BENEFIT MEASUREMENT OF METERING SIGNALS AT ROUNDABOUTS WITH UNBALANCED-FLOW PATTERNS IN SPAIN

1 1 1 1 1 0 1 Martín-Gasulla, Maria Dolores; Garcia, Alfredo; Moreno, Ana Tsui. (01). Benefit Measurement of Metering Signals at Roundabouts with Unbalanced-Flow Patterns in Spain In: Proceedings of th Annual Meeting of the Transportation Research Board (TRB), Washington, D.C., January 01. ABSTRACT Unbalanced flow patterns may be a problem even at medium demand levels on roundabouts. One single approach may cause queues on the downstream approaches; so, the average delay is highly increased and the roundabout can be collapsed. This operational problem can be smoothen through traffic regulation by metering signals, being one of the most cost-effective measures used in Australia, the U.K. and the U.S. Even though some of the Spanish roundabouts exceed capacity, it has not been studied the potential benefit on our local conditions. The aim of the research is to analyze capacity improvements and operational performance on roundabouts with metering signals using traffic microsimulation. It included a field study to characterize gap acceptance behavior on conditions close to capacity. Then, VISSIM was calibrated and validated and a signal control logic was implemented on its VisVAP module. The capacity from traffic microsimulation represents more accurately the traffic demands observed in the field because of the variable follow-up headways. Contrarily, the U.S Highway Capacity Manual underestimates capacity for almost all the observed conditions. Almost 00 combinations of design and control metering signals parameters were required to obtain the optimal model (location and timings) which presented the lowest average delay. Then, traffic demand scenarios were varied using the optimal metering system. The percentage of benefit, in terms of overall average delay, was calculated and could be up to 0 % depending on the combination of the controlling traffic demand and the conflicting traffic flow. The results allow to determine the need for metering and quantify the potential benefit from its application to single-lane roundabouts.

1 1 1 1 1 0 1 0 1 0 1 0 1 Martín-Gasulla, Maria Dolores; Garcia, Alfredo; Moreno, Ana Tsui. (01). Benefit Measurement of Metering Signals at Roundabouts with Unbalanced-Flow Patterns in Spain In: Proceedings of th Annual Meeting of the Transportation Research Board (TRB), Washington, D.C., January 01. INTRODUCTION Roundabouts show great advantages due to their geometry and their priority system, both in terms of capacity and road safety, compared to other intersections. However, complex interactions among geometry, driver behavior, traffic and control factors determine the roundabout capacity and level of service. More than 0,000 roundabouts have been implemented in Spain the last 0 years, with a wide variety of locations (rural, suburban and urban) and sizes (diameters from to more than 0 m) [1]. To calculate capacity of each approach at single-lane roundabouts, the Spanish empirical method considers their geometry and the conflicting flow []. However, the influence of the exiting flow may also affect a driver s decision to enter the roundabout. The U.S Highway Capacity Manual (HCM 0) [] is also used in Spain to evaluate capacity and level of service of roundabouts. The HCM 0 analysis procedure utilizes the critical headway and the followup headway to obtain capacity. Default values based on number of lanes on the roundabout are provided for locations where is not possible to collect data. For single-lane roundabouts, default critical and follow up headways are equal to.1 and. s, respectively. They are considered quite high compared to the critical headways between. and. s from Spain [1]. Moreover, the values are higher than the observed in Maryland with critical headways of. s and follow-up headways of 1. and.1 s []. It has been observed that some roundabouts reach capacity without high overall demand. Unbalanced flows patterns may not be a problem when the overall demand level is low. However, if there is a heavy movement of circulating drivers, one single approach may cause very long queues as the entering drivers at the next downstream entry may not be able to enter the roundabout. Among the possible solutions, such as constructing an interchange, modifying the intersection type, varying the roundabout geometry or installing metering signals, the latest is the most cost-effective measure []. Metering signals are based on creating gaps in the circulating stream in order to alleviate excessive delays on the main approach of the roundabout [-]. Consequently, they are used only on heavy demand conditions during peak hours. In fact, they should not be planned for metering unless unexpected demand dictates this need after construction []. On the metered approach, which causes problems to a main downstream approach (or controlling approach), metering signal is implemented. Red signal is controlled by a queue detector on the controlling approach, which benefits from the generated gaps; and the roundabout overall traffic operation is improved. Maximum red time is introduced to avoid excessive queue on the metered approach. The roundabout reverts to normal operation during a minimum blank/amber time