a driving simulator study on diverging driver

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A DRIVING SIMULATOR STUDY ON DIVERGING DRIVER PERFORMANCE ALONG TAPERED AND PARALLEL DECELERATION LANES

Alessandro Calvi, Corresponding Author Assistant Professor Roma Tre University, Department of Engineering, via Vito Volterra, 62 - 00146 Rome, Italy Tel: +390657333451, Fax: +390657333441, Email: [email protected] Francesco Bella Associate Professor Roma Tre University, Department of Engineering, via Vito Volterra, 62 - 00146 Rome, Italy Tel: +390657333416, Fax: +390657333441, Email: [email protected] Fabrizio D’Amico Assistant Professor Roma Tre University, Department of Engineering, via Vito Volterra, 62 - 00146 Rome, Italy Tel: +390657333412, Fax: +390657333441, Email: [email protected]

Word count: 5,243 words text (references excluded) + 7 tables/figures x 250 words (each) = 6,993 words Number of references: 35

Submission date: 14th November 2014

Calvi, Bella, D’Amico

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ABSTRACT Highway diverge areas are often characterized by high crash rates. All over the world, geometric design guidelines propose deceleration lanes with parallel or tapered layout in order to promote a free-vehicle condition, while neglecting the potential effects of traffic flow and risky driving behaviors that are induced by the wrong perception of road geometries and vehicle interferences. A driving simulation study is carried out to analyze the effects of traffic flow and deceleration lane geometries on the driving performance of diverging drivers. Two different types of deceleration lanes (parallel and tapered) are implemented in a simulator, and two different traffic conditions (low and high traffic flows) are simulated for each type. Thirty one drivers took part in the experiments. The effects of traffic flow on the driving performance whilst approaching the diverge area and during deceleration are investigated. The study found that lane type significantly affects the speeds of diverging drivers, independent of the traffic condition, with higher interferences with the through traffic on the tapered lane. Traffic condition was found to influence the driver’s trajectory along the tapered lane, and thus delaying the exiting maneuver under high traffic conditions. This effect was not found to be significant for parallel deceleration lanes on which, on the contrary, the deceleration rates were significantly affected by the traffic condition: higher decelerations were recorded under low traffic condition. No significant traffic effects on deceleration rates were found on the tapered lane.

Keywords: Driving simulator, Driver performance, Deceleration lane, Road safety


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INTRODUCTION Deceleration lanes provide an effective and a smooth transition from high-speed lanes to low-speed ramps, or turning roadways. They are meant to improve traffic operation, reduce interruptions and vehicular interferences, increase capacity, and improve safety. However, several studies conducted during the last decades on both the operation and the safety performance of deceleration lanes and exit ramps have noted high crash rates on these geometric elements (1) – (5). Studies on the relationships between crash rates and the geometrical features of deceleration lanes and exit ramps have demonstrated that the length of the deceleration lane (6) – (9), traffic flows and other geometric features (10) – (12), are important factors to consider in crash occurrence. However, despite an evidence of safety problems related to deceleration lanes, there are actually no guidelines or research outcomes that can provide designers with clear and updated criteria for appropriately designing deceleration lanes, whilst taking into account the driver’s actual behavior and the effects of different design variables on the driver’s performance. Moreover, previous findings on the impacts of deceleration lanes on the safety level are not quite consistent, or even contradictory. Therefore, a comprehensive study is needed to improve the understanding of the effects of different deceleration lanes on traffic safety and operation. The main objective of this study is to evaluate and model driver behavior on highway deceleration lanes. Accordingly, different types of deceleration lanes (tapered and parallel) and different traffic conditions on the highway (low and high traffic flows) are implemented in a driving simulator in order to observe how diverging drivers perform in terms of speed, deceleration and trajectory while approaching the deceleration lane and decelerating till the exit ramp. The effects of the deceleration lane type and traffic flow on drivers’ performance are then statistically analyzed. BACKGROUND Most of the design criteria for interchanges are based on studies that are about 60 years old (13), which indicated that: − crashes are more frequent and severe at interchanges (2), (4), (14); − crash rates on exit ramps are consistently higher than those at the entrance ramp (1); and − the highest percentages of exit ramp crashes are observed on the deceleration lane (1). Although several plans, programs, measures and activities have been developed during the last decades in the field of roadway safety, the safety problems at interchanges, and specifically on deceleration lanes, have not yet been solved, as demonstrated in recent crash analyses (5), (9), (12). A great effort has been made in the last years to identify the factors that mostly affect the safety performance along deceleration lanes and exit ramps. The typical approach is based on regression analyses aimed at identifying relationships between the crash rate and the design variables of the diverging area (15), (16). According to some regression models, the through and diverging traffic flows (7), (9), (10), the length of deceleration lane (9), (11), the exit ramp type and configuration (9), (10), (12), are considered to be the most significant contributory variables for crash occurrence along deceleration lanes and exit ramps. However, the findings of other crash-related analyses provide quite opposite conclusions, demonstrating the need for different approaches in studying the relationships between the safety and the design variables of deceleration lanes. In fact, some authors (17), (18) have found that the geometric design elements, such as configuration, type and geometry of exit ramp are not the determining factors of crashes, and do not yield significant effects on the severity of injuries at highway diverge areas. Using data collection from site observations, some authors (6), (19), (20), (21) found important results that were strongly correlated to operational and safety issues: exiting drivers adopt speeds that are significantly lower than the speeds of through traffic at the beginning of the deceleration lane; consequently, this difference in speeds is reflected backward causing interferences between diverging and through traffic on the highway. Garcia and Romero (8) observed several deceleration lanes with different lengths with the aim of integrating safety into the geometric design. The authors proposed a safer length for deceleration lanes, as lanes that were too short or too long reduced the overall safety of the


