Capacity and Delay Characteristics of Two-Way Stop-Controlled Intersections

Transcription

1 16 TRANSPORTATION RESEARCH RECORD 132 Capaity and Delay Charateristis of Two-Way Stop-Controlled Intersetions MICHAEL KYTE, CHRIS CLEMOW, NASEER MAHFOOD, B. KENT LALL, AND. ]OTIN KRISTY Three objetives are sought: (a) to present a data base for twoway-stop-ontrolled (TWSC) intersetions that an be used to investigate the fators that influene delay and apaity, (b) to identify some of the fators that affet delay and 1paity at a TWSC intersetion, and () to develop a set of preliminary models to estimate delay and apaity. Traffi flow, delay, and geometri data were olleted at nine TWSC intersetion sites in the Paifi Northwest enompassing a total of 13 hr of intersetion operation. A total of 97 minor-street (subjet approah) vehiles were observed and nearly 2, aepted and rejeted gaps were identified and reorded. Eah site has several ommon harateristis: four approah legs, single lanes on eah approah, and a 25-mph speed limit on the major street. Several fators were identified that may influene delay and apaity at a TWSC intersetion. The time waiting in queue (queue time) is affeted by the traffi flow rate on the subjet and opposing approahes, whereas the time pent waiting a.t the stop line (servie time) is affeted by the traffi flow rate on the onniting approahes. The apaily of the subjet approah is also affeted by the flow rote on the onfliting approahes. The size of the aepted gap is affeted by the length of time that a vehile has been delayed, the flow rate on the onfliting approahes, and the diretional movement of the subjet vehile. Literature desribing both empirial and theoretial studies of two-way stop-ontrolled (TWSC) intersetions is plentiful. One major soure is a ompendium of papers presented at the 1988 International Workshop, Intersetions Without Traffi Signals (1). These papers represent the latest researh efforts in Europe, Asia, and the United States on this topi. Common to muh of this work are two elements: (a) the determination of the ritial gap as a funtion of the intersetion geometry and major-street flow harateristis, and (b) the hierarhial lassifiation of onfliting traffi streams as the basis for omputing movement apaities. These two elements are essential omponents of the standard analysis methodology used in the United States, defined by the 1985 Highway Capaity Manual (HCM) (2). Despite its widespread use, there is a general onsensus that the HCM methodology requires substantial modifiation (3). Several empirial and theoretial limitations are most ommonly ited: 1. The measure of effetiveness (MOE) used in the HCM proedures is reserve apaity. Although reserve apaity is diretly related to delay in the German and Swedish proe- M. Kyte, C. Clemow, and N. Mahfood, Department of Civil Engineering, University of Idaho, Mosow, Idaho B. K. Lall, Department of Civil Engineering, Portland State University, Portland, Oreg C. J. Khisty, Department of Civil Engineering, Washington State University, Pullman, Wash dures (4,5), the HCM provides only a broad desription of the delay likely to be enountered for eah range of reserve apaity. Thus, it is not now possible to foreast the performane of an intersetion operating under various ontrol onditions (e.g., signal ontrol or two-way-stop ontrol) using a single MOE suh as delay. 2. The HCM proedure is based on a German method developed in 1972 and has been verified with only limited studies for U.S. onditions (6). There is an obvious need to develop a data base for U.S. onditions for TWSC intersetions. 3. The Germans have sine identified several theoretial problems with their original proedure and have now developed a new set of paper-and-penil proedures and a omputer simulation model (7). Also, the German proedures were developed only for single-lane approah sites. Multilane sites in Germany are always signalized. 4. There are several methodologial problems with the urrent HCM proedure, primarily related to the use of the ritial gap. First, the methodology used to estimate the ritial gap usually results in an overestimation of this parameter. Kittelson and Vandehey in a ompanion paper in this Reord propose a new method to obtain a more aurate estimate of the ritial gap; this method deserves onsideration. Seond, it has bt:en suggested that the ritial gap is not onstant and that drivers tend to aept smaller gaps the longer they wait in queue or at the stop line. This variability in the ritial gap needs to be quantified. Finally, it is likely that the ritial gap depends on the diretion of the minor-street vehile. The urrent HCM proedure assumes that the ritial gap varies only with the travel speed and number of lanes on the major street. One solution to this problem may be to adopt the approah used in the United Kingdom (8), where empirially based models for delay and apaity have been developed that do not rely on estimation of the ritial gap. Clearly, there is a need to address several major issues with respet to the understanding of traffi flow at TWSC intersetions. Three objetives begin to address some of these issues: 1. To present a data base for TWSC intersetions that an be used to investigate the fators that influene delay and apaity, 2. To identify some of the fators that affet delay and apaity, and 3. To develop a preliminary set of models that an be used to estimate delay and apaity.

