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Capacity Analysis of Interchange Ramp Terminals: Final Report (1997)

Chapter: APPENDIX A State-Of-The-Art

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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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Suggested Citation:"APPENDIX A State-Of-The-Art." Transportation Research Board. 1997. Capacity Analysis of Interchange Ramp Terminals: Final Report. Washington, DC: The National Academies Press. doi: 10.17226/6350.
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APPENDIX A STATE-OF-TlIE-ART A.1 SURVEY OF CURRENT PRACTICE A comprehensive evaluation of the state-of-the-art in areas related to interchange design and traffic operations was conducted as part of this research. The focus of this Valuation was on issues underlying the design and operation of interchanges In urban or suburban areas. More specifically, the issues related to the signal-controlled ramp terminals and traffic flow along the cross street through the interchange. Consideration was also given to the relationship between the interchange terminals and any adjacent, closely-spaced signalized intersections. One aspect of the evaluation involved a survey of transportation engineers. The intent of this survey was to gain insight into the current practices and concerns of engineers who evaluate interchange traffic operations. The survey was conducted In two stages. The first stage consisted of a one-page questionnaire. This questionnaire was intended to obtain basic types of information such as: 1. Common interchange types (geometric configurations) 2. Common operational problems 3. Common interchange operations analysis techniques 4. Common measures of effectiveness used for evaluation 5. Willingness of the respondent to participate in the second-stage survey. The second-stage survey was designed to obtain more detailed information about interchange operations. This survey asked the respondent to select one interchange that they were familiar with and then respond to detailed questions about its operation and any steps taken to alleviate flow problems at this Interchange. The respondent was also asked to describe the analysis techniques (or computer models) Hat they had successfully used to evaluate interchange operations. The findings from these two surveys are described In the next section. A.. F~rst-Stage Survey Distribution. The f~rst-stage survey was sent during the first week of February, 1994. More than 2,400 surveys were sent out to engineers In the U.S. and abroad. The members of the following groups were specifically targeted: AASHTO Subcommittee on Traffic Engineering; AASHTO Subcommittee on Design; AASHTO Special Committee on Transportation Systems Operation; ITE Urban Traffic Engineers Council; and ITE Consultants Council. A

Individuals in these groups include engineers responsible for planning, design, and operations of transportation facilities In the United States. In addition, several hundred surveys were sent to selected members of the Institute of Transportation Engineers OTE). After a thorough review of each returned questionnaire, a finalized total of 350 first-stage questionnaires were deemed completely responsive and valid. This group represents a 15-percent response rate, which is within the 10 to 20 percent rate expected prior to the survey. Overall, there were 146 responses from the public sector, Including 68 from state Dons, 63 from cities in 16 states, and 15 from counties In ~ states. Seventeen responses were received from outside of the United States (i.e., Canada - Il. Germany - 2, South Africa - 41. Responses were also received from IS7 consultants in 23 states. The geographical distribution of the responses is summarized in Table Apt. Results. The f~rst-stage questionnaire consisted of six questions, primarily requesting but not limited to multiple-choice replies. The results for Questions I, 3, 4, 5, and 6 are provided in Table A-2. The response format for Question 2 is somewhat different from the other questions and will be discussed separately. Question ~ inquired about the frequency of interchange operations analysis. An analysis of the survey responses shown in Table A-2 indicates that cities/counties and consultants evaluate interchanges about "3 to 6 times per year. " In contrast, most state DOTs evaluate interchanges "less Man 3 times per year." However, a relatively large percentage of state DOTs indicate that they evaluate interchanges as frequently as "once per week." Question 3 asked the respondents to rank the Operational problems listed in terms of their frequency of occurrence at interchanges that the respondent is personally aware of through their work experiences. As indicated In Table A-2, "~nadequatecapacity" was given the highest ranldng signifying it as the most frequently occurring problem. This problem was followed by "queue spillback" and then "weaving" in terms of Heir frequency of occurrence. Operational problems other than the three listed were described by twenty-two respondents. A common theme In these problems was a lack of effective signal coordination between the ramp terminals (or between the ramp terminal and adjacent signalized intersections. Question 4 inquired about He types of analysis methods used to evaluate Interchange traffic operations. In general, software methods were more frequently used than manual methods. The most commonly used software method is the Highway Capacity Software (HCS). PASSER IT and TRANSYT-7F were also found to be frequently used. Question 5 inquired about the most useful measure of effectiveness (MOE) for evaluating interchange traffic operations, particularly at ramp terminals. The most commonly selected MOE was delay, followed by spillback frequency, and volume-to~apacity ratio. The "other" category was infrequently used. Those that did use it indicated Hat speed or travel time measured along the cross street through the interchange would be most helpful. A - 2

Table A-1. Geographical distribution of responses to first-stage questionnaire. Number of Responses Alabama Alaska Arizona Arkansas California Colorado Connecticut Delaware noridla Georgia Hawaii Idaho Illinois Indiana Iowa Kansas Kentucky Louisiana Maine Maryland Massachusetts Michigan Minnesota Mississippi Missouri State Montana Nebraska Nevada New Hampshire New Jersey New Mexico New York Norm Carolina Nor h Dakota Ohio Oklahoma Oregon Pennsylvania Rhode Island South Carolina South Dakota Tennessee Texas Vermont . . I Vlrg~a |~ash~gu)n | West Virginia | Wisconsin 1 Wyoming Number of Responses A - 3

Table A-2. Summary ~ I Numbe - (Percent) of Responses Response | City &County i State DOT I Question 1. How often do you analyze some aspect of traffic operations at signalized intersections? [~ Once per Week l 11 (14%) | 18 (26%) l 0~ it- Ma at, 13 (17%) 14 (21%) . ~ w ~ Am. per Ye~r 28 (36%) 11 (16%) . [~;I:~Wu 24 (31%) 24 (35%) !~1~5~e 2 (3%) 1 (1%) 1 Question 3. Please rank the operational problem that you have encountered on the cross street at the most frequently used "Existing" interc Lange type. (1- no problem; ~ - serious problem) | Average Rank Inadequate Capacity 3.7 3.7 W~: ~ 2.2 2.3 t1~ Kid 1~:k 3.5 3.5 uestion 4. What analysis techniques do you use to evaluate traffic operations at the interchange ramp terminals? (mark all that apply) l Numbt (Percent) of Responses' Highway Capacity Manual 25 (32%) 20 (29%) Over Manual Methods 9 (12%) 9 (13%) Highway Capacity Software 38 (49%) 48 (71%) TRANSYT-7F 37 (47%) 30 (44%) PASSER II 35 (45%) 31 (46%) PASSER III 22 (28%) 20 (29%) JI:A~81M 8 (10%) 15 (22%) Other Software Methods 10 (13%) 9 (13%) Question 5. What measure of effectiveness would be the most useful in evaluating traffic operations at the ramp terminals? (Including ~:1b ADAM 1 ~30 (38%) 33 (49%) Queue Spillback Frequency 34 (44%) 39 (57%) Delay per Vehicle 52 (67%) 44 (65%) Thru Movement Bandwidth 30 (38%) 21 (31 %) Saw ~r `/:h~l: 17 (22%) 10 (15%) ~5 (6O ~1 (1%) Question 6. Would you be milling to respond to a more detailed questionnaire concerning the details of interchange ramp Junct_ ~ _~ Yes 1 36 (46%) 1 38 (56%) . . No 1 34 (44%) 1 28 (41 %) No Response 1 8 (10~) l 2 (3%) Consultant 32 (17%) 50 (27%) 70 (37%) 35 (19%) O (0%) 3~8 2.6 3.7 77 (41%) 11 tS~) 161 (86%) 57 (30%) 35 (19%) 35 (19%) 80 (43%) 99 (53%) 115 (61%) 34 (18%) - 29 (16%) 10 (5%) 105 (56%) 70 (37%) 12 (6%) Notes: 1 - Percentages for Questions 4 and 5 do not sum to 100% due to the "mark all that apply" nature of the questions. A - 4

Question 6 inquired about the willingness of the respondent to participate in the Second- Stage Survey. All total, 179 U.S. respondents indicated that they would be willing to participate. This represents about 51 percent of the 350 U.S. responses received. Question 2 was used to identify the interchange configurations Mat were most commonly being evaluated by engineers. The frequency of evaluation was further categorized by "existing interchanges, ~ "interchanges In design, " and "interchanges in planning. " This categorization was helpful In identifying current trends In Interchange design. Variations of the question were prepared for engineers employed in the public or private sectors. Engineers employed in the public sector were asked to assess the percentage of each type of signalized interchange in their jurisdiction that are "existing," "in design," or "in planning." Engineers employed in the private sector were asked to assess the percentage of interchanges that Hey typically evaluate as part of their consulting activities. The response to Question 2 is illustrated In Table A-3. As the data in this table suggest, the most commonly evaluated Interchange configuration is He Compressed Diamond. However, all of He interchange forms were selected with sufficient frequency as to suggest Hat none should be excluded from consideration in the development of melons to evaluate Interchange operations. CD TD TDw/F PC Other Responses Table A-3. Distribution of interchange type by agency and stage of project development. ,. ., Interchange Existing Interchanges Typlel City or County . 30% State DOT 59% 13% 8% . 0% . 0% . 52 Design or Construction Consultant City or Colmty _ State DOT _ _ ~ 45% 38% _ 13% 17% 5% 9% l ~ 21%0 30% _ . 6% 6% 32 76 COIlSllltaIlt City or County 37% 9% 27% 9% 8% Planning State Consultan DOT 52% 40% 4% ~ 16% 10% 6% 24% 32% 0% 6% . 11 1 21 1 104 Notes: 1 - InterchangeType Descriptions: CD - Compressed&amond (ramps 120 tO 240 m); TD - Tight diamond (ramps less Man 120 m); TDw/F - Tight diamond win frontage roads; PC - Partial cloverleaf of several variations. Discussion of Results. The first-stage survey results show that practicing engineers are concerned win the effective operation of interchanges. Questions 1, 2, and 3 were asked to determine how much and what is being done on interchange design and Aerations. Based on the replies given for Question 3, engineers frequently encounter operational problems at interchanges in urban areas. However, it appears that neither the reasons for the problems (e.g., lack of capacity, queue spilIback, weaving, etc.) are well understood nor are He solutions (e.g., ~nterchange-specific analysts techniques) readily available. These Innitations hinder an engineers ability to analyze interchange traffic operations. A - 5

