify potential lane group deficiencies, which should be considered in the recommendations regarding storage lengths. It is also very desirable to determine an LOS. An ICU may not adequately assess LOS. The HCM better addresses any questions that may arise regarding the design of identified necessary improvements. TRANSYT-7F, PASSER 87, and PAS- SER 88 use the HCM delay equations in their analyses and thus improve these analyses greatly. Moreover, these programs relate capacity of intersections and lane groups to these signal system evaluation models. These signal modeling softwares are widely used. The HCM should be understood to better evaluate the results of signal system modeling. The use of the Highway Capacity Software (HCS) is currently being recommended in conjunction with SOAP, PAS- SER, and TRANSYT-7F. It should be noted that the ICU methodology is much like that used in the HCM planning method. Both utilize critical lane analysis and disregard timing parameters. The HCM planning method, however, does distribute traffic for shared lanes. The principle of addressing capacity and not LOS, as with the HCM planning method, should be applied to an ICU. The measurement of capacity should also be considered approximate due to other possible considerations as identified in this paper. In the absence of shared left-through lanes, the ICU method maybe more appropriate than the HCM planning method. The ICU determines an intersection V/C ratio that can be better related to the HCM operational analysis than the planning analysis. An ICU should be used in the determination of an approximate intersection geometry and capacity. I would encourage the use of ICU in the determination of an approximate intersection geometry and capacity. I use computer spreadsheets to analyze intersection geometries using the ICU. Capacity and LOS, however, are calculated using the HCS. Conclusion Having described both the HCM and ICU methodologies and discussed their advantages and disadvantages, I hope I have provided a strong argument in favor of using the 1985 HCM. This, however, is not to say that an ICU should not be used. Its easy procedure and calculation of an intersection s V/C can assist the analyst in determining possible intersection geometries to be evaluated in more detail using the HCM method. This paper identified the major differences between the two methods to be the definition of LOS and the adjustments made to calculate flow ratios for their use in determining intersection capacity. The ICU method has been used for many years in Southern California, and there appears to be a reluctance to use the HCM. However, in order to address the weaknesses identified in Cromme- Iin s earlier paper and in this paper, greater consideration of the HCM should be given. Additional research continues to be conducted for the HCM, and further refinements and improvements can be expected. Existing capacity software continues to be improved, and new software is being developed. Current signal system modeling software also utilize different aspects of the the HCM and therefore require a greater understanding of the HCM. The traffic profession s credibility depends on the resolution of current inconsistencies. Analyses need to better represent the actual or anticipated LOS in order to evaluate project impacts and identify any existing deficiencies. It is hoped that the newly established committee within the Riverside/San Bernardino ITE Section can resolve many of the questions surrounding this issue, as well as collect additional data to be included in any future revisions of the HCM. The discussions at committee meetings have made it clear that many problems exist in the current application of capacity and LOS analyses. References Transportation Research Board. Highway Capacity Manual. Special Report 209. Washington, D, C.: TRB, Transportation Research Board, Circular 212. Washington, D. C.: TRB, Crommelin, Robert W. iemploying Intersection Capacity Utilization Values to Estimate Overall Level of Service. Trafjic,&gitreering (July 1974): Roess, Roger P. Development of Analysis Procedures for Signalized Intersections in the 1985 Highway Capacity Manual. Transportation Research Record 1112; John E Gould is a transportation engineer with J. E Davidson Associates, Inc., in Riverside, California. He currently manages the development of traffic.. studies and the design of various design projects. Gould worked with the City of Nashville s Traffic and Parking Commission while he completed his B. E. degree in civil engineering from Vanderbilt University. He is an Associate Member of ITE and a member of the newly established Riverside/ San Bernardino Section of ITE. ITEJOURNAL. AUGUST