or as long as the controlling approach does not present queuing conditions. There are not enough guidance available for designers in order to successfully implement a metering signal system on a roundabout. Most of the application of roundabout metering signals has been in Australia and some in the U.K. and the U.S [, ]. There are not metered roundabouts in Spain, even though some of them present unbalanced flows pattern, reaching up to 1,00 veh/h on the main approach. Akcelik [] presented the basic principles of operation of metered roundabouts. He studied one-lane, two-lane and three-lane roundabouts in Australia, UK and the USA [-]. He firstly developed the aasidra gap-acceptance model using field data from a large number of roundabouts in Australia; and it was then used to determine the queue length, signal timing and other operational parameters on metered roundabouts. In 0, he used SIDRA INTERSECTION for estimating capacity depending on signal timing. The results indicate that low cycle times produce better performance than longer cycle times []. Natalizio [] used a traffic microsimulation in aasidra to analyze traffic flow conditions at urban single-lane roundabouts without the influence of metering signals and tested the theoretical analysis model developed by Akcelik [] using a range of data to verify its operation and robustness. Finally, operational benefits were reported for certain flow conditions on the

1 1 1 1 1 0 1 0 1 0 1 0 Martín-Gasulla, Maria Dolores; Garcia, Alfredo; Moreno, Ana Tsui. (01). Benefit Measurement of Metering Signals at Roundabouts with Unbalanced-Flow Patterns in Spain In: Proceedings of th Annual Meeting of the Transportation Research Board (TRB), Washington, D.C., January 01. controlling approach. The observed traffic flows for the controlling approach were between 00 and 00 veh/h. The benefits of metering signals start when the combined volumes of the controlling and conflicting flows are 00-00 veh/h; and they decline once they are higher than 10-10 veh/h. However, the benefit in terms of other operational parameters besides capacity was not quantified. Natalizio also provided guidance on optimal metering signal timing based on his results. Hummer et al. [] developed a simple macroscopic model based on the HCM equations and validated it using VISSIM. The validation was set to replicate the roundabout capacity relationship of the HCM, and as a result the car-following parameter and speeds in reduced area were modified. Default values of critical and follow-up headways were considered. Analytical delays were computed for one cycle length and metering signal location, proving the benefits of metering. Criteria for signalized intersections were used (delay over 0 seconds) to indicate when metering is helpful and LOS was better than F. Both single-lane and two-lane roundabouts were analyzed. Research motivation The differences on the driver behavior (more used to roundabouts and/or aggressive) and entry traffic flow (higher demands), may be enough to indicate that the results are not completely suitable for application to Spanish conditions. Therefore, there is a need to document gap acceptance behavior on Spanish roundabouts with unbalanced traffic flow patterns. Then, driver behavior will be used to calibrate and validate a microsimulation program. The benefit of metering systems will be verified with traffic microsimulation. Moreover, it is intended to quantify the benefit of the metering system and serve as recommendation criteria to implement metering systems in our conditions. OBJECTIVES The aim of the research is to analyze capacity improvements and operational performance on suburban roundabouts with metering signals using traffic microsimulation for Spanish local conditions. To accomplish this objective, the following tasks were carried out: - To study the gap acceptance process in one Spanish single-lane roundabout. - To calibrate and validate VISSIM for the observed scenarios. - To generate multiple demand scenarios and obtain capacity from the simulations. - To implement the metering signal using VisVAP module, generate multiple scenarios varying the main variables of the metering system and define the optimal design. - To analyze the sensibility of the optimal design to demand variations. - To discuss the results with previous research. METHODOLOGY Field Study For the field study, two single-lane roundabouts on Valencia (Spain) were selected: the intersection of CV-00, CV-01 and CV-0 (El Saler) and intersection of CV- and CV-0 (Alzira). El Saler roundabout (Figure 1a) has four approaches and the main demand comes from CV-00. The outer diameter is equal to 0 m. Depending on the peak period (morning or afternoon; weekday or weekend), the most important demand is Northbound or Southbound. The other two approaches have medium traffic flow, and the Rambla is a local connection to the beach without much traffic. Alzira roundabout (Figure 1b) is a three-approach roundabout; the main flow comes from the East (CV-0) approach and is directed, mainly, to the South (Alzira access). They have to yield to a low demand from the Alzira access to CV-, due to a bypass lane that