As a means of examining the operational problems at interchanges in more detail, the second-stage questionnaire was developed and distributed to the interested f~rst-stage respondent. The results from the second-stage questionnaire can be found in the next section. Question 2 verified that the diamond interchange (either compressed or tight urban) was the most common existing, designed, and planned interchange. This fact is likely due to the reduced right-of-way costs associated with interchanges of the diamond familY. relative to those of He partial cloverleaf family. Ha, of, Question 4 was asked to determine which traffic models are being used by practicing engineers to evaluate interchange operations. The most common type of analysis used by the respondents is computer software models and, most often, He Highway Capacity Software (HCS). This may be due to its widespread acceptance, consistency with the Highway Capacity Manual, or the relative ease with which it can be used. As the current HCS is relegated to worksheet-based procedures that are sufficiently simple that they can be used in a manual fashion, it tends to be limited in its ability to evaluate traffic flc~v problems in interchange areas. As a result, several computer-baseds~mulation models were often cited by the respondents. Specifically, TRANSYT-7F was cited by nearly half of all the respondents. This may be due to the fact that TRANSYT-7F is sensitive to the proximity of adjacent ramp terminals or signalized intersections in its signal timing optimization routine. Another software model, PASSER-M was also cited by 40 to 50 percent of the respondents as being a usefill tool to analyze arterial traffic flow through interchange ramp terminals. This large response may be due to the fact that PASSER-II optimizes signal timing based on progression analysis. In order to better understand the strengths and weaknesses of these models, as perceived by the users, the second-stage questionnaire requested that the respondents expand upon their reasons for selecting a specific analysis tool. The results of the second-stage survey can be found In a later section. Question 5 asked the respondents to identify an MOE that they felt would be useful in evaluating traffic operations at an interchange. In order to better explain traffic operations at an interchange, MOEs must be selected that are comprehensible and practical. Thus, it is important that the MOEs selected to evaluate an interchange be those that are easy to observe and to comprehend (not something abstract In nature). Delay per vehicle was the MOE most often selected by respondents. This finding is probably due in part to the fact that the HCM uses delay to describe the level of service provided to motorists at intersections. It would appear to be a logical extension on the part of the respondents as a diamond interchange has the appearance of two arterial intersections rather than two closely-spaced ramp terminals whose individual operatic is highly dependent on the signal operation of the other ramp terminal. After delay, queue spillback frequency was the next most frequently cited MOE by the respondents. This is consistent with the findings regarding operation problems, as requested in Question 3. Queue spillback is recognized by many engineers as a significant problem at urban interchanges. It is likely that the length of the queues formed between the ramp terminals and the frequency that they spillback into the upstream ramp terminal (or closely-spaced adjacent A - 6

signalized intersection) could be used as a primary indicator of the quality of flow within the interchange area. Question 6 showed a willingness to respond to the second-stage questionnaire. The large positive response received in this regard is believed to represent the engineering commun~ty's overall level of interest in the topic of this research. A.~.2 Second-Stage Survey The findings from the f~rst-stage survey provided important information regarding the extent of operational problems at urban interchanges and the general thoughts of the practicing engineering community regarding techniques for evaluation of these problems. These findings were used to develop the format and content of the second-stage survey. This survey sought specific details of operational problems occurring at specific types of interchanges. The second- stage survey also inquired about the strengths and weaknesses of specific analysis techniques. Distribution. The second-stage survey was sent during the last week of March 1994. This survey was sent to 179 individuals who indicated a willingness to respond to it from the first survey. A total of 31 completed surveys were returned representing a 17 percent response rate, a rate that was somewhat lower than anticipated. The findings from the second-stage survey were generally consistent with those from the f~rst-stage survey. Therefore, it was concluded that the information obtained from Me second-sta~ survey would tee more representative then the small sample size would otherwise suggest. Possible reasons for the small sample size could include a combination of the following: (~) the survey may have been conducted during a busy time of the year for Me respondents, (2) respondents may have believed that the time required to complete the survey was excessive, and (3) the return date may not have allowed Me respondent enough time to adequately respond. Of the 3 1 surveys returned, 29 were determined to be valid responses In the context that they addressed the interchange types and issues described in the survey. Valid responses were returned from 10 state DOTs, ~ cities In 6 states, 9 consultants In ~ states, and 2 cities in Chnada. Overall, 21 states are represented among the 29 valid returned surveys. The response rate was about 29 percent for the DOTs, 25 percent for the cities, 0 percent ibr the counties, 9 percent for Me consultants, and IS percent for international replies. Results. The findings from the second-stage survey are described in the following paragraphs. These findings are presented in the following format: the individual question is repeated (in italics); then, the response to each question is summarized; finally, some observations and insights are provided to put Me findings in the proper context. In general, each respondent was asked to identify one interchange of the diamond or partial cloverleaf family and answer the survey questions as they relate to this interchange. The interchange that they selected was to have attribute s that were consistent with the objectives of this research and that were otherwise not unusual or geometrically constrained. Specifically, the A - 7

selected interchange was to be located in an urban or suburban setting, have signalized ramp terminals, and a distance between ramp terminals of 275 meters or less. The respondents were encouraged to complete additional survey forms for a second or third interchange, if tune permitted. ]. Please sketch the interchange. The types of interchanges sketched (and described In subsequent questions) ranged from the partial cloverleaf to the single-point urban interchange. In three instances, the respondent submitted a second survey describing a different interchange. As a result, descriptions of 32 interchanges were received; however, one interchange was described twice by two different respondents. As a result, only 31 unique interchanges are described in the summary statistics. These Interchanges are distributed among the seven interchange types listed below. 1. Tight Urban Diamond (less than 120 m between ramps): 2. Tight Urban Diamond with frontage roads: 3. Compressed Diamond (120 to 240 m between ramps): 4. Conventional Diamond (more than 240 m between ramps): 5. Single Point Urban Diamond: 6. Partial Cloverleaf (Type A): 7. Partial Cloverleaf (Type AB): 10 2 11 2 2 3 1 Vent is the distance between the two ramp terminals (as measured along the cross street from stop line to stop line) ? Average: 150 meters, Minimum: 60 meters, Standard Deviation: 90 meters Maximum: 410 meters These distances are not representative of all interchanges because the survey specifically requested information on interchanges whose ramp-to-ramp separation distance was less than 275 meters. However, they are representative of urban interchanges that tend to experience traffic operational problems because of short ramp separation distances 3. Wheat is the distance between the ramp terminal and the nearest downstream signalized intersection (as measured along the cross street from stop line to stop lined ? Average: IS0 meters, Minimum: 50 meters, Standard Deviation: 90 meters Maximum: 440 meters As with Question 2, these distances should not be taken as typical of all interchange locations; just those interchanges in urban areas with relatively close ramp spacings. The respondent was informed (in the survey) that one objective of the project was to address the operational impact of closely-spaced intersections. As a result' the respondents, tended to include A - 8

interchanges with closely-spaced intersections. These closely-spaced intersections often lead to problems such as queue spiliback between ramp terminals and left-turn bay overflow. 4. Coul~you provide a block diagram illustrating the phase sequence for one signal cycle? Twenty-four respondents provided phase sequence information. The open nature of this question led to a wide range of response formats. As a result, it was difficult to generalize the types of phasing based on the descriptions provided by the respondents. The problems with interpretation were grouped into three categories. First, very few of the respondents used Me block diagram format requested; many provided signal timing information fran plan sets or from manufacturer-specif~c controller printouts that could not be translated with any real certainty. Second, it was apparent that many respondents were only guessing at the phase sequence based on their observation rather than obtaining the actual sequence from the appropriate authority. Finally, many respondents described We phase sequence for each ramp terminal but did not convey the manner in which they were coordinated. After reviewing the phase sequences provided, the following generalizations were made. First, only 2 of the 25 diamond interchanges appear to be using me four-phase-with-overIap phasing. It was expected that this type of phasing would be more prevalent due to its ability to deal with high-volume left-turns and narrow ramp separation distances. Second, it appeared that most of the interchanges with two controllers used three-phase operation at each ramp terminal with, presumably, some type of signal offset tuning used to coordinate the two major street through movements at the ramp terminals. 5. Wheat type of signal control is used to implement the phasing dlescnbed in Question 4? About 59 percent of the respondents indicated that two controllers were used at the interchange (one controller for each ramp terminals. Another 31 percent of the respondents indicated that one controller was used for both terminals; the remaining 10 percent did not know the controller type. As diamond interchanges were the most common interchange type cited In Question I, it is somewhat surprising that so many sites had two controllers at the interchange. One common controller for both diamond interchange ramp terminals is generally best able to maintain the type of two-way traffic progression necessary to eliminate queues on the street segment Internal to the ramp terminals. The trend of using two controllers (with presumably signal offset timing) may possibly contribute to the queue spillback that many of the Interchanges exhibit because of the lower level of coordination it affords to the left-turn movements. 6. What control mode does the controller provide ? About 75 percent of the respor~ents indicated that semiactuated control was used at their interchange. Thirteen percent indicated that pretuned control was used and 9 percent indicated that fi~ly-actuated control was used. Comparison of the responses among Questions 5 and 6 indicate that Mere is no correlation between the number of controllers and the type of control mode. A - 9

7. Is the interchange controllerts) coordinated with the cross street signal system? As semi-actuated control implies coordination, it is logical that coordination was found at the same percentage of interchanges as those having semi-actuated control. In fact, this was Me case, 75 percent of the Interchanges described had semi-actuated control. The high percentage of coordinated interchanges suggests that, while efforts should be made elsewhere in Improving Interchange traffic operations, impacts on coordination should not be forgotten. 8. Describe the traffic flow problem which tends to be most disruptive to smooth traffic flow. Although this question asked about the most disruptive problem, most respondents chose to describe more than one problem. In general, they selected one or more of We traffic flow problems that were described in the survey. These problems are restated below along with the percentage of responses Mat identified a particular problem as berg Me most disruptive. 41% a. Capaciy restriction due to queue spillback between ramp terminals. 34% b. Capacity restriction due to queue spillback from a ramp terminal into the upstream signalized intersection. 31 % c. Capacity restriction due to cross street left-turn bay queue overflow into the through lanes. 25% d. Unbalanced lane volumes on the cross street approaches to the romp terminals due to h~gh-volllme downstream turn movement. 25 % e. Flow turbulence between a ramp terminal and an adjacent signalized intersection due tohigh-volume lane changing (i.e., right-turn at terminal followed by left-turn at intersection, or vice versa). 22% I. Capacity restriction due to queue spillback from a signalized intersection into the upstream ramp te~lllinal. 22% g. Capacitor restriction due to queue spillback from the off-ramp signal into the freeway main lanes. 19% h. Poor signal coordination between the two ramp terminals due to complex signal phasing, variability in hourly turning movement volumes, or minimal interior queue storage space. 16% i. Capacitor restriction due to queue spilIback from a ramp meter into the upstream ramp terminal. 6% j. Poor or nonexistent signal coordination between Me ramp terminals and adjacent intersections due to jurisdictional policies (i.e., Cider control of the intersection and State control of the interchange). 0% k. Poor or nonexistent signal coordination between the ramp terminal and ramp meter. Based on the percentages listed above, it appears that "queue spiliback between ramp terminals" is the most frequently found problem at interchanges in narrow-rights-of-way. When combined with "left-turn bay overflow, " it would appear that traffic flow problems et interchanges are most frequently found between the ramp terminals, where the volume of left-turns is highest. A- 10

