Observing Freeway Ramp Merging Phenomena in Congested Traffic

Transcription

1 Journal of Advanced Transportation, Vol. 41, No. 2, pp Observing Freeway Ramp Merging Phenomena in Congested Traffic Majid Sarvi Masao Kuwahara Avishai Ceder This work conducts a comprehensive investigation of traffic behavior and characteristics during freeway ramp merging under congested traffic conditions. On the Tokyo Metropolitan Expressway, traffic congestion frequently occurs at merging bottleneck sections, especially during heavy traffic demand. The Tokyo Metropolitan Expressway public corporation, generally applies different empirical strategies to increase the flow rate and decrease the accident rate at the merging sections. However, these strategies do not rely either on any behavioral characteristics of the merging traffic or on the geometric design of the merging segments. There have been only a few research publications concerned with traffic behavior and characteristics in these situations. Therefore, a three-year study is undertaken to investigate traffic behavior and characteristics during the merging process under congested situations. Extensive traffic data capturing a wide range of traffic and geometric information were collected using detectors, videotaping, and surveys at eight interchanges in Tokyo Metropolitan Expressway. Maximum discharged flow rate from the head of the queue at merging sections in conjunction with traffic and geometric characteristics were analyzed. In addition, lane changing maneuver with respect to the freeway and ramp traffic behaviors were examined. It is believed that this study provides a thorough understanding of the freeway ramp merging dynamics. In addition, it forms a comprehensive database for the development and Majid Sarvi, Institute of Transport Studies, Department of Civil Engineering, Monash University, Melbourne, Australia Masao Kuwahara, Kuwahara Lab., Institute of Industrial Science, University of Tokyo, Meguro-ku, Tokyo Avishai Ceder, Transportation Research Institute, Civil Engineering Department, Technion-Israel Institute of Technology, Israel Received: January 2006 Accepted: August 2006

2 146 M. Sarvi, M. Kuwahara and A. Ceder implementation of congestion management techniques at merging sections utilizing Intelligent Transportation System. 1. Introduction The freeway ramp merging process has been studied since the 1930s. Research on several elements of merging behavior during freeway merging maneuver has been carried out but has been mostly on gap acceptance behavior and ramp vehicle acceleration characteristics while focusing on free flow traffic situations. One of the earliest study of acceleration/deceleration characteristic was conducted in late 1930s (Beaky 1938, Loutzenheiser 1938) and was employed by AASHO (American Association of State Highway Officials) until early 1980s. Some modification was made to the survey method in 1984 (Olson et. al. 1984) which was then used by AASHTO (American Association of State Highway and Transportation Officials) in Some operational effects of entrance ramp geometrics (i.e. acceleration lane length, angle of convergence, and the ramp grade) on free flow merging were also studied in 1970s (Wattleworth et al. 1967). Drew et al. (1967) proposed a gap acceptance model to determine the capacity of merging sections utilizing a relationship between critical gap and entrance ramp geometrics. There are also few studies investigated the driver turn taking behavior, bottleneck behavior analysis, and traffic oscillation due to lane changing at freeway ramp merging (Mauch and Cassidy 2002, Bertini 2005, Cassidy and Ahn 2005). However, the extent of research into the dynamic aspects of traffic characteristics as well as drivers behavior such as lane changing maneuver upstream and within a merging section under congested traffic situations is rather small compared to the effort directed at analyzing freeway ramp merging phenomena under free flow traffic conditions. This is due, at least in part, to the limited availability of measured traffic data during congested traffic situations. In the research reported here that issue is overcome through use of extensive macroscopic and microscopic data obtained from data collection and observation at eight merging sections. The objective of the research reported in this paper is to empirically identify and examine traffic behavior and characteristics such as congestion forming, macroscopic lane changing behavior, and drivers interactions under congested traffic situations when the demand exceeds capacity. The paper is structured as follows. After the introduction, first