2 Kyte et al. DEFINITIONS OF TERMS Intersetion Approahes Queue ativity is monitored on one of the minor-street approahes. This approah is alled the "subjet approah." The other (faing) minor-street approah is alled the "opposing approah." The two major-street approahes are alled the "onfliting approahes." Mirosopi or Instantaneous Major-Street Flow Rate The mirosopi analysis requires a definition of the majorstreet flow as seen by eah subjet approah vehile. Consider the following sequene of events. A subjet vehile arrives at time t 1 Two onfliting approah vehiles pass by at times t 2 and t 3, respetively. The gaps t 3 - t 2 and t 2 - t 1 are rejeted by the subjet vehile. The subjet vehile departs at t 4, whereas the next onfliting vehile passes at ts. The definition of the onfliting flow rate as seen by this partiular subjet vehile is the number of observed onfliting vehiles divided by the observation time, or 3 Flow rate = -- ts - t1 This definition differs from the standard (marosopi) method of estimating flow rates, in whih averages are reported for some fixed period, usually 15 min or 1 hr. This distintion will be noted whenever the mirosopi or instantaneous flow rate is used. Aepted Gap The standard definition for the aepted gap is ls - t 3 However, it is ommon for the onfliting vehile representing the end of the aepted gap to pass through the intersetion long after the departure of the subjet vehile. Thus, the subjet vehile never sees this onfliting vehile, and the alulated aepted gap is not meaningful. Kittelson and Vandehey in a ompanion paper in this Reord propose that a maximum value be used for the aepted gap based on riteria of sight distane and major-street speed. They suggest in most ases that a maximum of 12 se is reasonable. (1) between the arrival at the end of the queue and the time when the vehile arrives at the stop line is defined as the "queue time." The time between the arrival at the stop line and the departure from the stop line is defined as the "servie time." COLLECTION AND REDUCTION OF TRAFFIC OPERATIONS DATA Data were olleted during nine different days at four different intersetions over a period of 13.1 hr. Eah of these days is lassified as a sample site. Eah site has four approah legs, a single lane on eah approah, and a speed limit of 25 mph on the major-street approahes. Eah sample site was videotaped with the amera in a position to observe vehiles on eah approah as they passed through the intersetion as well as the queue ativity on one minor-street approah. Data olletion software was used to reord flow rate and delay data while observing the videotapes from eah sample site (9). The time that eah vehile entered the intersetion was reorded and its diretional movement (left-tum, through, or right-tum) was noted. Flow rates were alulated on the basis of these data. For one minor-street approah (referred to as the subjet approah), the times that eah vehile entered the end of the queue, arrived at the stop line, and entered the intersetion were also noted. Delays were alulated on the basis of these data. OVERVIEW OF SITE CONDITIONS: THE DATA BASE The first objetive is to present a data base for TWSC intersetions. Table 1 presents some of the important geometri, traffi flow, and delay data for the nine sample sites. Sight Distane Data The sight distane data were assessed for eah of the intersetion approahes at the nine sample sites. Four ategories were'used in this assessment, inluding exellent, good, fair, and poor. Although admittedly qualitative, the assessment may be useful in determining if sight distane affets driver behavior and thus delay and apaity harateristis. 161 Delay Stopped delay was alulated for eah minor-street subjet vehile by measuring two separate omponents. The time Upstream Control and Platoon Charateristis The arrival patterns of the traffi on the onfliting approahes have an effet on the behavior of the subjet vehile. Up- TABLE 1 SITE DATA Site Site Distane Observed Major Subjet Volume, Oppooin, Volume, Confliting Mean Servie Mean Queue Stroot Platoon vpb vph Volume, vpb Time, se Time, se I Fair Cyli Fair Random Exellent Cyli Good Uniform Good Cyli Good Cyli F11ir Cyli Good Random Fair Cyli