Martín-Gasulla, Maria Dolores; Garcia, Alfredo; Moreno, Ana Tsui. (01). Benefit Measurement of Metering Signals at Roundabouts with Unbalanced-Flow Patterns in Spain In: Proceedings of th Annual Meeting of the Transportation Research Board (TRB), Washington, D.C., January 01. discharges right turn maneuvers. Both are suburban roundabouts with no pedestrian impedance. The roundabout circle was wide ( m) but only one single-lane was utilized by drivers. (a) Figure 1. Selected roundabouts. (a) El Saler; (b) Alzira (b) 1 1 1 1 1 0 1 0 On both locations, manual and video recording traffic counts every minutes were carried out to describe traffic flow patterns and verify the capacity variability. The traffic counts were developed at peak hour conditions, hours (.0-.0 a.m.) in a labor day and hours at weekend (.00 a.m. to 1.00 p.m.) for El Saler roundabout, and for hours (.00-1.00 p.m.) in a labor day at Alzira roundabout. Peak hours in El Saler exceeded capacity, with demands of 00 veh/h to 100vh/h. No pedestrians were observed during the recording time. The percentage of trucks was very low, around 0. %. To study the gap acceptance process, accepted gaps, rejected gaps and follow-up headways were measured at El Saler roundabout. These data were obtained through video recordings from one domestic camera and the surveillance camera from the Traffic Management Center, which was located on a pole at the roundabout. Some criteria were established before measuring headways and lags (Figure ). There was a section in the roundabout circle that accomplish the following: when rear-end of the circulating vehicle was on this section, most of the vehicles start and initiate the entering maneuver (red line in Figure ). Accepted and rejected headways were directly obtained. Lags were also obtained. They are defined as the time interval between the arrival of a vehicle on the approach to the stop line and the arrival of the next vehicle on the conflicting flow. Given that there is no stop line at roundabouts, the reference line was considered where most of the drivers slow down near the yield line (green line in Figure ). Accepted or rejected lags are measured from this position until the conflicting vehicle arrives to the previous section on the roundabout circle. A total of, headways were measured,,0 accepted headways (, accepted lags) and rejected headways (1 rejected lags). The recording time was 0 min. Given that available sight distance was unlimited, some gaps are not relevant to the gap acceptance process because they are too long [,, 1]. The gaps determined by the geometrical characteristics of the roundabout and the threshold was.1 s.

1 1 1 1 1 0 1 0 1 0 1 Martín-Gasulla, Maria Dolores; Garcia, Alfredo; Moreno, Ana Tsui. (01). Benefit Measurement of Metering Signals at Roundabouts with Unbalanced-Flow Patterns in Spain In: Proceedings of th Annual Meeting of the Transportation Research Board (TRB), Washington, D.C., January 01. Connectors and links were introduced, as well as reduced speed areas on the approaches to the roundabout [1, 1]. Priority rules were also introduced to reflect driver behavior on the approaches and roundabout circle. Stop lines and conflict markers associated to them were installed depending on the observed conditions. On this stage, critical headway was also introduced on VISSIM based on the field study results. Once the geometric design and priority rules were set, the program was tested to verify that drivers circulate following the rules and were able to detect all the vehicles around them. Afterwards, traffic demand from was selected from the observations. Given that the base scenario should be able to capture driving behavior at conditions close to or over capacity, it was selected the peak hour in labor days (:0-:0, Northbound). On those conditions, the congested main approach yielded to a medium demand. The entry volume at yield line and the routing decisions corresponded to the O/D matrices from the traffic counts. The goal of calibration is to minimize the differences between the observed behavior on the field and the simulation results. The queue length was used to verify the goodness of fit. The calibration parameters in VISSIM included: Stop lines and conflict markers position. Reduced speed area length. Dynamic queue condition. Number of observed vehicles, from the Wiedemann car-following model. Look ahead distance, from the Wiedemann car-following model. The twelve -minute scenarios from the heaviest traffic demand were used for the calibration. Each scenario was evaluated using 1 different random number seeds and the models were simulated for one hour plus previous minutes of warm-up period. The model was considered calibrated if the queue length in each approach did not differ more than 1%. Given that the differences on the queues were lower than 1 %, the model was calibrated. For the validation, the peak hour from the weekends (Southbound) was introduced. The differences on the queue lengths were lower than 1 %, so the model was validated. The same process was developed for the Alzira roundabout. With these adjustments, it was concluded that VISSIM simulated the reality for an unmetered roundabout. Further details can be found in Martin-Gasulla [1]. Unmetered Scenario In order to determine the benefits of installing the metering system, capacity on the unmetered (base) scenario should also be obtained. This capacity was defined as the maximum traffic flow that can reasonably enter the roundabout from an access with a maximum average delay of 0 s. The 0 HCM threshold for roundabouts were used []. Several combinations of entering and conflict flow rates were used. Every 0 veh/h of conflicting flow, the demand on the approach is increased until it reaches the average delay corresponding to the LOS F. Conflicting flow varied between 0 and 100 veh/h. 1 random seeds were used to account for the randomness of driver behavior. The total number of simulated scenarios was,0. Metered Scenario For the metered scenario, it was decided to implement an indirect, part-time and part-control metering system. Indirect metering because the control is set some distance from the yield line (circulating traffic is not controlled and therefore has priority); part-time only switched on by

Martín-Gasulla, Maria Dolores; Garcia, Alfredo; Moreno, Ana Tsui. (01). Benefit Measurement of Metering Signals at Roundabouts with Unbalanced-Flow Patterns in Spain In: Proceedings of th Annual Meeting of the Transportation Research Board (TRB), Washington, D.C., January 01. Table 1. Parameters to define the semi-actuated metering system design Traffic light at the metered approach Parameter Used values Reference values [] Distance to yield line (m) 0 Minimum blank time setting (s) - MIN BLANK 0; 0; 0; 0 0 0 Minimum red time setting (s) - MIN RED ; 1; 0 0 Maximum red extension time settings (s) 0; ; 0; 0 0 0 Maximum red time setting (s) - MAX RED 0-0 0 0 1 1 1 1 1 0 1 0 1 Queue detector at the controlling approach Parameter Used values Reference values [] Distance to yield line (m) 0; ; 0; 0 0 0 Loop length (m).. Queue gap setting (s) H..0. Queue occupancy setting (s) - O DETECTOR ; Among the timings indicated on the literature, minimum red and blank/amber times should be set high enough in order to have a credible system for the drivers. Therefore, very short timings, such as proposed by Hummer et al. [], are discarded. Certainly, metering signals should be placed at a specific distance from the yield line in order to ensure the right-of-way there, as well as the signal should never display green in that locations where more aggressive behavior has proved among drivers. On the other hand, maximum red time must be set to avoid excessive queue and delays at the metered approach. Different combinations of the variables were set in the VISSIM program and the VisVAP module. Almost 00 combinations of design and control metering signals parameters were required to obtain the optimal model for the heaviest demand scenario. The traffic demand scenario was the same as for calibration: the peak hour in labor days. The outputs from VISSIM were: average overall delay, average delay at the controlling approach, average queue length at the controlling approach, and average delay at the metering approach. The optimal design improved most the overall delay and the performance parameters for the controlling approach (average delay and average queue length). Moreover, it should not worsen much the delay on the metered approach. Finally, a sensitivity analysis was carried out to quantify the benefit of metering the roundabout. This analysis used the optimal metering design and varied the traffic demand. Overall delays were compared between the base scenario (unmetered) and the metered scenario. The conflicting flow varied between 0 and 1,00 veh/h in steps of 0 veh/h; and the entry flow between 0 and 1,00 veh/h. To create the different traffic flow scenarios, variations on traffic volume were based mainly on the metered approach and slightly modifications in the rest of access were carried out to avoid the quickly collapse of the metered approach. Turning percentages for each approach remained constant in all scenarios. 1 random seeds were used during the whole process to account for variability. Nearly 0,000 simulations were carried out in VISSIM to quantify the benefit of metering. ANALYSIS OF THE RESULTS Unmetered Scenario The capacity from VISSIM was compared to the field data and the capacity values from the HCM, for both default values (critical headway of.1 s and follow-up headway of. s) and calibrated values from the field data (critical headway of. s and follow-up headway of. s). The obtained capacity from a calibrated microsimulation program represents more accurately the demand observed in field data than any other analytical or empirical method (Figure ). The capacity from VISSIM can reflect more behavioral variables than the analytical