3 Observing Freeway Ramp 147 the data collection is presented. The next section explores the decision process of drivers and interactions between drivers. That is followed by an analysis of the congestion forming and developing pattern of merging sections. The macroscopic analysis of various lane changing occur in the vicinity of merging sections are then explored. Next, a comparison between capacity of merging sections and simple (i.e., non-merging) sections together with analysis of merging ratio is presented. The conclusions are summarized in the final section. 2. Data Observation of vehicles operational characteristics in freeway entrance ramps and detector data collection were conducted at several locations in Tokyo Metropolitan Expressway. In order to establish driver behavior concepts such as zone definition, drivers interaction, and driver decision process videotaping and surveys were used. Traffic flow was recorded using several video cameras mounted on the top of the buildings in the vicinity of the merging sections. The tapes were reviewed to identify different type of lane changing and steering maneuvers. The results of this part are presented in section 3. In this study both parallel and taper type acceleration lanes were included (see Figures 1). To estimate the capacity of merging sections and analyze its relation to the geometric and traffic characteristics detector data were utilized and results are presented in section 4. For this purpose, detectors collected traffic data including freeway and ramp flow rates, speeds, occupancy, and type of vehicles. Traffic count of each detector was verified by cross-examination of upstream and downstream detectors. Each detector collected speed, occupancy, flow rate, and type of vehicles in five-minute interval at eight merging sections over three years (1996 to 1998). From the detector data in Horikiri, Hamazaki-bashi, Hakozaki, Ikejiri, Kosuge, Kasai, Ryogoku, and Ichinohashi merging sections, the period is selected which is fully congested; i.e., queues reside on both freeway and merge approaches and there is no exogenous flow restriction from downstream (see Figure 2). Table 1 summarizes the detailed descriptions of studied merging sections in Tokyo Metropolitan Expressway.

4 148 M. Sarvi, M. Kuwahara and A. Ceder Figure 1. Sketch of taper type (left) and parallel type (right) entrance ramp. Figure 2. Study sites in Tokyo Metropolitan network.

5 Observing Freeway Ramp 149 Table 1 Specification of studied merging sections Merging sections Configuration type Length of acceleration lane (m) Hamazaki-bashi Direct taper (left side merging) 125 Kasai Parallel acceleration lane (left side 290 merging) Kosuge Direct taper (right side merging) 115 Ryogoku Direct taper (left side merging) 120 Horikiri Parallel acceleration lane ( left side 120 merging) Hakozaki Direct taper (left side merging) 80 Ikejiri Direct taper (right side merging) 125 Ichinohasi Direct taper (right side merging) Vehicle Interaction and Traffic Behavior 3.1. The decision process of drivers The tasks and decision-making processes required for drivers approaching a freeway merging point differ between free-flow conditions and congested-flow conditions. A comprehensive traffic survey and onsite observation have shown that the decision-making process of drivers in merging situations can be divided into three zones, as shown in Figure 3 (Sarvi 2000). The decisions required in each zone can be expressed as follows. Ramp Zone 1 (preliminary zone): A decision about how to arrive at Zone 2 (from lane one or two), Ramp Zone 2 (merging zone): A decision about which two vehicles to merge between, Ramp Zone 3 (downstream zone): A decision about at what distance and speed to follow the vehicle in front, Freeway Zone 1 (preliminary zone): Same as ramp Zone 1, Freeway Zone 2 (merging zone): A decision as to which vehicle from the ramp should be permitted to merge, Freeway Zone 3 (downstream zone): Same as ramp Zone 3. The first decision that a driver must make is greatly affected by the surrounding traffic situation (e.g., traffic volume in the two lanes, traffic