3 162 TRANSPORTATION RESEARCH RECORD 132 stream ontrol devies often have the effet of regulating this arrival pattern in some way. The type of ontrol and the distane upstream to this ontrol devie for traffi arriving on the onfliting approahes from both the left and the right of the subjet approah and the resulting observed platoon patterns were noted. Flow Rate Data Traffi flow rate data were olleted for eah intersetion approah at the nine sample sites. Intersetion flow rates varied from about 4 veh/hr at Site 2 to 1,2 veh/hr at Sites 3 and 15. Delay Data Queue and servie times were measured for eah minor-street subjet vehile. High mean delays, about 25 se/vehile, were observed at Sites 9, 11, and 14. The lowest delays, about 11 and 12 se, were measured at Sites 1, 3, 12, and 15. The distribution of the queue time and servie time omponents varies widely among the sites. Major-Street Gap Data Table 2 presents several important gap data for the nine sample sites. The ritial gap was alulated as the gap size at whih the perentage of aepted gaps is equal to the perentage of rejeted gaps. The ritial gap ranges from 5.5 to 7.5 se. Data from Table 1-2 of the HCM for major streets with 3-mph speed limits and single lanes on eah approah exhibit a range for the ritial gap of 5. to 6.5 se. Thus the data olleted in this study ompare favorably with ranges suggested in the HCM. The zero gap was alulated as the highest measured gap that was rejeted by all drivers. Aepted and Rejeted Gap Data All gaps seen by eah subjet vehile were measured. Eah gap was lassified into one of eight ategories. The first gap observed by a subjet vehile is a lag. All others are gaps. Aepted gaps or lags were used only by a subjet vehile. Shared gaps or lags were used jointly by the subjet vehile and one or more opposing vehiles. Used gaps or lags were used by an opposing vehile and were thus unavailable to the subjet vehile. Rejeted gaps or lags were not used by the subjet vehile or any opposing vehile. Table 2 presents the mean values for eah of these gap ategories for eah site. Several points an be made about the data presented in Table 2. First, expeted differenes between gap ategories are supported by the data. For example, gaps or lags shared by more than one vehi"le are larger than gaps that are aepted or used by only one vehile. Also, rejeted gaps or lags are smaller than aepted gaps or lags. Seond, the large values of aepted gaps or lags (all mean values are greater than 11 se) support the need to develop adjusted values for aepted gaps, as desribed earlier. Gaps greater than some value (suggested as 12 se) do not provide any useful information in the analysis of the gap-apaity relationship. IDENTIFICATION OF BASIC RELATIONSHIPS MACROSCOPIC ANALYSIS The seond objetive is to identify some of the fators that influene delay and apaity. In this setion, mean values for several variables from eah site are ompared to determine, on a marosopi basis, those fators that influene delay and apaity on the minor-street approah of a TWSC intersetion. Vehile Delay Stopped vehile delay data were olleted by measuring two omponents, queue time and servie time. Plots of servie time versus both onfliting flow and subjet flow are shown in Figures 1 and 2. These flow rates are the measured marosopi flow rates. Plots of queue time versus onfliting flow and subjet flow are shown in Figures 3 and 4, respetively. Servie Time Servie time, or the time spent waiting at the stop line, is affeted most diretly by the rate of flow on the major street. As major-street flow inreases, all other fators being equal, the servie time should also be expeted to inrease. The data shown in Figure 1 indiate a strong orrelation between servie time and major-street flow rate. If a linear relationship is assumed, the oeffiient of determination (R 2 ) is.94. Figure 2 shows a slight deline in servie time with inreasing subjet approah flow. One supposition that may be made TABLE 2 GAP DATA Sample Critial Zero Mean Aepted Shared Site Gap Gap Gap Gap Gap I II II.I Aepted Shared Used Rejeted Avg Used Avg Lag Lag Lag Lag Gap Rejeted Gap II.I 3,