1 1 1 1 1 0 Martín-Gasulla, Maria Dolores; Garcia, Alfredo; Moreno, Ana Tsui. (01). Benefit Measurement of Metering Signals at Roundabouts with Unbalanced-Flow Patterns in Spain In: Proceedings of th Annual Meeting of the Transportation Research Board (TRB), Washington, D.C., January 01. method from the HCM. For example, the uncertainty of whether a vehicle is exiting the same approach (in case the traffic island is not wide enough). Both HCM curves show the same capacity when there is no conflicting traffic flow because of similar follow-up headways. Then, the results differs due to the critical headway value of the HCM 0 is higher than the one in Spanish conditions. Moreover, none of the HCM capacity curves represents the majority of volume combinations observed in field. The reason why the VISSIM capacity curve is over more conflicting flow combinations is that the follow-up headway is not a value that can be implemented in microsimulation programs. The average value calculated in this study (. s) was obtained in congestion. The follow-up headway can be deducted from the VISSIM capacity curve based on the HCM formula when there is no conflicting flow, resulting a value of 1. s. Such a low value indicates that this variable in free flow (low conflicting flow) is relatively non-existent because the entering drivers with unlimited sight distance directly enter the roundabout with imperceptible slowing-down maneuver. The above result shows that the follow-up headway should not be considered as a constant value, it is a distribution that increases from low conflicting traffic flow to values like the one calculated at this study, in congestion. That s why the best scenario is the one calibrated by a microsimulation program that allows the user to implement critical headway as a distribution which is closer to the reality. 1 0 1 Figure. Capacity of single-lane roundabouts approach depending on the conflicting traffic flow Metered Scenario Optimal metering system Among the 00 combinations of design and control metering signals parameters, the optimal model was selected. The optimal design improved most the overall delay and the performance parameters for the controlling approach (average delay and average queue length). Moreover, it should not worsen much the delay on the metered approach. The optimal scenario is determined in Table.

1 1 1 1 1 0 1 0 1 Martín-Gasulla, Maria Dolores; Garcia, Alfredo; Moreno, Ana Tsui. (01). Benefit Measurement of Metering Signals at Roundabouts with Unbalanced-Flow Patterns in Spain In: Proceedings of th Annual Meeting of the Transportation Research Board (TRB), Washington, D.C., January 01. Table. Parameters to define the semi-actuated metering system design Traffic light at the metered approach Parameter Optimum value Distance to yield line (m) 0 Minimum blank time setting (s) - MIN BLANK 0 Minimum red time setting (s) - MIN RED 1 Maximum red extension time settings (s) 0 Maximum red time setting (s) - MAX RED Queue detector at the controlling approach Parameter Optimum value Distance to yield line (m) 0 Loop length (m). Queue gap setting (s) H. Queue occupancy setting (s) - O DETECTOR ; Under the optimal model, capacity was improved by % for the worst observed scenario on the El Saler roundabout and a % on the Alzira s one; representing an operational improvement changing from a level-of-service (LOS) E and F, respectively, to a LOS C. The optimal model reveals that shorter distances to the yield line for the queue detector benefits the controlling approach. Similar results, but with lower influence, were observed by to Natalizio []. Smaller minimum amber or blank times had better impact on the controlling approach because the queuing conditions were quickly accomplished. A minimum red time was needed to discharge the queue and depending on the controlling approach demand, red time could be prolonged until the queuing condition disappeared or until the maximum red time was reached. Similar conclusions were obtained by Akcelik [] and Natalizio [] for Australian conditions. The blank time from Hummer et al. [] was much lower, between and s, probably because constant metering signal timing was considered instead of activated green times. For those conditions, important amounts of extra delay may not be accounted and therefore the actuated green times are higher. Quantification of the benefits In order to quantify the benefit at different demand conditions, a sensibility analysis of traffic flows was carried out. Average delay of the whole roundabout was compared between the unmetered scenario (base scenario) and the metered scenario with the optimal metering design implemented. The percentage of benefit, in terms of overall average delay, is represented in Figure. To better reflect the sensitivity of the results, entry demand was represented on the x-axis instead of the y-axis. A higher demand would be able to enter the roundabout with less average delay values by installing metering signals for the same medium-high circulating flow in front of the controlling approach. Also, it is proved that metering signals could improve operational parameters at roundabout beyond peak periods. The entering demands are considerably higher than the corresponding to the base scenario, which proves the efficiency of the system. In fact, the benefit could be up to 0 %. Moreover, some of the volume combinations with high benefit represent most of the conflicting flows present at Spanish single-lane roundabouts. However, in case of higher volumes in front of the controlling approach, the metering system would not offer too much benefit and another systems should be considered to obtain better results. Two behaviors could be observed depending on the controlling traffic demand. For controlling traffic demands higher than 00 veh/h, the benefits of metering signals disappear when the combined traffic demand from the controlling and conflicting flows is higher than 1,0 veh/h.