6 150 M. Sarvi, M. Kuwahara and A. Ceder flow speed, desirable gap) and by the circumstances of the particular driver (e.g., attitude, vehicle type, familiarity with the area). The second decision, which involves the ramp driver searching for and accepting a suitable gap, has been extensively studied for the free-flow merging condition (Drew et al. 1967, Daganzo 1981, Makigami et al. 1988, Chang and Kao 1991, Ahmed et al. 1996, Kita 1999, Kurian 2000). The gap searching and acceptance maneuvers commonly observed under free-flow conditions do not occur under heavy traffic flow conditions, according to a macroscopic study by Sarvi and Kuwahara (1999) and Sarvi (2000). These macroscopic studies found no significant correlation between the acceleration lane length and the maximum flow rate in the merging sections. Heavy traffic conditions also lead to squeeze merging at the end of the merging section. Here, we define this type of merging as zip merging, which refers to the situation where vehicles from the ramp and freeway shoulder lane merge together one by one regardless of the available gap. Observations at the Ichinohashi and Hamazaki-bashi merging sections under congested traffic flow found more than 97% of merging maneuvers to be of the zip merging type. The third driver decision is related to the known car-following behavior, which describes how a pair of vehicles interacts with each other, and will not be dealt with in this paper. The reader can refer to Sarvi et al. 2002, 2004, and Figure 3. Definition of the three zones and two lane changing. Figure 3. Definition of the three zones and two lane changing.

7 Observing Freeway Ramp Drivers interactions Table 2 lists the possible interactions between vehicles approaching and engaging the merging area under congested traffic conditions, established through thorough observations as described in section 3. These interactions include lane-changing in Zone 1 before engaging the merging section, merging at Zone 2, lane-changing within Zone 2, and car-following behavior between vehicles. For example, driver i in freeway lane 1 (row 1) interacts with driver j in ramp lane 1 (column 3) by slowing down and provides a gap that is sufficient for the ramp vehicle to merge. Conversely, driver i in ramp lane 1 (row 3) interacts with driver j in freeway lane 1 (column 1) by forcing a merge in order to merge as early as possible. Research on lane-changing behavior has focused on gap acceptance behavior and its applications. In this study, lane-changing behavior in the merging area was investigated at the macroscopic (not individual vehicle) level. Two types of lane-changing behavior are frequently observed in the merging sections. In Zone 1, aggressive drivers force their vehicles onto the freeway/ramp lane 2 in order to avoid merging interactions (aggressive lane changing). In Zone

8 152 M. Sarvi, M. Kuwahara and A. Ceder 2, some drivers force their vehicles into the freeway lane 2 in order to avoid the delay of a second merging (avoidance lane changing). These lane-changing maneuvers affect the flow rate at the merging section, usually causing a decrease in the flow rate in freeway lane 2 and an increase in the flow rate of the ramp. A strong correlation between vehicle lane changing maneuvers and traffic oscillation were observed near ramp interchanges by Mauch and Cassidy (2002). 4. Macroscopic Aspect of Traffic Behavior and Characteristics 4.1. Congestion forming and development A typical flow and speed pattern of a traffic jam at the immediate upstream section of a merging section is illustrated in Figures 4 and 5. Traffic speed gradually decreases as volume increases until speed suddenly drops when the volume reaches a certain level, which is indicated as A in Figures 4 and 5. This is the initial stage of congestion and therefore, the volume level A can be called capacity before congestion. The traffic volume after this period is the capacity of the bottleneck because there is an excess demand in the upstream section of both freeway and ramp in the form of a queue. After the start of congestion, the volume decreases and fluctuates around level B which could be considered as capacity during congestion. The values of level A and B are around 1500 veh/hr/2-lane and 1200 veh/hr/2-lane for Hamazaki-bashi merging section (freeway leg) and around 1300 veh/hr/lane and 920 veh/hr/lane for Ichinohashi merging section (ram leg). In Figures 4 and 5 the time periods associated with the flow rates at level A and level B are denoted as T t and T c respectively. The capacity reduction process from level A to level B takes some amount of time, varying widely from case to case. In the case shown in Figures 4 and 5, this transition period was approximately an hour between 7.30 a.m. to 8.30 a.m. and one hour and half between 7 a.m. to 8.30 a.m. respectively. The capacity during congestion fluctuates around level B and again may increase toward the end of congestion. A number of similar results have been obtained at the major bottlenecks on Tokyo Metropolitan Expressway, as shown in Table 3 (capacities in Table 3 refer to flow rate at level B). It has been shown that capacities during congestion differ from one bottleneck to another and range between 1679 to 2068