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interchange. Recently, El-Basha et al. (22) examined the speeds of drivers at highway diverge areas by using data collected on several exit ramps. The authors found that the speeds of diverging vehicles are highly dependent on the highway’s average speed, the deceleration length and the diverging traffic flow. However, it is widely recognized that the use of a driving simulator to study the interactions between the driver, the vehicle and the road environment, is an effective tool to overcome several problems of on-site observations. Numerous applications demonstrate the great potential of interactive driving simulations in road safety investigations and evaluations (23) – (26). However, few simulation studies of the driver’s performance on deceleration lanes have been made. Van Winsum et al. (27) analyzed the relationship between perceptual information and motor response during lane-change maneuvers in a fixed-based driving simulator. The results suggested that steering actions were consequences of the previous actions in such a way that safety margins were maintained. Bella et al. (28) studied the validation of the simulator used in this study to verify its reliability for designing deceleration lanes as a function of the lane length. Their analysis revealed that the average trajectory in the field agrees with the trajectory in the simulation. Before arriving at the deceleration lane, the speeds observed in virtual reality were higher than those in the field measurement; along the deceleration lane simulated speeds were similar to field data. No relationship between the deceleration rates and lane length were found in reality or in the driving simulator. Recently, Calvi et al. (29) studied the driver’s performance while approaching a diverge area on a highway (dual carriageway with two lanes) and decelerating during the exiting maneuver on a parallel deceleration lane. Three different through traffic conditions were investigated in order to analyze the influence of traffic volume on driving performance. Their findings demonstrate that there are considerable differences between the main assumptions of models generally used to design deceleration lanes and the actual driving performance. In particular, diverging drivers begin to decelerate before arriving at the deceleration lane, causing interferences with the through traffic. Moreover, the authors demonstrated that a lower traffic volume results in high exiting speeds and decelerations, and that diverging drivers begin to decelerate earlier on the through lane when the traffic volume is low. This was the pilot study of a wider research (of which the current study is a part) that we are carrying out during the last years. The research has the final objective of providing to practitioners effective guidelines for designing deceleration lanes and exit ramps, whilst taking into account the driver’s actual behavior and the effects of different design variables on the driver’s performance. OBJECTIVE OF THE RESEARCH The main objective of the current study is to investigate the driving performance of diverging drivers along highway deceleration lanes under different traffic and geometric conditions. Specifically, the effects of different through traffic conditions (low and high traffic volumes) and deceleration lane type (tapered and parallel) on the speed, trajectory and deceleration of diverging drivers are investigated, in order to provide a better knowledge of the behaviour of drivers whilst approaching a deceleration lane, during lane-change maneuver, and along the deceleration lane until the beginning of the exit ramp. A multi-factorial experiment with the above was conducted using the advanced driving simulator of the Inter-Universities Research Centre for Road Safety (CRISS) at Roma Tre University. METHOD Apparatus The CRISS fixed-based driving simulator consists of a complete car positioned in front of three angled projection surfaces able to provide 135 degree field of view using a resolution of the visual scene of 1024×768 pixels with a refresh rate of 60 Hz. The hardware and software make the simulator an effective tool to study traffic safety and road design problems, specifically for the evaluation of driving performance in terms of speed, acceleration and trajectory under different driving conditions (24), (30), (31). The simulator allows modeling the road in accordance with the traditional road engineering constraints, and thus providing an authentic driving experience to drivers as demonstrated by several previously performed validation studies (25), (32). Moreover, previous studies validated CRISS simulator