One response from a consultant in Portland, Oregon reported a lack of capacity between the terminals of a compressed diamond interchange. This interchange has ore controller for both terminals and operates In a three-phase sequence. The respondent indicated that the restricted capacity "results in a queue spilIback into the adjacent cross street signalized intersection. " This spilIback, In turn, "results in little or no capacity for local circulation" at the adjacent intersection. Another response from a consultant in New York identified problems associated with turning movements. The respondent reported that the tight urban diamond exhibited left-turn bay queue overflow at one of the ramp terminals and severe turbulence associated with high-volume weaving on the cross street between the ramp and adjacent signalized Intersection. The maneuver that caused most of this turbulence was the off-ramp right-turn movement becoming a left-turn movement at the downstream intersection. Further examination of the responses to this question revealed that all of the reported flow problems related to queue spilIback between the ramp terminals were associated with tight or compressed diamond interchanges. Single point diamond interchanges, conventional (wide) diamond Interchanges, and partial cloverleaf interchanges were not associated with queue spillback-related flow problems. The single point diamonds do not experience queue spiliback because Hey combine the two ramp terminals into one intersection. TO conventional and partial cloverleaf interchanges do not experience spillback because of the relatively large distances separating Be two ramp terminals. 9. What treatments have you applied! (or would apply) to alleviate the traffic flow problem described in Question S? A wide range of treatments were described by the respondents. There were no definitive trends although it appeared that geometric changes were commonly seen as the only available treatment. Typical geometric treatments included adding a second left-turn lane or an additional through lane to the cross street. In some instances, the respondent recognized the difficulty of adding lanes to (i.e., wideIiing) an existing bridge. One of the more Interesting signal timing treatments was the use of signal phasing at the adjacent Intersection to separate the traffic movements accessing the on-ramp so as to prevent the congestion associated with a high-volume of weaving vehicles. Many respondents indicated that improved or updated signal timing and coordination helped mitigate some traffic problems. 10. If you were asked to evaluate and quantify the problem described in Question 8, what Abyss technique (or techniques) would you presently use ? The analysis techniques cited by most (60 percent) of the respondents can be described as those developed for isolated signalized intersections. These techniques were used for the analysis of the individual ramp terminals and adjacent intersection. Of those techniques identified, that described in Chapter 9 of the 1985 Highway Capacity Manual (HCM) was cited as being most frequently used. PASSER IT was identified by 33 percent of the respondents as being helpful in coordinating the two ramp terminals and the adjacent signalized intersection. Other, less frequently noted techniques included the use of the NETSIM and TRANSYT-7F computer models. . A- 11

71. With regard to the analysis technique described in Question 10: a) What is its main technical strength? The most frequently cited strength of the "Chapter 9" HCM technique was Hat it is easier to use than multiple-intersection software programs (e.g., PASSER IT, TRANSYT-7F, NETSIM, etc.~. In general, the HCM technique was used to evaluate the individual ramp terminals with appropriate calibration of the progression adjustment factors to account for nearby intersections. The PASSER ~ program was credited with being the easiest multiple-intersectionprogram to use. This program was used when a Trough traffic progression solution and/or queue length estimate was desired. NETSIM was noted to be the only program that accurately modeled queue spill~ck and congested flow conditions. TRANSYT-7F was noted to consider upstream queue length and left-turn demand when determining the "optimum" traffic progression solution. b) What is its main technical weakness ? The weaknesses cited for the HCM technique were that it did not accurately model the effect of closely-spaced upstream intersections and that it did not yield queue length estimates. The weaknesses cited for He PASSER IT program were that it did not provide progression solutions for left-turn movements, did not consider upstream queue length when deterring the progression solution, did not allow the user to enter some types of interchange phasing, and did not consider right-turn demand. NETSIM was noted to be very tune consuming to use due to its microscopic simulation formulation. A couple of respondents noted Hat none of the techniques dealt with the coordination of a ramp meter win He ramp terminal. c) Describe how you have overcome arty weakness described in Question 11-b. In general, the respondents Indicated that they used engineering judgement and field observation to manually adjust the signal offset or tunings to optimize traffic progression and minimize queue lengths. A few respondents indicated that they used a second analysis technique; however, they did not elaborate on which supplemental techniques were used and under what . . conditions. S=nmary. The results of the second-stage survey indicated that queue spillback, left-sum bay overflow, and weaving between the off-ramp and downstream intersection were significant operational problems at interchanges with closely-spaced ramps or adjacent signalized intersections. Of these problems, queue spillback tends to degrade He smooth flow of many interchange traffic movements and thereby, aggravate mild inefficiencies into significant capacity constraints. Thus, the indirect solution to many ~nterchange-related problems appears to be related to devising analysis techniques that are sensitive to the proximity to downstream queues, the propensity of these queues to spillback, and the relationship between queue-clearance-lime end the signalization of He interchange ramp terminals and adjacent intersection. The Implementation of He findings of this research could be facilitated by Heir incorporation into one or more of the existing capacity analysis techniques (e.g. Highway Capacity Software, PASSER II, etc.). A- 12

A.~.3 Researchers Field Observations The research team studied a dozen interchanges during the field studies and spent many hours observing traffic operations at the sites. Traffic congestion was routinely observed at all of these interchanges. Comparisons were rapidly made among interchange types, types of operational problems observed, and the probable cause of these problems. Our summary ofthese field observations are noted below: I. Design life of older interchanges usually exceeded so traffic demand exceeded interchange capacity during rush hours. Many older interchanges noted above have a predominant number of s~ngle-lane left turn lanes within the interchange and/or have single lanes at ramp terminals assigned to serve heavy left and/or right turning movements. Deficient turning capacity exists. 3. Due to urban growth, four-lane crossing arterials need to have six lanes. Cross street has functionally become a major urban arterial. 4. 6. Traffic management of queueing and spillback is difficult at some interchanges due to high volumes and high percentages of turning traffic having typical lane distribution problems. Some approaches along the crossing arterial and within the interchange have almost constant demand within the cycle so queueing can not bem~tigated using traditional arterial signal coordination techniques. Random flow should be assumed, as a minimum, for queueing analysis. Parclo A's seem to be more susceptible to constant demand conditions. However, all off ramp terminals having free right turning operations Will be more prone to overloading downstream arterial storage areas. Many arterial links connecting the freeway interchange with the "next" downstream signalized intersection experience high traffic demands to/from the freeway (interchange) and the flows are frequently nearly constant over He cycle. These adjacent intersections often have four-phase signals that provide less arterial capacity then the three-phase signals at the interchanges. For these conditions, many of these connecting links appear to be too short to provide good storage and operating conditions. Longer intersection spacings and better design policies are needed for interchange planning and design. Traffic control strategies employed appear to be based on undersaturated flow conditions which may lose efficiency during oversaturated conditions now being more commonly experienced. Better management of queue spilIback to mitigate the onset of congestion is needed together win the need to transition to downstream bottleneck control strategies once oversaturation has occurred. A- 13

A.2 EXISTING CAPACITY AND LEVEL OF SERVICE MODELS A.2.! Overview This section presents a state-of-the-art summary of current traffic models for assessing the capacity, delay, and level of service of traffic operations at the signalized interchanges shown in Figure A- ~ . Most interchange forms have two signalized intersections per interchange. The primary focus of previous research has been on diamond interchanges because they are the predominant signalized interchange form Ail. Partial cloverleaf (parclos) interchanges have similar phasing strategies and can be modeled using the same general capacity analysis methodology provided for diamond interchanges. Current interchange capacity analysis essentially treats each intersection within the interchange as a separate entity, with minimal consideration given progression effects and spilIback. The significant number of users of the Highway Capacity Software (HCS) for interchange studies, ~ . , . . ,~ . . . . . . .. . . .. . . . based on the held survey ot practlcmg engineers previously clescnnea, snows tnls technology limitation since there is no generally accepted standard analysis methodology for interchanges. The Arterials (Chapter ~ ~ ~ methodology of the ~ 994 Highway Capacity Manual (HCM) (29 is sometimes used for interchar~ges,but it assumes that the intersections are widely spaced and traffic operations are ur~dersaturated. Because the signalized ramp junctions at an urban interchange are usually less than 300 meters apart, and most urban diamonds are less than 200 meters, the effects of closely- spaced signals should be identified and modeled in interchange analysis. Current capacity analysis for intersections also assumes that the output (saturation) flow from a signal is independent of downstream traffic conditions. This is a major deficiency for interchange analysis due to the high traffic volumes and closely-spaced signals. Even the most highly utilized macroscopic computer-based signal timing optimization models (PASSER IT, PASSER Ill and TRANSYT 7F) presently fail to reliably address oversaturation issues at signalized intersections. A.2.2 SignalPhase A signal phase is a period of time provided by the signal controller unit to an approach permitting legal entry of vehicles into the intersection. The entry may be described as being protected(from conflicting vehicular arid pedestrian traffic), permitted(to legally enter but exposed to other potential conflicting movements), or combinations of the two (protected-plus-permitted). Other descriptive terms are used such as exclusive/permissivephasing or combinedphasing. A basic protected through or left turn phase would have the following signal interval times - = G + Y + RC (A-~) A- 14

d En CO I of An CD I at: I a: U.! o c \ _ ~ a mk~ ~· ~ l-=1 . J JO o O~ I, Ad c: C] FEZ To Sit of _ ~ ~ at; _ t YY / 03 Z of I Cat L Figure A-1. Common Two-Level Signalized Interchanges. A- 15

where: G y R - -C total duration ofthe signal phase, see, green signal interval, see; yellow warning interval, see, and red clearance interval, sec. Assuming that a long line of cars is trying to use the phase, the flow profile measured at the stopline shown in Figure A-2 would be expected, assuming that the phase is protected from conflicting traffic and unimpeded by downstream queue spilIback. Following onset of green, flow reaches a maximum or saturation flow rate and would be expected to maintain this flow until the green interval ends. Some usage of the yellow change interval occurs, perhaps as much as 2.5 seconds on the average. The ~ 994 HCM assumes through phases are never blocked or impeded by downstream storage conditions. A.2.3 Phase Capacity The capacity of a traffic signal phase is the maximum number of vehicles that an be expected to enter the intersection per cycle (assuming one phase per cycle) from the lane group being analyzed under prevailing roadway, traffic and control conditions. The Highway Capacity Manual (HCM) expresses this concept of phase capacity at maximum flow as being the total area under the saturation flow curve per phase n = Js(t)dt o (A-2) Assuming the saturation flow s(t) is a known constant, sg (ups), during the effective green portion, g, of the phase and there is one phase per cycle, then the phase capacity per cycle then becomes n = g sg (A-3) It is assumed that the phase is protected and unblocked dunng the effective green penod, g, while serving a waiting queue. A representative capacity flow profile of the signal was shown in Figure A-2 for unimpeded/unblocked saturation flow. Phase capacity is usually expressed in equivalent flow rate units of vehicles per hour consistent with traditional volume counting practice. Assuming there is only one phase of interest per cycle and noting that the numbers of cycles per hour are k(C) 3600 A- 16 (A-4)