9 Observing Freeway Ramp 153 veh/hr/lane. For estimating the capacity of the merging sections from the detector data, the period of time is selected which is fully congested for at least 15 minutes; i.e., queues reside on both freeway and merge approaches upstream and there is freely flowing traffic downstream. Subsequently, the capacity is simply four times of the flow rate during the fifteen minutes peak period. According to Highway Capacity Manual the fifteen minutes is selected as the minimum congestion period that needs to be maintained in order to obtain stable flow rate (HCM 2000). More explanation and typical speed-flow curves are presented in section 4.4. Table 3. Geometric and maximum flow rate of major bottleneck merging (level B volume) Merging Section Configuration type Capacity (veh/hr/lane) Capacity (pcu/hr/lane) Hamazakibashi Direct Taper Kasai Parallel acceleration lane Kosuge Direct Taper Ryogoku Direct Taper Horikiri Parallel acceleration lane Ichinohashi Direct Taper Hakozaki On Ramp Ikejiri On Ramp

10 154 M. Sarvi, M. Kuwahara and A. Ceder Figure 4. Typical example of Hamazaki-bashi merging section congestion. Figure 5. Typical example of Ichinohashi merging section congestion.

11 Observing Freeway Ramp Transition from free flow to congested flow The time period of T t in Figure 4, during which the flow rate drops from level A to level B, is transition period from free flow to congested flow. The time period T c in Figure 4 is congestion period from the beginning to the end of congestion. The drivers seem to switch their behavior from the free flow manner to congested flow manner during the time period T t. The transition period T t varies widely as illustrated by Figures 4 and 5. Figures 6 demonstrate the plots of the departure flow rates during T t and T c against duration of the average time that the drivers have spent in the queue for the Hamazaki-bashi and Ichinohasi merging sections. The total travel time, T, for a vehicle passing through a merging section at time t, can be computed as follows: T = (Waiting time in queue) (t ) Where: T : travel time at time t (t) V α ( )] C β + t + (1) t 0 : the free flow travel time V : the volume of traffic at time t C : the capacity of the merging section α, β : constant parameters Delay in Eq (1) or the waiting time spent in queue is calculated from the arrival and departure curves of cumulative vehicles counts versus time measured across all freeway/ramp lanes at upstream and downstream detectors as shown in Figure 7 (Newell 1993). Based on Figure 6, there are two groups of plots that are associated to the transition period (T t ) and the steady congestion period (T c ). The transition of the driver behavior from free flow to congested flow is completed around 15 minutes for the Ichinohashi merging section and it is around five minutes for the Hamazaki-bashi regardless of the values of transition period T t, which are one hour for Hamazaki-bashi and one hour and half for Ichinohashi. Based on Figure 6, it could be concluded that drivers start switching their behavior (e.g. car-following) from free flow mode to congested flow mode when they are caught in the queue, and complete the switching when their time in the queue reaches around 15 minutes (comprehensive car-following study is carried out but it lies beyond the scope of present work however, is the subject of other studies 0[1

12 156 M. Sarvi, M. Kuwahara and A. Ceder summarized in Sarvi et. al. 2002, 2005). Figure 6, further illustrates the reduced departure flow rates during congestion as the time in queue increased. Drivers seem to gradually change their pattern of carfollowing becoming less sensitive and cautious as they are caught in the queue for longer periods. During periods of heavy freeway congestion, unstable or stop-and-go traffic flow appears which may be well due to unresponsive driving behavior of sluggish drivers who prefer to adjust their speed and distance to the vehicle in front only from time to time rather than continuously in response to conditions ahead. This phenomenon (unresponsive driving behavior) has also been reported for the congested freeway sections by Daganzo et al. (1999). 4.3 Macroscopic lane changing behavior Extensive detector data available in this study made it possible to have a thorough macroscopic analysis of lane changing phenomena (during the morning and afternoon peak periods) which frequently takes place in the vicinity of merging sections as it is discussed in section 3. Presentation of these finding for a typical day follows below. Figure 8 displays the maximum throughput of merging section in addition to the maximum discharged volumes of the ramp and the freeway approaches. This data indicates that, the freeway traffic volume increased from 2000veh/hr/2-lane in the morning to 2500veh/hr/2-lane in the evening, whereas the ramp traffic volume decreased from 1800veh/hr/2-lane in the morning to 1500veh/hr/2-lane in the evening. It can be seen that the total maximum discharged flow rate of the bottleneck stay constant around 4000 veh/he/2-lane. In a comparative sense, this could be attributed to the two competing traffic streams entering the merging section. The lower traffic volume of the ramp stream during evening may be associated to the hesitation of the ramp drivers to merge due to insufficient natural light together with other reasons associated to the variation of traffic volume by time of day.