4 Kyte et al :g 5 ~ 4 (/) ~ 3 i= ~ 2 ;; ~ 1 D 1 " <DO D Major Street Flow, VPH FIGURE 1 Servie time versus major-street flow rate. 6.:g 5 ~ ~ o ~ 3 i= ~ 2 ~ ~ IQ o ~ o a Subjet Flow, VPH FIGURE 2 Servie time versus subjet approah flow rate. - D 1 ~so +-~~~~~~~~~~~~~~-< " ~4 1-~~~~~~~~~~~~~~---< (/) ~3Q +-~~~~~~~~~~~~~~-l E i= "., w " w2o +-~~~~~~~~~~~~~~-l 1 +-~~g~~~~~~~~~~~~-1 from the data in Figure 2 is that the pressure from inreasing subjet approah flow rates results in drivers aepting smaller major-street gaps and thus experiening lower servie times. Queue Time Queue time, or the time spent waiting in queue until arriving at the stop line, is a funtion of both the number of vehiles that are waiting in the queue and the rate at whih the vehile at the stop line is able to depart. The data shown in Figure 4 indiate a good orrelation between queue time and the subjet approah flow rate. Although other variables suh as arrival patterns may also be important, the subjet approah flow rate may be a good indiator for the number of vehiles likely to be in a queue on the minor street. If a linear funtion is assumed, the oeffiient of determination (R 2 ) for this relationship is.7. The data shown in Figure 3 do not, at first glane, support the ontention that queue time is affeted by major-street flow. However, a more detailed analysis may, in fat, bear out this supposition. If, for example, the three points in the lower right quadrant of Figure 3 are eliminated from the plot, the data indiate that queue time does inrease with servie time. The elimination of these data may in fat be justified beause these three data points (sample Sites 1, 3, and 15) are from sites with the lowest subjet approah flow rates (less than 15 veh/hr). Beause these are ases in whih subjet approah queuing is low or nonexistent, it would not be expeted that a variation in the major-street flow rate would have any effet on the queue time. However, the limited number of data points prevents a definitive onlusion at this time. Minor-Street Capaity Minor-street (subjet approah) apaity was alulated using Equation 2 and the servie time for eah subjet approah vehile. FIGURE 3 flow rate. R Major Street Flow, VPH Queue time versus major-street. 3,6 Capaity = S. T ' erv1e 1me Figure 5 shows a strong linear orrelation between the minorstreet apaity and the major-street (onfliting approahes) (2) " - s o 1--~~~~~~~~~~~~~~ ~4 1-~~~~~~~~~~~~~~ (/) ~3 +-~~~~~~~~~~~~~---< E i= w2 I-~~~--~~~~~~~~~~ " w "' 1 +-~---='----~~~~~~~~~~---< " Subjet Flow, VPH FIGURE 4 Queue time versus subjet approah flow rate. 1 I 11. > i 8 u D. 6 u D Q; 4 ~ iii I- 2 'i 5 1 Major Street Flow, VPH FIGURE 5 Minor-street apaity versus major-street flow rate. 15

5 164 flow rate. Although other variables are likely to affet apaity (e.g., major-street approah speed and turning proportions), the oeffiient of determination (.91) indiates the importane of major-street flow rate. Figure 6 shows a plot of aepted gap versus major-street flow rate. The aepted gap data shown in this figure have not been modified as desribed earlier. One possible hypothesis that an be suggested on the basis of Figure 6 is that the size of the aepted gap first inreases as major-street flow inreases, and then dereases. The hange point may represent the point at whih a driver's frustration level aused by the delays experiened at higher major-street flow rates begins to affet the deision-making proess. IDENTIFICATION OF BASIC RELATIONSHIPS MICROSCOPIC ANALYSIS In order to further meet the seond objetive, a mirosopi analysis of the data was undertaken. In this mirosopi analysis, the onditions faed by eah subjet approah vehile were taken into aount; that is, onsideration was given to the major-street flow rate as seen by the driver, the gaps that were presented, and so on. Several other points should be made about this analysis. 1. The gap aeptane and delay data for eah minor-street subjet vehile were alulated and the gaps were lassified aording to the definitions desribed earlier. 2. Only those ases in whih a subjet vehile did not fae any opposing approah vehiles were onsidered (in effet ~ u ~ '----~----~- I ~ r---"'- ~~~~~~---, " ~ ~ I ~~~-1 Q. " u ~ 5 +-~~~~~ ~-t O +-~~~~~~~r--r--~,---, Major Street flow, VPH FIGURE 6 Aepted gap versus major-street flow rate. TRANSPORTATION RESEARCH RECORD 132 simulating the operation of a one-way minor street rossing a two-way major street). 3. The mirosopi major-street flow rate and the aepted gap were alulated as desribed earlier. 