Saturation Flow Phases for Red Movement ; i' Effective I,' ActualFlowCu~ve Flow Curve :/ Effective Green Time, g Saturation Flow, s s~sst 1 \ iL2 7 End Gain \ i\ , 1' ,\ End Loss, L :\ / I\ /~' my, Time Signal Green Time, G Yellow | Red Green Figure A-2. Basic Saturation Flow Mode} for Unimpedec! Traffic Conditions. then the hourly phase capacity is the product of the capacity per phase times the number of phases (cycles) per hour, or c = ~ q substituting for n from Equation A-3 yields Rearranging terms yields (A-S) 3600 c = gsg C g s 3600 C g A- 17 (A-6) (A-7)

g s 3600 C g (A-7) Letting the saturation flow be expressed in vehicles per hour, as s = 3600 sg, results in the more traditional hourly-based phase capacity formula used in the ~ 994 HCM (2), of c = g s (A-8) where: c g C n k(C) sg s A.2.4 Saturation Flow phase capacity for the subject lane group, vph; elective green time of phase, see; cycle length, see; phase capacity, vein; number of phases (cycles) per hour; average saturation flow during phase, vpsg; and average saturation flow during phase, vphg. The ~ 994 HCM provides a module for calculating saturation flow for signalized intersections which is su~.nmar~zed belong 62,}. The HCM defines saturation flow rate as the flow in vehicles per hour that could be accornrnodated by the lane group assuming that the green phase was always available to the lane group, that is, the green ratio (g/C) was I.0. Computations begin by selecting an "ideal" saturation flow rate, usually 1,900 passenger cars per hour of display green time per lane (pcphgpl), arid then adjusting this value to prevailing conditions which may not be ideal. Following Eq. 9-12 of the 1994 HCM for signalized intersections, the adjusted saturation flow is s 50 Nfw fry fg fp fbb fa fRT fL,T where: (A-9) . _ ~. . . . . . . ~... s = saturation flow rate for the subject lane group, expressed as a total tor all lanes in the lane group under prevailing conditions, vphg; so = ideal saturation flow rate per lane, usually 1,900 pcphgpl; N = number of lanes in the lane group; fW = adjustment factor for lane width (12-ft larches are standard), given in Table A-4; A- IS

fHV = adjustment factor for heavy vehicles in the traffic stream, given in Table A-5; fg = adjustment factor for approach grade, given in Table A-6, fp = adjustment factor for the existence ofaparkinglane adjacentto the lane group and the parking activity in that lane, given in Table A-7, Ebb = adjustment factor for the blocking effect of local buses that stop within the intersection area, given in Table Am; fa = adjustment factor for area type, given in Table A-9; fRT = adjustment factor for right turns in the lane group, given in Table A-IO; and flit = adjustment factor for left turns in the lane group, assumed to be 0.95. A.2.5 Saturation Flow Adjustment Factors The use of adjustment factors is a common feature throughout the HCM. Each factor accosts for the impact of one or several prevailing conditions that are different from the ideal conditions for which the ideal saturation flow rate applies. The factor represents the average adjustment needed over the entire duration of the displayed effective green time. Lane Width. The lane width adjustment factor,fw, accounts for the deleterious impact of narrow lanes on saturation flow rate and allows for an increased flow on wide lanes. TweIve-foot lanes are the standard. The lane ~ factor may be calculated with caution for lane widths greater than 5 m (16 ft), or an analysis using two narrowlanes may be conducted. Note that the use oftwo lanes wall always result in a higher saturation flow rate than a single wide lane, but in either case Me analysis should reflect the way in which Me width is actuary used or expected to be used. In no case should the lane width factor be calculated for lane widths less than 2.5 m (8 flc). Table A-4. Adjustment Factor For Average Inane Width (fw) Average Lane Width, W(ft) Lane Width Factor, [w 0.867 0.900 0.933 0.967 1.000 1.033 1.067 1.100 1 1 ~ 8 9 10 11 12 13 14 15 16 NOTE: fw = 1 + W - - W 28 (if W >16, consider two-lane analysis) A- 19

Table A-S. Adjustment Factor For Heavy Vehicles (fHv) Percent Heavy Vehicles, % HV o 4 6 8 10 15 20 25 30 35 40 45 50 75 100 Heavy Vehicle Factor, fHV 1.000 0.980 0.962 0.943 0.926 0.909 0.870 0.833 0.800 0.769 0.741 0.714 0.690 0.667 0.571 0.500 100 He 100 +°/OHV (ET where ET = 2.0 passenger cars per heavy vehicle. - 1) O < To HO < 100 Table A-6. Adjustment Factor For Grade (f) Grade %G Type Percent Grade Factor (f:) Do~Thil1 -6 or less 1.030 -4 1.020 -2 1.010 Level O 1.000 Uphill +2 0.990 +4 0.980 +6 0.970 +8 0.960 +10 or more 0.950 NOTE: f = 1 - o/O g 200 -6 < To G ~ + 1 0 A - 20

Table A-7. Adjustment Factor For Parking (fp) No. of Lanes . No. of Parking Maneuvers Per Hour N., In Lane Group, N No Parking 0 10 20 30 40 1 ~ 1.000 ~0.~00 ~ 0.850 ~ 0.800 ~ 0.750 2 ~ 1.000 ~0.~50 ~ 0.925 ~ 0.900 ~ 0.875 3a 1 1.000 1 0.`l67 T 0 950 1 0.933 1 0.917 0.700 O.850 0.900 N-0.1 -1 8N /3600 m NOTE: fP = N 0 ' N m < 180 fp 2 0.05 ause formula for more than 3 lanes, or more than 40 maneuvers per hour. Table A-X. Adjustment Factor For Bus Blockage (fbb) No. or Lanes No. of Buses Stopping Per Hour. NR In Lane Group,N 0 1020 30 1 11.000 1 0 9601 0.920 0.880 2 1l.OOb 1 0 9801 0.960 1 0.940 3a 1 ~ 000 1 0 987 1 0.973 0.960 40 0.840 0.920 0.947 N - 1 4.4NR/3600 NOTE fbb = 0 < NR < 2s0 N - fbb 2 0.05 ause formula for more than 3 land or more than 40 buses stopping per hour Table A-9. Adjustment Factor For Area Type (fa) Type Of Area COD All other areas Area Type Factor, (fa) 0.90 1.00 A - 21

Heavy Vehicle and Grade. The effects of heavy vehicles and grades are treated by separate factors,fg andfHv respectively. Their separate treatment recognizes that passenger cars are affected by approach grades, as are heavy vehicles. The heavy vehicle factor accounts for the additional space occupied by these vehicles arid for the negative differential in the operating capabilities o heavy vehicles with respect to passenger cars. The passenger car equivalent (ET) used for each heavy vehicle is 2.0 passenger car units (pcus) and is reflected in the formula. This value was increased from ~ .5 used in the ~ 985 HCM. The grade factor accounts for the effects of grades on We operation of all vehicles. Parking. The parking adjustment factor,fp, accounts for the frictional effect of a parking lane on flow in an adjacent lane group, as well as for the occasional blocking of an adjacent lane by vehicles moving into and out of parking spaces. Each maneuver (either in or out) is assumed to block traffic in the lane next to the parking maneuver for an average of IS sec. The number of parking maneuvers used is the number of maneuvers per hour in parking areas directly adjacent to the lane group and within 76 m (250 ft) upstream from the stop line. If more than ~ 80 maneuvers per hour exist, a practical limit of ~ 80 should be used. If the parking is adjacent to an exclusive-turn- lane group, the factor only applies to that lane group. On a one-way street, parking on the left side wall affect the left most lane group, as in a one-way street with no exclusive-turn lanes, the number of maneuvers used is the total for both sides of the lane group. Note that parking conditions with zero maneuvers are not the same as no parking. Bus Blockage. The bus blockage adjustment factor,fb',, accounts for the impacts of local transit buses that stop to discharge or pick up passengers at a near-side or far-side bus stop within 76 m (250 ft) of the stop line (upstream or downstream). This factor should only be used when stopping buses block traffic flow in the subject lane group. If more than 250 buses per hour exist, a practical limit of 250 should be used. When local transit buses are believed to be a major factor in intersection performance, more precise methods may be needed. The factor used here assumes as average bus blockage time of 14.4 sec. during a green indication. Area Type. The area type adjustment factor,fa, accounts for the relative inefficiency of business area intersections in comparison with those in other locations, primarily because of the complexity and general congestion in the business environment. Right-Turn. Turning factors depend upon a number of parameters. The most important characteristic is the manner in which turns are accommodatedin the intersection. Turns may operate out of exclusive or shared lanes, with protected or permitted signal phasing, or way some combination of these conditions. The impact of turns on saturation flow rates is very much dependent upon the mode of turning operations. A - 22

Table A-10. Adjustment Factor For Right Turns (fRT) Cases 1-6: Exclusive/Shared Lanes and Protected/Permided Phasing fRT= 1.0 - PRT [0.15 ~ (PEDS/2100) (1 - PRTA) 0.O < PRT < 1 0 Proportion of RT in lane group = 1.00 for eel. RT lane (Cases 1 3~; <1.00 for shared lane (Cases 4-6) 0.O < PRTA < ~ ·0 Proportion of RT using protected phase = 1.00 O < PEDS < ~ 700 Volume (peddler) of peas conflicting with RT fRT ~ 0~05 (if PEDS > 1700, use 1700) - Case 7: Singl - Lane Approach (all traff~c on approach in a single lane). fRT 0.90 - PRT [0.135 + (PEDS/2100)] O < PRT< 1.0 | Proportion of RT in lane group. O < PEDS < 1700 Volume (peds/hr) of peas conflicting with RT (use O if RT is completely protected). PRT 1.00 if PRT = 0.0 FRT > 0~05 Range of Variable Values Case PRT PRTA PEDS Simplified Formula 1. Excl. RTlane; pros. RT phase 1.0 1.0 0 0.85 2. Excl.RTlane;perm.RT phase 1.0 0.0 0-1700 0.85-(PEDS/2100) 3. Excl. RT lane; pros. + Penn. RT phase 1.0 0-1.0 0-1700 0.85 - (PEDS/2100) (1-PRTA) 4. Shared RT lane; pros. RT phase 0-1.0 1.0 0 1.0 - PRT[O 151 5. Shared RT lane; perTn. RT phase 0-1.0 0.0 0-1700 1.0 - PRr[O-I5 + (PEDS/2100~3 6. Shared RTlane;Prot.+perm.RT phase 0-1.0 0-1.0 0-1700 1.0-PRT[0.15+(PEDS/2100~] 7. Single-lane approach 0-1.0 0-1700 0~9 - PRT[O.135 +(PEDS/21001] A - 23