13 Observing Freeway Ramp 157 Figure 6. Departure flow rate vs. time in the queue for Ichinohashi (left) and Hamazaki-bashi (right) merging sections. Figure 7. Calculation of delay utilizing cumulative count of vehicles.

14 158 M. Sarvi, M. Kuwahara and A. Ceder Figure 8 displays the maximum throughput of merging section in addition to the maximum discharged volumes of the ramp and the freeway approaches. This data indicates that, the freeway traffic volume increased from 2000veh/hr/2-lane in the morning to 2500veh/hr/2-lane in the evening, whereas the ramp traffic volume decreased from 1800veh/hr/2-lane in the morning to 1500veh/hr/2-lane in the evening. It can be seen that the total maximum discharged flow rate of the bottleneck stay constant around 4000 veh/he/2-lane. In a comparative sense, this could be attributed to the two competing traffic streams entering the merging section. The lower traffic volume of the ramp stream during evening may be associated to the hesitation of the ramp drivers to merge due to insufficient natural light together with other reasons associated to the variation of traffic volume by time of day. Figure 9 shows the total numbers of lane changing occur in the freeway zone one and two according to the definition presented in the section 3. It is clear that the average volume of the freeway lane two (median lane) increased from 1450 to 1700 veh/hr/lane and the average volume of the freeway lane one (shoulder lane) increased from 520 to 800 veh/hr/lane. Consequently, the total numbers of the lane changing decreased from 650 to 500 veh/hr. This suggests that increased flow rate of the freeway lane two will reduce the lane changing opportunities of the vehicles in the freeway lane one which in turn leads to the reduction in the number of lane changing to freeway lane two. Subsequently, the flow rate of the freeway lane one increased. Additional insight can be obtained by observing the interaction between the freeway lane one (shoulder lane) and the ramp lane one as it is shown in Figures 10. The data demonstrate that while the total flow rates of freeway lane plus ramp lane one stays invariable around 1400 veh/hr/lane, the freeway lane one flow rate increased by reduction of the ramp lane one flow rate. The maximum discharged flow rates of each lane for the freeway and the ramp are shown in Figures 11 and 12 respectively. These figures highlight extremely unbalanced lane utilization for both the freeway and the ramp approaches. Notably, the average volume of the freeway lane one (shoulder lane) is one third of that for the freeway lane two (median lane) and the average volume of the ramp lane one is around half of the volume of the ramp lane two. These results led us to propose the strategy of lane one closure of both the freeway and the ramp in order to maximize the flow and minimize the potential conflicts between vehicles (Sarvi et al. 2003).

15 Observing Freeway Ramp 159 Traffic volume (veh/hr/2lane) Total bottlenck discharged flow rate Freeway flow rate Ramp flow rate Time of day Figure 8. Total discharged flow rate, ramp flow rate, and freeway flow rate at Hamazaki-bashi (peak period only). Traffic volume (veh/hr/lane) Freeway lane 2 Freeway lane 1 Total lane changing Time of day Figure 9. Freeway lane two flow rate, freeway lane one flow rate, and total lane changing at Hamazaki-bashi (peak period only).

16 160 M. Sarvi, M. Kuwahara and A. Ceder Traffic volume (veh/hr/lane) Freeway lane 1 plus ramp lane 1 Ramp lane 1 Freeway lane Time of day Figure 10. Interaction of freeway lane one and ramp lane one at Hamazaki-bashi (peak period only). Traffic volume (veh/hr/lane) Ramp lane 2 Ramp lane Time of day Figure 11. Distribution of ramp lane one and two flow rates at Hamazaki-bashi (peak period only).