4. Not all of the sample sites are shown in every assessment: a sample site is only shown if all ells for a partiular table had at least 1 observations. Aepted Gap Four assessments of aepted gap are presented here. These assessments are intended to identify whether the following variables influene the size of the gap aepted by a driver: whether a vehile waited in queue or not, the major-street flow rate, the queue time, the servie time, and movement diretion of the subjet approah vehile. Several onlusions may be stated from the following assessments. The most important is that the aepted gap is not a onstant, but varies as a result of several fators: queue time, servie time, number of gaps that have been rejeted, major street flow, and diretional movement of the subjet vehile. Aepted Gap as a Funtion of Number of Rejeted Gaps and Presene of Queue The assessment presented in Table 3 indiates that most vehiles that wait in a queue aept shorter gaps than vehiles that do not wait in a queue. Table 3 also indiates that most vehiles that rejet one or more gaps eventually aept a shorter gap than vehiles that aept the first gap. One possible inferene is that the longer a vehile waits, the greater the driver's frustration, and a gap that previously seemed too short now beomes aeptable. Another possible inferene is that a learning proess takes plae as the driver waits at the intersetion. The longer the wait, the better a driver an estimate the size of an aeptable gap. Both of these inferenes are tentative onlusions, and they should reeive further study. Aepted Gap as a Funtion of Major Street Flow Table 4 indiates that the size of the aepted gap dereases as the mirosopi major-street flow rate inreases. With inreasing major-street flow rates, the size of the available gaps TABLE 3 ACCEPTED GAPS, NUMBER OF REJECTED GAPS, AND QUEUE TIMES Aepted Gap, Seonds Zero Gaps Rejeted One or More Gaps Rejeted Sample Site No Queue Queue No Queue Queue I S

6 Kyte et al. 165 TABLE 4 ACCEPTED GAPS AND MICROSCOPIC (INSTANTANEOUS) MAJOR-STREET FLOW RATES Aepted Gq, Seonds Sample Site M1jor St Flow Major St Flow Major St Flow Major St Flow > 9 ().3() vph 3-6 vpb (J()().9 vph vph I IS S IO.S S S beomes smaller. But these results may also mean that when faed with higher flow rates, drivers may antiipate the need to aept what is available, even if the available gap size is smaller than desired. Aepted Gap as a Funtion of Servie Time Table 5 indiates a slight derease in the size of the aepted gap as the servie time inreases. More data are needed to determine if these results are statistially signifiant. Again, the trend may point to two fators disussed earlier: the aepted gap may be influened by driver frustration level or by learning, or both. Both fators diretly relate to the length of time spent at the intersetion stop line (servie time). Aepted Lag as a Funtion of Diretional Movement Table 6 indiates the importane of the diretional movement of the minor-street vehile and the size of the lag that is aepted by the subjet vehile. Beause a through-vehile must only lear the intersetion, the size of the required Jag is smaller than for a turning vehile, whih must also merge into the onfliting-vehile stream. The data presented in Table 6 seem to onfirm this assertion. Delay Two assessments of vehile delay are presented here. Both assessments tend to onfirm the onlusions reahed in the marosopi analysis. Servie Time as Funtion of Major-Street Flow Table 7 indiates that, for most sites, servie time and total delay inrease as major-street flow rates inrease. This relationship was also observed in the marosopi analysis. Total Delay as a Funtion of Major-Street Flow Table 8 indiates that, for most sites, total delay inreases as major-street flow rates inrease. DEVELOPMENT OF PRELIMINARY MODELS FOR CAPACITY AND DELAY On the basis of the analysis presented earlier, a set of equations has been developed that indiates some important fa- TABLE 5 ACCEPTED GAPS AND SERVICE TIMES Aepted Gap, Seonds Sample Sites Servie Time, Servie Time, Servie Time, -S se 5-1 se > 1 se 3rd/Hayes JO.I (Sites 2, 8, 12) 29th/Freya (Sites 9, 11, 14) TABLE 6 ACCEPTED LAGS AND DIRECTIONAL MOVEMENT Sample Sites Aepted Lag, Seonds Left Tums Throughs Right Turns All I I I I.I II I I.I 1.