1 1 1 1 1 0 1 0 1 0 1 0 Martín-Gasulla, Maria Dolores; Garcia, Alfredo; Moreno, Ana Tsui. (01). Benefit Measurement of Metering Signals at Roundabouts with Unbalanced-Flow Patterns in Spain In: Proceedings of th Annual Meeting of the Transportation Research Board (TRB), Washington, D.C., January 01. Based on the results of the study, the following conclusions were made: The critical headway in Spain was equal to. s; which is lower than the values reported from the Maryland and the HCM 0. Greater experience on driving through roundabouts, conditions close or over capacity and likely more aggressive behavior can cause the differences. The capacity from VISSIM represented more accurately the traffic demands observed in the field. The HCM capacity model was lower than the actual observed traffic demands because the follow-up headway was constant, and therefore for low conflicting traffic flows is not representative. Variable follow-up headways reproduce better driving behavior, such as observed on the VISSIM program. Using the optimal metering system design on the observed roundabouts in Spain, capacity would be improved by %, varying the level-of-service from E to C. It has been verified the utility of metering systems on our selected roundabouts. Shorter distances to the yield line for the queue detector (0 m) benefits the controlling approach. Smaller minimum amber or blank times (0 s) had better impact on the controlling approach because the queuing conditions were quickly accomplished. A minimum red time (1 s) was needed to discharge the queue and depending on the controlling approach demand, red time could be prolonged until the queuing condition disappeared or until the maximum red time was reached ( s). The percentage of benefit, in terms of overall average delay, could be up to 0 % depending on the combination of the controlling traffic demand and the conflicting traffic flow. Contour lines of the benefit were generated for the conflicting traffic combinations. For controlling traffic demands higher than 00 veh/h, the benefits of metering signals disappear when the combined traffic demand from the controlling and conflicting flows is higher than 1,0 veh/h. With this study, an analyst would be able to determine under what traffic conditions a specific roundabout may require a metering system and the expected benefit enabled by the optimal design determined in this research. Then, the analyst could easily determine the potential metered approach. Therefore, the system is ready to be used by practitioners and analysts. The conclusions of the study are limited to the observed and simulated conditions: singlelane roundabouts; unbalanced flows; and, similar behavior in terms of gap acceptance behavior (aggressive and medium-high experience on circulating in roundabouts). It would be interesting to compare the results with other driver behavior and/or with signalized roundabouts. Moreover, it could be calibrated an analytical model considering the distribution of follow-up headways rather than considering a constant value obtained from congested conditions underestimates the capacity for low conflicting flows. ACKNOWLEDGMENTS The authors would like to thank Spanish General Directorate of Traffic (Dirección General de Tráfico del Ministerio del Interior), Valencian Regional Department of Transport (Conselleria de Vivienda, Obras Públicas y Vertebración del Territorio de la Generalitat Valenciana) and the Valencia City Traffic Management Center (Centro de Gestión de Tráfico del Ayuntamiento de Valencia) for their collaboration during the field study. REFERENCES 1 Romana, M. Roundabout practice and research in Spain. Presented at Roundabout workshop during the th International Symposium on Highway Capacity and Quality of Service. Stockholm, Sweden, 0.

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