17 Observing Freeway Ramp 161 Traffic volume (veh/hr/lane) Freeway lane 2 Freeway lane Time of day Figure 12. Distribution of freeway lane one and two flow rates at Hamazaki-bashi (peak period only). 4.4 Capacity of the merging sections Traffic data from studied merging sections were collected during the periods when queues were forming upstream from the merging sections and downstream were free, so that the capacity could be observed. A typical flow and speed pattern which is used for the capacity estimation and evaluation of a traffic jam is illustrated in Figures 13 to 15 for the immediate upstream and downstream sections of the Hamazaki-bashi merging section. In these figures, capacity points are referred to the capacity definition presented in section 4.1. When merging section (bottleneck) reaches capacity, there will be a backup of traffic on the freeway and ramp streams, resulting in stop-and go traffic in upstream of the bottleneck. Theses vehicles can be considered to be in queue, waiting their turn to be served by the bottleneck section immediately downstream of the entrance ramp. These ranges of congested data are referred to as capacity points concentrated in area of the lower part of the speed-flow curves of Figures 13 and 14. These data point are also characterized with speed lower than 40km/h. The volumes of freeway stream upstream the bottleneck in Figure 13 show the capacity flow at merging section less the entering ramp flow. At bottleneck location, the full range of uncongested flows is observed, right-out to capacity, but the location

18 162 M. Sarvi, M. Kuwahara and A. Ceder never becomes congested, in the sense of experiencing stop-and-go traffic. The data that are observed at bottleneck (at front end of the queue) are on the top portion of the speed-flow curve, and are characterized by speed between 40km/h to 60 km/hr (see Figure 15). This segment of the speed-flow curve has been referred to as queue discharge flow (Hall et al 1992). The specific speed observed at this location will depend on how far it is from the front end of the queue (Persaud and Hurdle 1988). The observed 15 minutes volumes were multiplied by 4 to obtain hourly rates, which were then adjusted to passenger-car units the highest and lowest capacity is obviously significant. To compare the capacities of the merging sections with capacities of simple (i.e., non-merging) sections, capacity observation were carried out on three simple sections of the Tokyo Metropolitan Expressway as shown in Table 4. Traffic volumes for five-minute intervals over three days were counted by traffic detectors. Average traffic volumes in the three simple sections during the two-hour peak period (from 7am to 9am) and capacities adjusted for heavy vehicles are also shown in Table 4. The capacity of the Gaien- Yoyogi segment is about 15% lower than the other two simple sections. This is due to the presence of an S-shaped curve and sag downstream from this section. The capacity of a simple section on the Tokyo Metropolitan Expressway would thus generally be about 4600 pcu/hr/2- lane based on observations on the other two simple sections. Comparing capacity of merging and simple sections, it is clear that the capacity of 1889 Pcu/hr/lane in Ikejiri is about 18% lower than the capacity of simple section. Although the capacity of 2458 pcu/hr/lane in Kasai is about 7% greater than the simple section, however, the average capacity of all eight sections of 2146 pcu/hr/lane is about 7% lower than the capacity of a simple section. In other words, among the eight sections, presented in Table 3, six sections showed capacity lower than 4600 pcu/hr/2-lane.

19 Observing Freeway Ramp 163 Figure 13. Flow rate speed of the freeway approach upstream of the Hamazaki-bashi. Figure 14. Flow rate speed of the ramp approach upstream of the Hamazaki-bashi.