7 166 TRANSPORTATION RESEARCH RECORD 132 TABLE 7 SERVICE TIMES AND MAJOR-STREET FLOW RATE Sample Sites Servie Time, Seonds Major St Flow Major St Flow Major St Flow < 3vph 3<i vph > 6 vph S S TABLE 8 TOTAL DELAYS AND MAJOR-STREET FLOW RATE Total Delay, Seonds Sample Sites Major St Flow (}.3 vph I S IS 7.1 Major St Flow 3{i()() vph Major St Flow > 6vph S I S tors that affet minor-street apaity, total minor-street delay, queue time, and servie time. Minor-Street Capaity A orrelation analysis was undertaken to identify the fators that affet minor-street apaity. The strongest linear orrelations are between minor-street apaity and the volume on the onfliting approahes (R 2 =. 91 ), the ritial gap (R 2 =.42), the mean gap (R 2 =.88), and the volume on the subjet approah (R 2 =.31). Equation 3 shows the best model resulting from a regression analysis with minor-street apaity as the dependent variable. C = q (3) The model of Equation 3 has been ompared with apaity models estimated in two other ountries. Equation 4 was developed from simulation data in Poland by Marion Traz (1). Equation 5 was developed by the Transportation Road Researh Laboratory in the United Kingdom (11). There is a strong similarity between the three equations in both interept and slope, indiating that the apaity model developed with data from this urrent study ompares favorably with models developed in Poland and the United Kingdom. C = q (4) C = q (5) Total Minor-Street Delay A orrelation analysis was undertaken to identify the fators that affet total minor-street delay. The strongest linear or- relations are between total minor-street delay and the volume on the subjet approah (R 2 =.6), the volume on the opposing approah (R 2 =.83), and the minor-street approah volume-to-apaity ratio (R 2 =.68). Equation 6 indiates the best model resulting from a regression analysis with total minor-street delay as the dependent variable and the volumes on the subjet and opposing approahes as the independent variables. d, = q, +.55q (6) Minor-Street Queue Time A orrelation analysis was undertaken to identify the fators that affet minor-street queue time. The strongest linear orrelations are between minor-street queue time and the volume on the subjet approah (R 2 =.7), the volume on the onfliting approahes (R 2 =.3), the volume on the opposing approah (R 2 =.74), and the volume-to-apaity ratio (R 2 =.7). Equation 7 indiates the best model resulting from a regression analysis with minor-street queue time as the dependent variable and the volumes on the subjet and opposing approahes as the independent variables. dq =.428q +.245q, (7) Minor-Street Servie Time A orrelation analysis was undertaken to identify the fators that affet minor-street servie time. The strongest linear orrelations are between minor-street servie time and volume on the onfliting approahes (R 2 =.94), and the volume on the subjet approah (R 2 =.43).