The nght-turn adjustment factor,fRT, depends upon a number of vanables, including Whether the right turn is made from an exclusive or shared lane; Type of signal phasing (protected, permitted, or protected plus permitted) a protected r~ght-turn phase has no conflicting pedestnar~movements and a permitted phase has conflicting pedestnar1 movements; Volume of pedestrians using the conflicting crosswalk; Proportion of right-turning vehicles in the shared lane, and Proportion of right turns using the protected part of protected-plus-permitted phase. 3. 4. Item 5 should be determined by field observation but can be grossly estimated from the signal timing. This is done by assuming that the proportion of r~ght-turning vehicles using the protected phase is approximately equal to the proportion of the turning phase that is protected. If PRTA = I .0 that is, the right turn is completely protected from conflicting pedestrians, a pedestrian volume of zero should be used. The right-turn factor is ~ .0 if no right turns are present on the lage group. When RTOR is permitted, the right-turn volume may be reduced as described in the discussion of the Volume Adjustment Module of the HCM. Left-Tu~n. The left-turn adjustmentfactor,f~T, is based on similar variables to those for the right-turn adjustment factor, including 1. 2. 3. 4. Whether left thins are made from exclusive or shared lanes; Type of phasing (protected, permitted, or protected plus pennitted), Proportion of left-tu~ning vehicles using a shared lane group, and Opposing flow rate when permitted left tutus are made. The left-turn adjustment factor is ~ .0 if the lane group does not include any left turns. When a left turn is not opposed at any time by through vehicles but encounters conflicting pedestrian movements, the left turn should be treated using the adjustment procedure for right turns. If no conflicting pedestrian movements are present, a normalprotectedleft-turn adjustment is performed. Basically, turn factors account for the fact that these movements are not made at the same speeds arid saturation flow rates as through movements. They consume more ofthe available green time and consequently more of the lane group's available capacity. The turn adjustment factors reflect seven different conditions under which turns may be made, as follows: Case I: Exclusive lane win protected phasing,fi, = 0.95; Case 2: Exclusive lane with permitted phasing, see below; Case 3: Exclusive large with protected-plus-permitted phasing; Case 4: Shared lane with protected phasing; Case S: Shared lane with permitted phasing; Case 6: Shared lane with protected-plus-permitted phasing; and Case 7: Single-lane approaches (rtght-turn factors only). A - 24

A.2.6 Assessment of HCM Capacity Methodology A critical assessment of the 1994 HCM capacity analysis methodology for signalized intersections should consider all aspects and issues related to its application to interchanges. Some factors appear to be directly usable as is for interchanges. Other factors would generally appear to be unneeded. The need to consider honing radius at interchanges seems apparent due to the high volumes of turning traffic and range of turning radius that might be encountered. Also, cost of some interchange designs are sensitive to turning radius. Because interchanges usually have closely- spaced intersections,and often are oversaturatedw~th queue spilIback blocking upstream flow, queue impediments should be considered. The definition and resulting application of effective green, g, is judged a major problem for general use. In addition, technology issues arise as to how the HCM uses capacity analysis in the estimation of performance measures and level of service using vehicular traffic delay. Adjustment Factors. Geometnc, location and environmentalaspects are more identifiable as to transferability from intersections to interchanges. These aspects are considered first for all of the adjustment factors noted in Equation A-9 for saturation flow. fLT fD, fv Factor Recommendation - fw = same lane w~dth factors should be used, although larger vehicles on the average may be found at interchanges; fHY = same heavy vehicle factor and E T ECUS should be used, although larger and heavier vehicles may use some interchanges, and if so, larger ET should be applied based on field observations; fg = adjustment factor for grades should be similar for interchanges, fp = parking wait not likely be permitted in/around interchanges, so this factor is not needed; fig = buses may be stopping on the cross street, so this factor should be retained; fa = area-type factor is not needed for interchange environments; fry = right-turn factor should be retained, but give additional consideration to · .- . ~ . · . . .. ~ . .. . ~ ,- ~ ~ turning radius, number or mrmng lanes, arid the fact that very Iew t1I anyJ phases are permissive from shared Cartes. Moreover, this factor should only adjust for time periods when the traffic actually moves, unlike in the 1994 HCM, so that delays can be better estimated; left-turn factor should be retained only for protected phasing as a base case. Further adjustments based on turning radius should be made ; and new adjustment factors for downstream queue spilIback impediments and traffic pressure, as described further in this report. A - 25

Permissive Left Turn Operations. Permissive/permitted left turn operations occur at signalized interchanges when left turns, after yielding to opposing queues, subsequently find acceptable gaps in the opposing traffic flow and then complete their intended led turn maneuvers. The period of time the opposing queue blocks the left turn during green, gin, must be known and can be estimated from rv ro gbq S ~ V go go (A-10) Assuming undersaturated conditions with no queue spillback, Me unblocked green time, go is the time the pennided left turns can safely maneuver across We opposing flow during the lager portion of the phase for a time of c' = 0 - O Mu hop Abe where: g2. gp gbq = = Yro vgO sgo (A-11) unblocked green time of the chase serving the lane grouts see; original effective green time of the phase, see; time opposing queue blocks the permitted phase from serving the ~... .. . permitted led turns, see; red time of opposing lane group per cycle' see, opposing arrival rate on red for lane group, vph; opposing arrival rate on green for lane group' vph' and nominal saturation flow for opposing lane group; vphg. The left turn saturation flow possible during the permitted green interval, au, should be calculated from the following relationship: T-vg 5 ~go ~-ego Hr where: vgO TL HL opposing lane croup volume during permitted green, vph, , ~ ~ ~ . left turn critical gap, sec/3600; and left turn minimum headway, sec/3600. A-26 (A-12)

Recommended values for TL and HL depend on several factors, including whether the left turns are made from a dedicated lane or a shared lane. Table A-1 1 provides recommended values of the factors TL, HL based on We 1994 HCM applications and other references 62, 3). The resulting relationships of saturation flow for permitted left turns versus opposing volume are estimated by Equation A-12 are shown in Figure A-3 for two types of led turn lane use. Table Apt. Parameters for Permitted Left Turn Phases Factor ~Left-Turn Lane ~Shared-Turn Lane TL(sec) ~4.5 (see) 4.6 (see) TL ~0.00125 0.00128 HL(sec) ~2.4 (see) | 4.5 (see) . HL ~0.00067 0.00125 Saturation Flow vs Opposing Flow 1400 1200 ~| Separate Lane. ~ t \ IN same 3 &00- . \ .,' \ to 600- . '''- ., ~ .,, eS 400' ~ U) ~., ~ 200 o O 200 400 600 800 1000 1200 1400 1600 1800 2000 Opposing Flow on Green (vph) Figure A-3. Permitted Left Turn Saturation Flow as Related to Opposing Volumes. A - 27

The modeling of permissive left turns raises a majortechnologicalissue web the ~ 994 HCM. The question is "How is it best to estimate capacity and delay using the basic HCM methodology?" The ~ 994 HCM uses the following general methodology to estimate phase capacity, c, as Equations A-8 and A-9 have noted c = g Sa = c sO N fit (A-13) where g is the phase' s effective green, including times during the phase when flow may be blocked, e.g. by opposing queues for permitted left turns, Sa is the adjusted saturation flow, end f,: is the product of all relevant adjustment factors to existing conditions during the phase. The recommended method for estimating phase capacity per cycle essentially sums component maximum allowable flows over the cycle according to ~gf Sf C (A-14) for di~enng flow conditions ~ over the cycle, C. For permitted led turn phases, this can be implemented in either of two ways, both of which yield the same estimate of capacity: HCM Method: c = gs = g N ~-f ·-~ sp (A-IS) c =gNfpm sP (A-16) where fpm is the resulting HCM capacity adjustment component for permitted left turns due to opposing queue blockages and turn flow reductions due to opposing traffic flow from g S f = f f (A-17) pm g S p arid sp is the adjusted saturation flow for protected left turns for existing conditions. A - 28

and sp is the adjusted saturation flow for protected left turns for existing conditions. Recommended Method: where: SL SP = so = C = gb O gf f C = gfSf C = gf ·fo · Sp fo = .£ UP (A-18) (A-l9) (A-20) (A-21) equivalent permitted left turn saturation flow rate Trough opposed flow from Equation A-12; saturation flow rate for protected left turn operations for otherwise existing conditions; and base ideal saturation flow for signalized intersections, pcphgpl. Both the HCM and proposed flow modeling methods yield the same phase capacity, but they produce significant differences in delay when used in traditional HCM delay models as noted below. The major difference between the two methods is in delay estimation. These differences arise due to how the first term of the HCM delay equation calculates queuing delay for left turning vehicles arriving on red and opposed green. Considerate following analysis for permitted left turn operations from a separate left turn bay. It has been shown that the phase capacity of the two methods would be equal to C = gsa gf f (A-22) The HCM effective green definition exceeds the actual green period when led In flow occurs A - 29

but the saturation flow during turning is greater than the HCM adjusted saturation flow Sf > 5 Since the capacities of the two me~ods are the same, for convenience let (A-24) Sa =-f Sf = asf, a =-f < ~(A-25) HCM Delay Estimation Method: Recommended Delay Estimation Method: C (1 - g/C)2 ~ _ . 2 (1 - v/Sa) C (1 gf/C) ~ = _ 2 ~ 1 - V/Sf) Define the delay ratio, e, to be the ratio of the HCM delay to the recommended method for calculating the first term of the delay equation such that AH (} - g/C)2 (1 - av/s) (A-26) OF (1 - a g/C)2 (1 - As) Let ~ = g/c; y = As, and x = y/1 to simplify the comparisons (i _ \~2 (] _ Aye `i _~2 (] - FOXY (A-27) (1 - of (1 - y' (1 - ~2 (1 - ~X) A - 30

Consider the following two volume cases, low and high volumes, for three green splits, with a = 0.5 for assumed moderate opposing volumes. Case I. Low Left Turn Volumes: yew, v ~0, x~O (1_~2 (1 - ai)2 Case Il. High Left Turn Volumes: x~ = ~ (degree of saturation equals I.0) eII = ~ . (l-la) (1 -id (A-28) (A-29) These delay comparisons are presented in Table A-12. At extremely low volumes and high green splits, the HCM method would underestimate the average delay by over 55 percent (a delay ratio of 0.~). At high volumes, the error in delay would be less, but still practically significant. The HCM Method (the combination of effective green definition and resulting saturation flow adjustment memos) uphill consistently underestimate the signal delay incurred for permitted left turns even when calculated turn capacities are the same as the proposed method. Table A-12. Comparison of First-Term Errors in HCM Delay Estimation [ Green Ratio ~ = g/C | Low Volumes X= 0 | High Volumes X= 1.0 | 1/40.250 T 0735 1 0.858 1/3 0.333 1 0.641 0.801 [ 1/2 0.500 1 0.444 1 0.667 Blockage. Output flow from the stop line may be blocked and otherwise impeded by several conditions. Permitted left turns are blocked by opposing queues from using a portion of the displayed green interval. All turning movements may also be blocked by queue spiliback from other movements storing behind the downstream signal, even though the downstream signal is undersaturated. This research has identified that queue blockage is a major consideration during oversaturated conditior~s, and spilIback blockages must be identified for meaningful capacity and delay analyses to be conducted. Thus, the true "elective green" should be used. A - 31

oversaturated conditions, arid spillback blockages must be identified for meaningful capacity and delay analyses to be conducted. Thus, the true "effective green" should be used. A.2.7 HCM Vehicular Delay Methodology --of- ~ ---rid ~ Vehicular delay is recognized by the 1994 HCM as being a significant traffic performance measure and, consequently, is used as the sole cr~tenon for the level of service provided, for isolated intersections. Federal Highway Administrationhas sponsored a research project in coordination the Highway Capacity Committee of TRY to provide a recommended update for HCM chapters on isolated signalized intersections (Ch.9) and on coordinated signalized arterials (Ch.11~. This research has just recently been completed and is being reviewed by HCQS committee of TRB. The following is a summary of the current arterial recommendations provided by the cited researchers (4). Generalized Delay Model. The proposed generalized delay mode] for signalized intersections end arterial streets (interrupted ~aff~c flow facilities) for a subject lane group (phase) is f49. ~ = ~! + 42 ~ ~3 where: (A-30) a' = average total delay per vehicle for vehicles arriving during the analysis period, sec/veh, ah = uniform delay, sec/veh; ~2 = increments delay due to random arrives and overflow queues, sec/veh, md ~3 = incremental delay due to non-zero queues at the start of the analysis period, sec/veh. where He uniform, or so-called first-term, delay is d1 = 0.5 C (1- g/C) 1 - (g/C ~ min(X, 1 .0) PP arid the second-term of delay is ~2 = 900 T (X-1)+ ,` (X_l)2 + 8kIX Tc A - 32 (A-31) (A-32)