20 164 M. Sarvi, M. Kuwahara and A. Ceder Figure 15. Flow rate speed of the freeway downstream of the Hamazakibashi merging section. Table 4. Capacity observations of simple sections Sections Average flow for saturated two hours(veh/hr/2-lanes) Percentage of heavy vehicles Capacity (Pcu/hr/2Lanes) Hakozaki to Ryogoku Showajima to Hanada Gaien to Yoyogi Merging Ratio to Affect the Merging Capacity The volume of traffic, which can merge at freeway ramp connection, is dependent on a number of variables associated with geometric and traffic characteristic. One important variable is the relative proportion of two traffic volumes, which are combined to form the merge volume. Merging ratio is defined as the ratio of ramp flow rate (QR) divided by

21 Observing Freeway Ramp 165 freeway plus ramp flow rates (QR+QF). Figure 16 shows the relation between the merging ratio and the maximum flow discharged flow rate at the Hamazaki-bashi merging section. It is clear that the capacity of the merging section is not affected by the merging ratio. In other words, the wide range of the merging ratio fluctuation of 0.35 to 0.67 showed no significant effect on the merging capacity. Similar trend is observed for the Ikejiri merging section. Hess (1963) showed, when either freeway flow or ramp flow were dominant the free flow merge volumes were higher than when the two flows approximated each other in volume. It means that, one cannot expect the ease of merging to remain constant, regardless of how the two volumes are distributed. For instance, where 1600 vehicles must merge and all other variables are held unchanged, it appears easier to merge 400 ramp vehicles with 1200 freeway lane one vehicles than to merge 800 ramp with 800 freeway lane one vehicles. In contrast with the free flow conditions, based on the results of this research, the merging capacity is constant when the merge flow is either lower or approximately equal to the freeway flow. In other words, regardless of the merging ratio the maximum discharged flow rate of the bottleneck is quite invariable. In a comparative sense, this could be attributed to the two competing traffic streams entering the merging section. This result can serve as an initial guidance for introducing traffic management strategies (including intelligent transport systems strategies) at merging bottlenecks. For instance, knowing the fact that the capacity of merging sections is not affected by the merging ratio could assist in local or network traffic management by providing priority to one of the approaches (freeway or ramp approach) with predetermined higher priority. The distribution of the ramp and freeway volumes of the Hamazakibashi section is shown in Figure 17. It is expected that these volumes are distributed along a line with minus one slope (capacity line), whereas for free flow merging conditions the volumes of ramp and freeway are distributed along a line with slope of less than minus one as reported by Hess (1963). Figure 17 highlights that when the freeway volume is dominant, the merging volumes are slightly above the capacity line nevertheless, this difference is not significant and the findings are consistent with a theory of merging proposed by Daganzo (1995).

22 166 M. Sarvi, M. Kuwahara and A. Ceder Capacity (veh/hr/lane) Merging ratio (QR/QR+QF) Figure 16. Relationship between the merging ratio and the merging capacity at Hamazaki-bashi Ramp volume (veh/hr/2lane) Freeway volume (Veh/hr/2lane) Figure 17. Distribution of the ramp and freeway volumes at Hamazaki-bashi.

23 Observing Freeway Ramp Conclusions This paper has investigated the traffic behavior and characteristic of freeway ramp merging sections under congested traffic conditions. The literature has been reviewed and found to be mostly associated to the study of gap acceptance and ramp vehicle characteristics of freeway ramp merging sections under free flow traffic conditions. This is due, at least in part, to the limited availability of measured traffic data under congested traffic conditions. The analysis undertaken in this study was conducted on eight study sites using mostly recorded videotapes and detector data. This large quantity data supplies a sound and robust database for performing traffic characteristics analysis of merging sections under congested traffic conditions. Key findings from the studies described in this paper were as follows: Utilizing an arrival and departure curves of cumulative vehicles counts versus time, we studies the bottleneck discharged flow rate and the average time that drivers spent in the queue. We observed that the departure flow rate from the queue head, which is the capacity during congestion, was further reduced, as the queue grew longer. This could be associated to the driver s transition behavior (i.e. car-following) from the free flow mode to the congested flow mode. Drivers seem to gradually change their pattern of car-following becoming less sensitive and cautious as they are caught in the queue for longer periods. The transition period of the driver behavior from free flow to congested flow was observed around fifteen minutes for Ichinohashi and five minutes for the Hamazaki-bashi merging sections however, this time varying widely from case to case. Using data from the upstream and downstream detectors at merging sections we examined lane changing and lane utilization at merging sections. It was found that increased traffic volume of freeway lane two leads to a less total number of lane changing. The results also highlighted extremely unbalanced lane utilization for both freeway and ramp approaches. The average volume of freeway lane one was around one third of that for the freeway lane two and the average volume of ramp lane one was around half of the volume of ramp lane two. These results have important implications for practitioners and model developers. It could provide a basis for introducing intelligent transport systems traffic management strategies such as lane closure or lane changing restrictions strategies in order to increase the merging capacity.