8 Kyte et al. Equation 8 indiates the best model resulting from a regression analysis with minor-street servie time as the dependent variable and volume on the onfliting approahes as the independent variable. ds =.48q (8) (subjet approah) depends on traffi volumes or the volumeto-apaity ratio on the subjet and opposing approah. Qu ue time depends on traffi volumes or volume/apaity ratio on the subjet and opposing approahe. Servie time depends on the traffi volume on the onfliting (major-street) approahe. 167 SUMMARY AND CONCLUSIONS The first objetive was to develop a data base for traffi operations at TWSC intersetions. To meet this objetive, nine TWSC sample sites were studied overing 13 hr of traffi operations. A total of 97 minor-street subjet vehiles and nearly 2, gaps that were aepted or rejeted were studied. The seond objetive was to identify some of the fators that affet delay and apaity. Several onlusions an now be stated on the basis of the marosopi and mirosopi analyses. When the average delay and flow rate data for eah of the nine sites are ompared several general delay and apaity relationships an be identified. 1. The average time pent waiting in queue (queue time) inreases as the subjet approah flow rate inre;1 es. 2. The average time spent wailing at the stop line (servie time) inreases as the flow rates on the onfliting approahe inrease. 3. The minor-street apaity dereases as the major-street flow rate inreases. A mirosopi analysis of the behavior of individual minorstreet vehiles, effetively operating as a one-way minor street rossing a two-way major street, onfirms some of these same onlusions and suggests several additional ones: 1. The mean aepted gap tends to be lower for drivers who have waited in a queue than for drivers who have not waited in a queue. 2. The mean aepted gap tends to derease as the majorstreet flow rate inreases. 3. The mean aepted gap tends to derease as the queue time or servie time inreases. 4. The mean aepted gap is lower for through vehiles than for turning vehiles. S. The time in queue is not orrelated to the major-street flow rates. 6. Servie time and total stopped delay inrease as the major-street flow rates inrease. The third objetive was to develop a set of equations to foreast delay and apaity. The equations for foreasting minorstreet apaity depend on traffi volumes on the onfliting and opposing approahes. Total delay on the minor street ACKNOWLEDGMENTS Finanial support for this study wa provided by the Univerity of Idaho, the Idaho Tran portation Department and TRANSNOW. The a sistane of Robert Smith, re earh engineer for the Idaho Transportation Department, and Dan M omb, Elmer Kas n, and Sott Guernsey of the Idaho Transportation Department Traffi Setjon was greatly appreialed. Aknowledgment is also gratefully given to Wayne Kittelon, for his new and hallenging ideas in trying to solve a diffiult problem; to Martha Ford, for her substantial ontributions in the development of this paper; and to Joseph Shaher, for his assistane in the data olletion and redution. REFERENCES 1. W. Brilon (ed.). /111f!rsetions Without Traffi Signals. Springer Verlag, Berlin, Speial Report 29: Highway Capaity Manual. TRB, National Researh Counil, Washington, D.C Tr1111sporr111io11 Researh Cirular 319: Researh Problem State me111s. TRB, National Researh Counil, Washington, D.C., June ' W. Brilon. Reent Developments in alulation Method for Unsignalizd Intersetion in We t Germany. 1n lmerse1io11s Without Traffi Signals, Springer-Verlag, Berlin, J. Anveden. wedish Researh on Unsignaliz.ed lnter. tion. In Lmerseti<ms Without Traffi Signal, Springer-Verlag, Berlin, J. D. Zeger. Status of Un ignalized Intersetion apaity Reearh in the United tat. Jn /111ersetio11s Wit!rout Traffi Sig- 11<1/s, pringr-verlag, Berlin, W. Brilon and M. Grossman. Reommendation for a New Computational Proedure in Uns.ignnlizd lnter etion Design. Lhr- 1uhl fur Verkehrswesn, Ruhr Univer ital Bohum, July R. M. Kimber and R. D. Coombe. Tire Traffi Capaity of Major/Mi11or Priority J1mtio11. upplememary Report 582, Transport and Road Researh Laboratory, Crowthorne, England, M. Kyte and A. Boesen. Traffi Data Input Program, Program Doumentation and User' Manual, Ver-ion 3., Department of Civil Engineering, University of Idaho, Mosow, M. Traz. Researh on Traffi Performane of Major/Minor Priority Inter etions. l111erse1io11s Wi1ho111 Traffi Signals, pringer Verlag, Berlin, R. M. Kimber. The Design of Un ignalized Intersetions in the UK. /11terse1ions Without Traffi Signals, Springer-Verlag, Berlin, Publiation of this paper sponsored by Committee on Highway Capaity and Quality of Servie.