If X< I, a residual queue of size ni exist at the start of the analysis period, and a zero queue exists at the end of the analysis period, then d3 = 1 3600 ni 1 O.5ni ~c ) Tc(l -X) If X< ~ and non-zero queue exists at the end of the analysis period, then d = 3 13600 ' J 1800 T(1 -X) If X ~ I, or oversaturation is present, then n . d3 = 3 600 ' ~ c, If X< ~ and zero queue exists at the stay ofthe analysis penod, then d3 = o where: C = average cycle length, see; g = average effective green time, see; X = degree of saturation for subject lane group; PF = progression adjustment factor; fpp = early/late arrival adjustment factor; T = analysis period in hours, in which the mode! parameters are fixed; k = delay parameter for given arrival and service distributions; = variance-to-mean ratio of aITivals/cycle at a point; c = capacity of the lane group, veh~r, arid n, = queue at the start of the analysis period. A - 33 (A-33) (A-34) (A-35) (A-36)

Isolated Intersections. Considering isolated signalized intersections for a 15-minute analysis period under pretimed control and no initial queue, which essentially descnbes the nominal analysis conditions of Chapter 9 of the 1994 HCM, the following default values would be used: PF = 1.0,fp = I.0, T= 0.25 hours, k= 0.50, I= I.0 and ~3 = 0. Me resulting total delay equation for a lane group for an isolated approach (or one having random arrivals) would be ~ = ~ ~ ~2 d =O.SC (~-g/C)2 +225 (X-1~+ ~`X_~2 ~16X 1 - (g/C) min (x, 1.0) - i\| c (A-37) (A-38) which is almost the same model for pretuned control as has been used in the HCM since ~ 985. Only the X: tea in all has been dropped from the overall delay equation. Signalized Arterials. The generalized delay model for interrupted flow facilities can be used for coordinated approaches by selecting appropriate values for coordinated conditions and type of traffic control. Table A-13 provides the progression adjustment factor (PF7 for Me first ted of Me delay equation teased on arrival type (AT7 together with the early/late arrival factor Ups, which also depends on the degree of saturation of the lane group. penods For coordinated intersections, the following equation is proposed for IS-minute analysis = o.sc (} - g/C)2 PFf + 225 1 - (g/C)min (X,l.O) PP (X - l) ~ ~ (X - iy2 + (32 kit A - 34 c (A-39)

Table A-13. Uniform Delay Adjustment Factors Progression Adjustment Factor (PF) Green Ratio (~C) AT-1 Arrival Type (AT) | AT-2 | AT-3 AT-4 AT-5 AT-6 · 1 0.20 0.30 0.40 0.50 0.60 0.70 1.167 1.268 1.445 1.667 2.001 2.556 1.083 1.143 1.222 1.333 1.500 1.778 1.000 1.000 1.000 1.000 1.000 1.000 0.917 0.857 0.778 0.667 0.500 0.222 0.833 0.714 0.555 0.333 0.000 0.000 0.750 0.571 0.333 0.000 0.000 0.000 Default Rp 0.333 1 0.667 1 1.000 1 1.333 1 1.667 1 2.000 PF = (1 - P)/(1 - g/C); Rp = R/(g/C) Early/Late Arrival Factor (fpp) Degree of Saturation I) AT-1 AT-2 1 AT-3 Arrival Type (AT) | AT-4 | AT-5 0.2 0.4 0.6 0.8 1.0 1.000 1.000 1.000 1.000 1.000 0.880 0.910 0.940 0.970 1.000 1.000 1.000 1.000 1.000 1.000 1.240 1.180 1.120 1.060 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 fpp (x = 0, 1.000 1 0.850 1 1.000 1 1.300 1 1.000 1 1.000 fop =fpp (x = o) + ( 1 -fpp (x = 0) )X Source: Reference 4. A - 35

Delay Results. For isolated approaches having random amval flow the model (Eq. A-38) predicts that vehicular delay increases as volumes and resulting v/c ratio (degree of saturation) increase, as depicted in Figure AN for an isolated intersection approach. At low volumes (v/c ratio), delays are caused primarily by vehicles arriving on red. At higher volumes nearing saturation (X= 1 .0), some cycles fail to completely clear the number of vehicles arriving (randomly) per cycle and these vehicles, which would previously not be delayed hardly at all (because they arrived on late green), are now being delayed a full red duration plus adding to the delay of subsequent arrivals. At even higher arrival volumes that routinely exceed capacity, queueing and delay continue to increase for each cycle that oversaturation exists with theoretical delays being limited only by the ending of He overs-aturationpenod, T. Extremelylarge delays are theoretically possibleif adequate queue storage is available. A practical maximum X value of ~ .2 is employed by He HCS software to limit He delay calculated, but no magnum delay value has been selected. A.2.S HCM Capacity Analysis Recommendations The general HCM capacity, delay and level of service methodology used for isolated signalizedintersections end arterial street systems should be generally applicable for interchanges. However, the effects of signal coordination and limited queue storage available due to the closely- spaced signals having high turning traffic should be more precisely identified. Calculations of queueing delay and spillback should be specifically related to signal offset and available queue storage. Effects of ]0~5he~ h1ocka~e should be identified Ed assessed. Delay vs Volume for ECM Mode! 300 2so - C= 100 see g = 50 see ,s.,200 - s = t800 vph~pl _ lSO AS 100 SO O 0 2SO SOO PESO 1 ~1250- lSOO 1 Volume (vphgpl) Figure A-4. Proposed HCM Model for Estimating Vehicular Delay for Random Arrivals. A - 36

The operational dichotomy between capacity and delay should be recognized. Capacity of a phase fundamentally depends on the forward motion of a platoon. For capacity to be provided during a cycle, nght-of-way must be provided, flow through the intersection must be unblocked, and downstream storage/processing must be available. Delay, on the other hand, is the antithesis of motion. Delay occurs because vehicles have arrived that can not continue their forward motion at their desired speed. Normally, delay occurs because vehicles arrive on red, not green. Capacity depends on motion; delay depends on restriction to motion. The preferred set of analysis variables should be the optimal choice when considering both capacity and delay. There are several ways to precisely calculate signal capacity. There are only two ways to precisely calculate delay: integration of vehicle travel through the queue, or integration of vehicular queue over time (the cycle). All other delay models are approximations. The precise estimation of traffic performance at interchanges and other closely-spaced intersection depends on several important assumptions and estimates being true and accurate. Central to this issue is Nat of phase capacity per cycle. It is recommended that the phase capacity per cycle, n, be estimated from n = ~ gf Sf (Z) C (A-40) where f signifies flow (motion) occulting. Both go and Sf must be correct for We period and prevailing conditions. The preferred definitions for g and s are as follows: g -- gf = effective green time during phase (cycle) when platoon flow can occur at rate Sf, see; and s -- Sf (zJ = maximum average platoon flow that can occur dunnggf, considering the distance z to the back of the downstream queue at start of green, vpsg/vphg. Note that only when there is one motion period per cycle/phase does n =gfSf =g¢S ~(A-41) and only dunug a Filly protected, unimpeded, unblocked phase does as assumed in the ~ 994 HCM. n = gs A - 37 (A-42)

Given that the effective green is defined as the time when a platoon can be in motion across the stop line, then the saturation flow rate, and all related adjustment factors, must be defined accordingly. For undersaturated conditions without spilIback, this primanly affects only permitted left turn operations. Protected-plus-permitted operations should be evaluated using, as a minimum level of approximation, linear flow approximations during green/red and summations of polygons of queueing during the cycle. As Appendix D watt show and document, the above new robust definition of"effective green" can readily be used to characterize the capacity during output blockage conditions caused by queue spilIback and general oversaturation conditions. In this case, the effective green watt be further defined as the joint occurrence of the unblocked green with the "clear period" of the cycle when no spiliback blockage of the subject intersection occurs, or g =gfn CP where: (A-43) g = effective green time where platoon motion (flow) cart occur, see; gf = effective green time where platoon motion can occur during unblocked and . ~ .... undersaturated flow conditions, see, arid n = set theory "intersection" or joint occurrence in time and space; and CP = clear period during cycle when output flow can occur from the subject lane group considering downstream spilIback conditions from all connecting intersections, sec. A.2.9 Interchange Operations Signal Timing. A two-level signalized interchange is essentiality composed of two closely spaced intersections (except for a SPUN) connected by an internal crossing arterial link. As noted in Figure A-5, each intersection have an arterial input phase (a), and most will have a ramp input phase (b) together with an arterial left turn phase (c). All diamond interchanges (having one-way frontage roads/ramps) and all two-quadrant parclos wall have all three phases in some configuration. Some four-quadrar~t parclos do not need the ramp phase (Parclo BB) and others do not use the arterial left turn phase (Parclo AA). Thus, for most interchange cases, three separate protected phases will serve each ramp terminal. In 1973, Munjal (~) graphically examined the three critical conflicting phases at each intersection of a diamond interchange. This phase notation was adopted by the widely used diamond interchange computer program PASSER Ill, developed by Texas Transportation Institute in 1977 (59. - Almost all signal timing plans used at two-level interchangestoday can be described by using the a,b anc!c phases in four sequence combinations together with a related signal offset between the two intersections. Table A- ~ 4 illustrates these phase sequence combinations. Various phase overlap timing plans are also possible, depending on the interchange type. A - 38

~1 -it SINGLE - POINT a c b a /k ~ , 'I ~ = ~ rev PARCLO AA - 2Q '_ ' my :: _ ~ ., PARE) BB - 2Q I' tic b - b ~ a a _ ~ 1 b a a Figure A-S. Interchange SignalPhasing. A - 39 c b DIAMOND - FR a I :1 a PARCLO AA - 4Q me 1\l ~ PARCLO BB - 4Q ~ a