24 168 M. Sarvi, M. Kuwahara and A. Ceder The data from eight study sites indicate that in a comparison sense capacity of merging sections was found lower than an ordinary highway section (e.g., 18% less at Ikejiri section and 7% less as average of all eight sections). The average merging capacity of eight merging sections was found around 2146 Pcu/hr/lane. Observing the capacity of merging sections over a long period confirmed that the merging capacity was not correlated with the merging ratio (when both the ramp and freeway approaches are queued). The ramp and freeway volumes are distributed along a line with slop of minus one (capacity line). References Ahmed, K.I., Ben-Akiva, M.E., Koutsopoulos, H.N., and Mishalani, R.G., (1996). Models of freeway lane changing and gap acceptance behavior. International Symposium of Traffic and Transportation Theory, J. Lesort (ed.), Elsevier Science & Pergoman, pp Beaky, J., (1938). Acceleration and deceleration characteristics of private passenger vehicles. Proceedings of the 18 th Highway Research Board Annual Meeting, Washington, D.C., pp Bertini R.L. (2005). Detecting Signals of Bottleneck Activation for Freeway Operations and Control. Journal of Intelligent Transportation Systems: Technology, Planning, and Operations Publisher: Taylor & Francis Issue: 9(1), pp Cassidy, M. J. and Ahn, S. (2005). Driver Turn-Taking Behavior in Congested Freeway Merges. Proceedings of the 84th Annual of the Transportation research Board, Washington, D.C. Chang, G., and Kao, Y.M., (1991). An empirical investigation of macroscopic lane changing characteristics on uncongested multilane freeways. Transportation research Part A, 25(6), Daganzo, C., (1981). Estimation of gap acceptance parameters within and across the population from direct roadside observation. Transportation Research Part B, 15(1), pp Daganzo, C., (1995). The cell transmission model, Part 2: network traffic. Transportation Res. 29B, pp Daganzo, C., Cassidy, M. J., and Bertini R. L. (1999). Possible explanations of phase transitions in highway traffic. Transportation Research Part A, 33(5)

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26 170 M. Sarvi, M. Kuwahara and A. Ceder Sarvi, M and Kuwahara, M (1999). Comparative study on evaluation of merging capacity in Tokyo Metropolitan Expressway. Seisan- Kenkyu, 51, No. 2, pp Sarvi, M. (2000). Freeway ramp merging phenomena observed in traffic congestion. Ph.D. dissertation, University of Tokyo. Sarvi, M., Ceder A, and Kuwahara M. (2002). Modeling of freeway ramp merging process observed during traffic congestion. In: M. Taylor (ed.) Transportation and Traffic Theory, Elsevier, pp Sarvi, M., Kuwahara, M. and Ceder, A. (2003). Developing control strategies for freeway merging points under congested traffic situations using modeling and a simulation approach. Journal of Advanced Transportation 37(2) Sarvi, M., Kuwahara, M. and Ceder, A. (2004). A study on freeway ramp merging phenomena in congested traffic situation by traffic simulation combined with driving simulator. Journal of computeraided civil and infrastructure engineering 19(5), pp Sarvi M, Ceder A, and Kuwahara M. (2005). Freeway ramp merging process observed in congested traffic: Lag vehicle acceleration model. In: H. Mahmassani (ed.) Transportation and Traffic Theory, Elsevier, pp Wattleworth J.A., Buher J.H., Drew D.R., and Geric F.A., (1967). Operational effects of some entrance ramp geometrics on freeway merging. Highway Research Record, No. 208 pp