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specifically for highway deceleration lane (28), and investigated driver behavior along deceleration (29) and acceleration lanes (30). Several objective parameters (e.g. longitudinal speed, lateral position, acceleration) are recorded during the tests at a frequency of 20Hz, which meets the data accuracy requirements of this study. Scenario Set-Up Two different geometries of the highway deceleration lane were designed and implemented in the virtual reality environment: one-lane exit with tapered design and one-lane exit with parallel design (Figure 1). The geometric elements of the deceleration lanes were designed according to Italian (tapered (33)) and Spanish (parallel (34)) guidelines. In particular, the tapered lane consisted of two parts: the first, defined as the maneuvering segment or taper, was 90 m in length; the second, aimed at reducing the speed of exiting drivers before the exit ramp, was 150 m in length. The parallel lane consisted of a taper (100 m) and a deceleration lane (240 m). The widths of the deceleration lanes were 3.50 m. The crosssection of the highway was made up of a divided carriageway with a central greenery lane of 2.50 m. Each carriageway had three lanes (3.50 m wide) and a paved shoulder (1.00 m wide). As the objective of this study was to investigate the diverging drivers’ behavior whilst approaching the deceleration lane, during lane-change maneuver, and along the deceleration lane, the exit ramp terminal conditions were not implemented in the scenario. In fact, each simulation test was stopped when the driver reached a section on the exit ramp that was far enough from both the exit point (in order to not affect the diverging driver’s behavior) and the exit ramp terminal endpoint (in such a way that the driver could not see the type of terminal endpoint). In future studies the effects of the type of terminal endpoint and the distance from the exit point to the terminal endpoint on the diverging drivers’ performance will be investigated. For each deceleration lane type, two different through traffic conditions were investigated, for a total of 4 tests per driver: tapered lane with low traffic (TL); tapered lane with high traffic (TH); parallel lane with low traffic (PL); and parallel lane with high traffic (PH). The two conditions were designed so as to simulate a low traffic volume (200 vehicles/h, almost a free-vehicle condition) and a high traffic volume (800 vehicles/h on lane 1 and 1,600 vehicles/h on lanes 2 and 3). These traffic volumes on the highway lanes fully agreed with the results of previous field observations (8), (28). The average speed of through vehicles was 120 km/h. The studied section was seven-kilometer in length, consisted of curves with wide radii and tangents, randomly combined between the tests. Specifically, a tangent of 1,000 m was implemented before the beginning of the deceleration lane (tapered or parallel) in order to limit the driver’s performance to the geometry of the exit lane and traffic conditions whilst approaching the deceleration lane, and not affect it by previous geometric elements.

FIGURE 1 One-lane exit with tapered layout (on the left) and parallel layout (on the right).