Table A-14. Basic Signal Phase Sequences at Interchanges Phase ~Left-Side ~Right-Side ~Signal Combination Intersection Intersection Sequence ~T a:b:c T a:b:c T Lead-Lead ~ ~2 ~a:b:c ~a:c:b ~Lead-Lag 3 ~a:c.b T a:b:c T hag head 4 ~a:c.b ~a:c:b T ragtag Figure A-6 presents typical phasing sequences for some common interchange types for illustrative purposes. The four-phase strategyis depleted for diamond interchanges. As can be seen, partial cloverleafs (parclos), in contrast to traditional diamond interchanges, provide some application variation but do not change the basic concepts. Two-quad parclos have three phases per intersection,whereas, four-quad parclos have only two phases per side, deleting Phase b. The two- quad parclos may have Phase c in the outbound direction (Parclo BB) or in the inbound direction (Parclo AA). Moreover, the ramp phases (Phase by may be on the approach side (Parclo AA) as in a conventional diamond, or on the opposite side (Parclo BB). Parclos provide one distinguishing phasing difference to diamonds in that ramp right-turns (sometimes free) signal overlaps are common. Single-point urban (diamond) interchanges (SPUN) basically employ a conventional intersection phasing sentience. using dual-rina operations on the crossing arterial and a single chase _ ~ , _ ~ ~ ~ . . . - _ ~ ~ . e e ~ ~ ~ ~ tor the ramps. Longer phase lost times arise due to the longer clearance intervals generally employed to clear the large intersection area. Arterial right-t~n capacities at SPUl's are typically larger than for the off-ramps due to the significant increase in protected (overlap) turning time. Traffic flow through an interchange depends on the signal timing, traffic mix and geometric features. Throughput depends on the capability to first enter the interchange and then exit the facility. The more restrictive case is usually entry into the interchange, although cross street left turn lanes can become overloaded under some traffic patterns. Seldom is the arterial outbound phase a restraint undess it is blocked or impeded by queue spilIback from a closely-spaced downstream intersection. Operations at some two-quadrant parclos may be an exception. In(lepen~lent Operations. At low traffic volumes and/or wide intersection spacings, the two intersections (u,a) could operate independently. Let ~ be the total phase duration (green + yellow ~ red clearance intervals) such that the effective green is g = ~ - ~ where ~ is the total phase lost time. At one intersection, u, the three conflicting phases (a, b, c) must add to one cycle Qua Sub Us cu (A-44) and at the second intersection,a7, the sum of Tree conflicting phase times may add to another cycle A -40

i. t 1 it' ~1 . -. o SINGLE - POINT l /1 DIAMOND - FR ~ MA fib ! t T t a ~a ~ b - a:b:c a:b:c be ~ b al b a:b:c a:b:c b t PARCLO BE - 2Q c c 4~) a a 1 a:c:b a:b:c Figure A-6. Interchange Signal Phase Sequences. A-41

length of ~ a + 44d~b Jr tie = Cdf (A-45) Subject to each phase, m, satisfying its minimum green requirements ltm > min In As long as the two signals operate independently, cycle times can be different to accommodate variable traffic demands, green splits can be provided without constraints to better satisfy those demands, and capacities are at a maximum. Capacity principles described in Chapter 9 - Signalized Intersections of the HCM would apply, as follows. For a representative intersection and cycle, Cj, the sum of critical phases is Via ~ Sib + Tic = Ci (A-46) which is equivalent to (g l Via (g [)ib (g Chic Ci (A47) Letting [3 equal the sum of the three lost times per phase, the total effective green time per cycle is gia gib gic Ci [3 And since the v/c ratio for a phase or related lane group "m" is given by X. = (A-48) ~ VC 1 im 1` Sg) m (A-49) Solving for We effective green, am' and substituting into Equation A-48 for the available effective green time, yields a more general expression of vanables (, sx~J ia t~ sx! fib ~ SX) ic (A-50) Dividing by the cycle results in the fundamental capacity equation for intersections of A -42

j Sat ia ~ SX) ib ( SX) ic C, (A-51) Letting Y = As be the flow ratio of demand flow to saturation flow, then ~ X ! ia ~ X) ib ~ X] ic ci (A-52) Defining a new term, the pie ratio for a critical phase m, to be Him at,, xNJ It, sXl/1 (A-53) zm Im which is the proportion of the available green time needed (used) to serve the demand volume for a given geometry and degree of saturation, then for each intersection, i, the pie ratio must equal the proportion of the cycle time available for moving traffic rr`-~ Ci - L, ~ Hi = ~ brim = 3 = ~ _ 3 (A-54) m=} ci i For planning and design purposes, the fundamental capacity equation (A-5 1) may be solved for critical service volumes for given average/design assumptions. First, a signal timing strategy is usually assumed that provides equal v/c ratios for all critical phases such that X. = X. = X. = X. = X. a zb IC am I ~ ib ~~ ic ~~ im (A-55) following the welI-known Webster strategy which also tends to minimize intersection delay. Thus, for equal degrees of saturations for all critical movements m, X im = X `, Equation A-5 ~ becomes X~ ~ s\J ia X~ ~ s\J ib X it 5~11 ic Ci (A-56) A - 43

so that for the three critical phases at intersection i, the total intersection flow ratio is m=3 m=3 L Y = Yim v = Xi 1 - - m=1 m=1 (sJ im ~c,J (A-57) For planning arid design purposes, it is also convenient to work with locally adjusted average saturation flow values, and per lane volumes, to estimate the resulting flow ratios; y _ (v) ~ S J or on a per lane basis = 1 (~i' - J zm SO Rj (by) ~ NJ im (A-58) y 1 ( v ~Via (A-S9) such that the equivalent critical lane results are Vila } Vilb + ilc = X ~ 1 - 3: Sila Silb silo ~ci ) (A-60) Assuming that S'la = Silb = Silk and that v`/b and vile have been adjusted slightly to equivalent through volumes (TV,, CV'~c) to keep the flow ratios the same, then the sum of the "equivalent through vehicle" critical lane service volumes for a given Xj would be m =3 CV. = ~ CV. = s. X. al zlm zla z m =1 L3 1 -- C. \ 1 The average allowable service flow per critical phase, Pa, would be f P il 3 Sila Xi ~ 3 C A - 44 l (A-61) (A-62)

where ~ is the average lost time per critical phase. The equivalent through vehicle can represent passenger cars only, or the average fleet mix for the locale by adjusting S'`a acccordingly. Assuming that s,,,, = ~ 800 vphgp} for the local traffic mix, X, = ~ .0 for capacity flow, ~ = 4.0 sec/phase, and C = 100 see; then the total critical lane capacity flow, Cry, for intersection i from Equation A-61 would be CVji = IS00 X 1.0 ~1.0 - ~ )~ = 1584vphpl (A-63) and the resulting average critical lane capacity per phase from Equation A-62 is Pil = 1 8 00 x 1 .0 ( 0 3 3 3 - -1 = 52 8 Vphpl (A-64) 100) Other critical lane service volumes can be calculated for the degree of saturation selected. In addition, an operational analysis could be conducted to determine the overall degree of saturation (X,) on the intersection produced by the critical volume loading present. Coordinated Intersections. Traffic signals at most interchanges are coordinated to improve overall traffic operations because the intersections are closely spaced and traffic volumes are often high. Under these conditions, coordination generally improves operational reliability and reduces internal queueing, queue spilIback into upstream intersections, and the threat of operational gridlock. At lower traffic volumes, cross arterial progression can also be provided in most cases to reduce queueing. The overall quality of operations depends on the features of the signal system deployed, the progression provided, arid other factors. Coordinated signal operation implies Cat the two intersections no longer can operate independently. The first coordination constraint applied usually is that the two cycles, Cu and Ca5 must be the same length at a particular point in time. This cycle constraint is true whether Me interchange is controlled by one controller unit or by two. Thus (A-65) Delays and queueing incurred on the two external approaches to the lower-volume intersection will increase due to operations at a suboptimal (higher) cycle, assuming that the interchange would operate at the cycle of the higher-volume intersection. Signal coordination within the interchange can almost eliminate outbound delays, but arterial coordination is required to mitigate arterial approach delays incurred on the inbound Phase a.

Cycle timing is cr~ticalto interchange operations for many complex arid interrelated reasons. Capacity analysis should recognize this central control parameter. The first consideration in establishing cycle time is to determine whether interchange control is coordinated with the cross street arterial or not. If yes, then cycle times wall be fixed for various coordination time periods. The interchange' slocal control may be coordinated pretimed or coordinated(semi) actuated, but the system cycle length is the same in either case. The second consideration in establishing cycle time, given that cross arterial coordination is not effected, is whether the interchange' scontroller~s~is pretimed or coordinated (semi) actuated. Local interchange coordination is presumed. Pretimed systems are fixed to prescribed durations regardless of current traffic demands and local capacity provided. Presumably forecasted traffic demands and estimated roadway capacity were considered in the initial selection of cycle times, but conditions may have changed. For locally coordinated actuated control, field measurements are highly recommended over unproven analytical models or engineering judgement. Actuated control is reactive to queues and tends to be unstable in congested operational environments and, therefore, may generate extremely long cycles and undesirable green splits based only on maximum green settings, which may further exacerbate the congestion. Another critical operational feature affecting interchange traffic operations is whether the green splits at the two intersections depend on one another. If two separate controllers are used (or independent rings are provided using one controller), then capacity estimates are best given by Equation A-S I. If phasing is dependent (e.g. only one controller is used with dependent phases), then more complex demand analysis is required consistent with the type of interchange and phasing used. Operations at "four-phase" diamonds is a classic example, but others abound. Coordinated Diamond Interchanges. A popular diamond interchange signal timing strategy is "four-phase with two overlaps." In addition to having many of the above timing features (e.g., same cycle, a fixed sequence, and a fixed offset), this strategy also provides quality platoon progression for the arterial traffic in both directions of flow through the interchange by either special controller design, or by judicious signal timing. By either method, the following signal timing relationship must occur 66J for four-phase with two overlaps signalization to result Gua GUb + Gda ~ Glib = Ci + ~ (A-66) where it is presumed that Go and Go, are the thru and ramp phases, respectively, and 4) is the total interchange "overlap" for both directions of flow through the interchange (5~. This operational requirement provides great progression for the arterial traffic passing through the interchange. However, the sum of the four external phases serving traffic input to the interchange is fixed for a constant cycle and does not have full flexibility to optimally adjust to all possible traffic patterns that might arise at the interchange. In addition, this constraint (Equation A-66) further implies that the sum of the two internal left turns within the interchange is also fixed 669 at . . . A -46

GUC ~ Gdc = Ci ~(A-67) because the sum ofthe conflicting phases must equal two cycles (Equations A-66 and A-671. This fact may be a significant constraint on the optimal solution depending on the cycle time, minimum green times required, and traffic pattern being serviced 664. The computer signal timing program PASSER Ill, developed at Texas Transportation Institute, contains these strategies for developing optimal signal timing for all fonns of two-level signalized diamond interchanges. PASSER ITI also contains delay/difference-in-relative offset aIgonthms, somewhat like TRANSYT 7F, to evaluate traffic performance for given signal timings fly. A.2.10 Interchange Capacity Shoddies Most operational studies reported in the literature have been conducted on signalized tight urban diamond interchanges (TUDI), probably because of operational problems due to the closely spaced ramp terminal signals. These signal spacings in many urban areas are often on the order of 70-] 00 meters. Capelle and Pinnell wrote In We early ~ 960's f7) that: "After studying the problem of evaluating the capacity of diamond interchanges, it was determined~at it would be necessary to consider the two signalized intersections as a single unit. This is due primanlyto the requirements of signalization which should perform two basic functions. These functions are as follows: (a) all highway volume conflicting movements at both intersections must be separated, and (b) storing of vehicles between the two intersections must be kept to a minimum due to limited distance between them." The above study was conducted at diamonds that were "locking up" using three-phase operation. Capelle and Pinnell (7J then tested a new phasing plan that has since become known as "four-phase with two overlaps" to improve operations. They proposed a method for calculating interchange capacity for this strategy which they termed the critical lane capacity, CVc as being CV = (C + 4 + 4D + 8) 3600 (A-68) where C represents the cycle length, D the starting delay, and H the saturation headway. The starting delay used for their calculations was the time required for the first two vehicles in a lane to enter the intersection. The critical lane capacity, CVc, represents the maximum sum of the four critical lane approach volumes from the four external approaches to the interchange. Capelle and Pinnell (79 computed critical lane capacity using H= 2.] seconds/vehicle and D = 5.8 sec. A 2.] seconds saturation headway is equivalent to a saturation flow of 1,714 vphgpl in the HCM. In a recent HCQSC Literature review, Lee (~9 updated the Capelle/PinnellMethodto include recent interchange studies in the Phoenix area using Hook's data (99. The equivalent Hook's value for starting delay and average saturation headway. weighted bY the volumes of the movements were , , . . ~ , A - 47

7.1 and 1.89 seconds, respectively. Capelle and Pinnell assumed that the starting delay was incurred by the first two vehicles, while Hook assumed the third vehicle in queue. Following some additional modifications, Lee developed the equivalent sum of critical lane volumes shown in column 3 of Table A-15. Column 4 was derived by TT] from work on NCHRP 3-40 (109 described below. TABLE A-15. Critical Lane Capacity of Diamond Interchanges by CapeNe/Pinnell, Lee, and NCHRP 3-40 Cycle ~OriginalCritical ~Updated Critical Length Lane Capacity Lane Capacity(2) 40 1,611 1,821 50 1 1,635 1 1,838 60 1,650 1,849 70 I 1,660 T 1,857 80 1,668 1,863 100 1 1,674 T 1,872 180 1,692 1,886 . f~' Based on 1961 values for starting d. .ay, D, of 5.8 seconds and average headway, H. c 2.1 seconds (7). (2) Based on current values of D = 5.2 seconds and H = 1.9 seconds, (8). IT ! (3) Based on studies published in References 9,10,11,12, and 13. NCHRP 3-40 Cntical Lane Capacity(3) 1,980 - 1,984 1,987 1,989 1,990 1,992 1,996 About the time Lee was performing his analysis, Messer and Bonneson were publishing a more updated version ofthese formulations in NCHRP 345 (109 based on the Phoenix data (9,11) plus additional data collected in Florida and Texas by Bonneson (12,13,149. The basic "saturation flow" model followed the earlier work by Messer in 1975/76 (15) described in the previous section. The critical movement model for interchange capacityis, however, basically a reciprocal formulation ofthe headway model presentedby Capelle and Pinnellin 1961, but with slightly different saturation flow and start-up delay calibration factors. Messer, et al, showed in NCHRP 3-40 (10) that He sum of the four critical external inputs to a diamond interchange operating with four-phase with two external overlap signal timing has a critical lane capacity of CV = CV + CV + CV + CV Ic ua ub da db A-48 ila I C C ~ . (A-69)

where: CV,c sum of interchange critical input volumes, vphpl; adjusted saturation flow for all critical input phases, vphgpl; (= 3600/H above) total interchange overlap, see; number of critical input phases, n = 4; average phase lost time, see; and cycle length, sec. The total interchange overlap is a function of the center-to-center spacing between the ramps, arterial grade, and quality of cross street coordination. An equation for estimating ~ for nominal conditions of no grade nor coordination is given ~ the PASSER Ill users manual as 4) = 2 [0.sO ~ 40.137L ] (A-70) for art intersection spacing of ~ meters subject to a maximum speed of 50 km/in (30 mph). Higher maximum speeds and lower overlaps may be appropriate urlder well coordinated cross street operations. Messer's updated interchange/intersectioncapacity model (109 can be applied to other types oftwo-leve! signalized interchanges operating in a coordinated system/common cycle mode. The model estimates the sum of critical lane volumes, CVc' that can be input into the interchange from the four external approaches according to the following equation Chic = sofa [1 ~ ~/C n(Zs [e)/C] (A-71) where sea equals the mean saturation flow rate for all critical phases, (s = 3,600/H above), ~ is the total phase overlap in the interchange, h, is the number of critical phases, Is is the platoon startup lost time (about 2.0 seconds) per phase, and le is the end lost time associated with ending the phase as related to the width of the interchange and the duration of We signal change intervals (the yellow warning and (all?) red clearance intervals). The average phase capacity, P., can be estimated by dividing by the number of critical input phases, n, on the external approaches to yield Pa = s~atl/n + ~/nC ~ (Is ~ Ic)/c] (A-72) Equations developed in the earlier research projects to estimate the above parameters are given in the following section. A -49

The capacity of an interchange depends on several operational parameters, as Equation A-71 illustrated. Signalized interchanges are character~zedby the number of critical phases that may exist (usually 2 for 4-quad parclos, 3 for 2-quad parclos arid many diamonds, or 4 for many TUDIs), by the total phase overlap 4) that may be present (usually 0.0 except for four-phase overlap systems), arid by the total amours of clearance time used to safely terminate phases. Operational Parameters. TTI used the above referenced reports (12, 13, 14J to develop several useful relationships to study interchange capacity presented in NCHRP 345 (109. Saturation flow for protected left tutus at interchanges was estimated from: Sin = 3,600/~1.50 + 1 1l/r0,245) (A-73) where so' is the saturation flow per larle for left turns, pcphgpI, and r is the average left turn radius ofthe tuning maneuver, ft. A so, value of 2,000 pcphep] is predicted at a turning radius of 200 feet ~ . . ~ ~ ``l ~ ~ ~ ~A ~ ~ (60 m). This was the maximum value of saturation flow recommended,which was also the assumed value for through movements under ideal conditions (] 09. The application of Equation A-73 to signalized interchanges could easily follow HCM procedures where an adjustment factor for left turns, based on the radius of turn, is applied to the saturation flow for "ideal conditions" of 2,000 pcphgp] for interchanges. A protected left turn factor,fp as derived from Equation A-73, would then be (109 fit = 1 .0/~0.83 3 + 0.6 1 7/r 0.245' (A-74) Quantifying the effects of trucks and other heavy vehicles on saturation flow at traffic sign~s is another matter. The passenger car equivalency (PCE) used in the ~ 985 HCM was ~ .5 for through traffic only, but Molina (169 showed that the PCE for typical through moving urban truck traffic averages about 2.7, web a range from I.7 for small trucks to 3.7 for five-axIe trucks. The 1994 HCM provided an small increase in the average PCE of heavy vehicles at signalized intersections to 2.0. Heavy truck volumes turning left from off-ramps under tight geometric conditions could have an even larger impact on interchange capacity. Estimation of phase clearance lost times for interchanges is somewhat complicated by the diverse intemal clearance paths taken by vehicles traveling through the interchanges and the different forms of interchanges. Poppe (~]J estimated the clearance lost time, for a SPU} as related to the signal change interval, CT, of the phase as 1 = 0.95CI - 2.3 c (A-75) with an it-square of 0.97, suggesting an "end use" of the initial portion of the yellow interval of 2.3 seconds. These results compared well with Bonneson's Flonda data (129 and with those collected within this study and reported in Appendix C. A - 50

REFERENCES Munj al, P.K. "An Analysis of Diamond Interchanges." Transportation Research Record 349, Transportation Research Board, Washington, D.C. (1971) pp. 47-64. Highway Capacity Manual." Special Report 209, Third Edition, Transportation Research Board, Washington, D.C. (1994~. Fambro, D. B., Messer C.~., and Andersen, D.A. "Estimation of Unprotected Left-Turn Capacity at SignalizedIntersections." Transportation Research Record 644, Transportation Research Board, Washington, D.C. (1977), pp.1 13-1 19. 4. 6. 8. 10. 11. Fambro, D.B., Roupha~l, N.M., Sloup, P.R., Daniel, J.R., Li, J., Anwar, M., and Engelbrecht, R.J. "Highway Capacity Revisions for Chapters 9 and 1 1 ." Federal Highway Administration, FHWA-RD-96-088, Washington, D.C. (1996~. Messer, C.J., Fambro, D.B., and Richards, S.H. "Optimization of Pretimed Signalized Diamond Interchanges." Transportation Research Record 644, Transportation Research Board, Washington, D.C. (1977) pp. 78-84. Messer, C.~., Whitson, R.H., and Carvell, I.D. "A Real-Time Frontage Road Progression Analysis and Control Strategy." Transportation Research Record 503, Transportation Research Board,Washington,D.C.~1974) pp.l-12. Capelle, D.G. and Pinnell, C. "Capacity Study of Signalized Diamond Interchanges." Highway Research Board Bulletin 291, Highway Research Board, Washington, D.C. (1961~. Lee, J.C. "Review of Diamond Interchange Analysis Techniques: Past and Present." Synthesis Paper for Co~runittee A3A10. Transportation Research Board, Washington, D.C. (1992). Hook, D.~. and Upchurch, I. "Companson of Operational Parameters for Conventional Diamond Interchanges and Single-Point Diamond Interchanges," Transportation Research Record 1356, Transportation Research Board, Washington, D.C. (19921. Messer, Cal., Bonneson, J.A., Anderson, S.D., and McFarland, W.F. "Single-Point Urban Interchange Design and Operational Analysis." NCHRP 345, Transportation Research Board, Washington, D.C. (1991~. Poppe, M.~., Radwan, A.E., and Matthias, I.S. "Some Traffic Parameters for the Evaluation of the Single-Point Diamond Interchange." Transportation Research Record 1303, Washington, D.C. (1991~. A - 51

Bonneson, I.A. "A Study of Headway and Lost Time at Single-Point Urban Intercharlges.'' Transportation Research Record 1365, Transportation Research Board, Washington, D.C. (1992~. Bonneson, J.A. "Factors Affecting Bridge Size and Clearance Time of the Single-Point Urban Interchange." Journal of Transportation Engineering, Vol. ~ ~ 9, No. I, American Society of Civil Engineers, New York (] 993~. 14. Bonneson, J. A., "Operational Efficiency of the Single-Po~nt Urban Interchange." ITE Journal, VoT 62, No. 6. Institute of Transportation Engineers, Washington, D.C. (! 9921. 15. Messer, CJ., and D.J. Berry. "Effects of Design Alternatives on Quality of Service at Signalized Diamond Interchanges." Transportation Research Recorc! 538, Transportation Research Board, Washington, D.C. (19751. 16. Molina, C.A. "Passenger Car Equivalents of Trucks at Signalized Intersections." ITE Journal, Vol 54, No. 9. Institute of Transportation Engineers, Washington, D.C. (1988). A - 52

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