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Vehicles also travelled on the opposite carriageway, and no other vehicles diverged from the highway: the driver was the only one who moved to the deceleration lane, simulating in all tests the freediverging-vehicle condition. Future research will investigate the condition of multiple vehicles exiting the highway. Procedure The research protocol submitted to each participant consisted of the following steps: 1) once the participant arrived at the laboratory, he/she was first briefed about the requirements of the experiment and asked to fill out a form containing personal demographic and driving information; 2) prior to performing the tests, an illustration of the vehicle setting was made to the participant who was trained on the operation of the simulator. The training period lasted for about 10 minutes during which the driver experienced acceleration, deceleration, turning and braking maneuvers; 3) following simulator familiarization, the subject drove the first two tests in a set sequence (e.g., PL and TH); 4) then the driver got out of the car, took a rest for about 1 hour in order to reestablish the psychophysical conditions of before performing the tests; 5) the subject completed the other two tests (e.g., PH and TL) and then filled out an evaluation questionnaire about the discomfort that was perceived during driving (nausea, dizziness, fatigue, etc.). This helped to eliminate from the sample any drivers who performed the tests under abnormal conditions. The order of the tests was counterbalanced across drivers to avoid influences due to the repetition of the same order in the experimental conditions. The participants were not aware of the study purpose prior to taking part in the experiment. All the drivers were only instructed to drive as if they were driving in the real world, diverging from the highway at the first available exit that was indicated by vertical signs. The first vertical exit sign was 2000 m before the beginning of the taper and did not specify the type of the deceleration lane. Participants Thirty one participants, 17 males and 14 females, took part in the study. They were recruited from the staff and students at the Department of Engineering at Roma Tre University. Participants carried an Italian driving license, had no previous experience with the driving simulator, and have driven for at least 5,000 km during the last year. Two participants did not complete the experiment as they experienced simulation sickness, like nausea and headache. Afterwards, the simulation outputs of the drivers were validated using the Chauvenet criterion (35), by which any drivers, whose speed values whilst approaching the deceleration lanes (at 500 m and 100 m before the taper) were greater than three SDs (Standard Deviations) from the sample’s average speed, were dismissed from analysis. In this case, two drivers had to be excluded. Consequently, the sample used for analysis consisted of 27 drivers, 15 males and 12 females, with an average age of 28.8 years (SD = 5.1 years) and ranged between 22 and 45 years. Their average driving experience was 8.7 years and they have driven an average of 9,600 km during the preceding year. Data Collection In order to study the driver’s behavior whilst approaching the deceleration lane, during the lane-change maneuver, and along the deceleration lane, the driver’s speeds were recorded at five measurement points (see Figure 1): two points were fixed respectively at 500 m and 100 m, respectively, before the beginning of the taper; one point at the beginning of the taper; one point (site A), which depended on the driver’s lateral position (considering the center of gravity (CG) of the driver’s vehicle), at the site where the driver’s trajectory crossed the line between the right through lane and the deceleration lane; the last point (site B) at the site where the vehicle was completely within the deceleration lane; that is where the vehicle’s trajectory crossed a line parallel to the line between the right through lane and the deceleration lane, at 0.85 m from its right hand side (the vehicle was 1.70 m wide).


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Consequently, an analysis of the trajectories (lateral position of the vehicle’s CG along the roadway) of the diverging driver’s vehicle was performed at points A and B for each exiting maneuver, as well as their distances from the beginning of the taper (dA and dB respectively) were investigated. There was a need to do the analysis at points A and B since, as demonstrated elsewhere in Calvi et al. (29), if speed data are solely studied where the vehicle is completely within the deceleration lane, then the speed values would be significantly different, meaning that diverging vehicles would remain partially in the right through lane for a considerable part of the exiting maneuver (especially for parallel deceleration lane). Finally, an analysis of the deceleration rates of exiting drivers was carried out, with respect to the average (aav) and maximum (amax) decelerations that are adopted by the driver on the deceleration lane, and the locations (Site C) where the maximum deceleration was recorded in relation to the beginning of the taper. Table 1 summarizes the results of the speed analysis. The average values of trajectory and deceleration indicators are shown in Table 3. RESULTS AND DISCUSSION In order to analyze the effects of the layout of the deceleration lane and traffic conditions on driving performance of each diverging driver, the following parameters were investigated: speed (at five measurements points), trajectory (exit sites) and deceleration.

TABLE 1 Average and Standard Deviation of Speed for Every Combination of Deceleration Lane Type, Traffic Condition and Measurement Point Speed (Km/h) Average SD

Deceleration Lane Type

Traffic Condition

Measurement Point

Tapered

Low

-500 -100 0 A B

121.03 94.90 75.22 66.53 60.23

17.33 16.73 12.17 11.13 11.51

High

-500 -100 0 A B

112.48 94.49 84.26 75.16 70.85

19.93 10.83 10.32 11.95 11.88

Low

-500 -100 0 A B

122.56 105.36 97.49 89.83 85.03

13.94 14.31 14.44 17.25 15.75

High

-500 -100 0 A B

117.24 96.12 92.93 87.04 84.03

10.38 15.07 12.80 12.47 14.19

Parallel

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TABLE 2 Main and Interaction Effects p

Partial Eta squared

Observed Power

Main Effects

F

Deceleration lane type Traffic condition Measurement point

F(1,26) = 33.49 F(1,26) = 0.04 F(1.79,46.66) = 228.39