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EXPERIMENT 5: The effects of deceleration on braking reactions as a function of preferred time-headway

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EXPERIMENT 5: The effects of deceleration on braking reactions as a function of preferred time-headway

 

This is chapter 8 from the thesis “From adaptive control to adaptive traffic behaviour” about traffic psychology and behavioural adaptation of drivers, by Wim van Winsum. The thesis is from 1996. It describes a number of behavioural experiments into car driving that were performed in a research driving simulator.

Other chapters of this thesis can be found here:

 

Abstract 

The manoeuvre of braking for a decelerating lead vehicle was separated into three sequential processes that were manipulated independently. The initial time-headway to the lead vehicle at the moment it started to decelerate affected reaction time. Primary deceleration of the lead vehicle manipulated the duration of the open-loop phase. From the moment the driver touched the brake pedal, the deceleration of the lead vehicle was changed. This secondary deceleration was assumed to affect the closed-loop phase of braking. The hypothesis was that drivers who prefer a small time-headway during car-following (short followers) differ from drivers who prefer to follow at a large time-headway (long followers) in both the open- and closed-loop phases. In that case an interaction is expected between following group (short vs. long follo­wers) and primary deceleration on the duration of the open-loop phase and between following group and secondary deceleration on the duration of the closed-loop phase, the maximum brake force exerted and the number of movement corrections. In general terms, these predictions could not be confirmed. The lack of confirmation of the hypothesis is explained in terms of task characteristics that resulted in startle reactions and vigilance effects.

 

8.1 Introduction

 

A number of studies of car driving have shown that behaviour on the tactical level such as speed choice and choice of time-headway in car-following is sensitive to a variety of factors that affect operational perfor­mance. For example, marijuana affects operational car driving performance while it also results in driving with a lower speed and in choosing a larger time-headway (THW) during car-following (Smiley, et al., 1981; Smiley et al., 1985; Smiley et al., 1987). Time-on-task results in the choice of a larger time-headway during car-follo­wing, accompanied by verbal reports of performance decrements, drowsiness and exhaustion (Fuller, 1981). Brookhuis et al. (1991) reported an increase in THW when using a car telephone while driving. These fin­dings suggest that behaviour on the tactical level is used by the driver to compensate for effects on operational performance. For the case of car-following this means that any factor that may lead to performance decrements in braking for lead vehicles may result in a compensatory increase of time-headway. From the same perspective it was studied whether individual differences in preferred THW are related to individual differences in braking performance in a number of experiments (Van Winsum, 1996; Van Winsum and Heino, 1996; Van Winsum and Brouwer, 1996). In Van Winsum and Heino (1996) and Van Winsum and Brouwer (1996) preferred THW proved to be consistent within the driver. This means that drivers are consistently short or long followers and, thus, that individual differences in THW are consis­tent.

In Van Winsum (1996) it was found that, under the instruction to brake as fast as possible as soon as a deceleration of the lead vehicle was detected, reaction time and motor-response times are not different between short and long followers. Also, no evidence was found for differences in the informa­tion processing stages of stimulus encoding of a braking lead vehicle and motor adjustment. However, when the driver did not know the deceleration of the lead vehicle in advance, short followers generated a faster motor response, i.e. they moved the foot faster from the accelerator pedal to the maximum brake position when the lead vehicle decelerated. This suggests that short followers differ from drivers with a larger preferred THW in the transformation of visual feedback to a required motor response.

In Van Winsum and Heino (1996) it was found that the initiation and control of braking are both affected by time-to-collision (TTC) at the moment the lead vehicle starts to brake. This suggests that TTC information is used for judging the moment to start braking and in the control of braking. No evidence was found for differences between short followers and long followers in the ability to accurately perceive TTC. However, short followers were better able to program the intensity of braking to required levels and tuned the control of braking better to the development of criticality in time during the braking process. It was concluded that short followers may differ from long followers in programming and execution of the braking response as a function of TTC information.

Van Winsum and Brouwer (1996) analyzed the braking response in terms of three sequential phases in the braking process. The first phase consists of the interval between the moment the lead vehicle starts braking and the moment the driver releases the accelerator pedal. This is measured by the reaction time (RT). The second phase consists of the open-loop ballistic motor response. It is measured as the interval between the moment the accelerator pedal is released and the moment the brake pedal is touched and referred to as Brake Initiation Movement Time (BIMT). The third phase is a closed-loop motor response during which visual feedback is used to control the braking response while braking. The duration of the open-loop phase was strongly determined by the TTC at the moment the accelerator pedal was released, while the duration of the closed-loop phase was strongly determined by the number of movement corrections in the brake pedal signal. It was found that short followers exhibited a faster open-loop motor response which was not caused by a smaller TTC at detection time. Also, short followers generated a faster closed-loop response which was caused by fewer movement corrections. These results again supported the hypothesis that short followers differ from long followers in the efficiency of programming and execution of the braking response.

According to Van Winsum and Brouwer (1996), the duration of the three phases is affected by different factors. The RT interval is assumed to be affected by the THW at the moment the lead vehicle starts braking. This means that RT is expected to be faster if the momentary time-headway at the moment the lead vehicle starts to decelerate is smaller. The open-loop interval (BIMT) is assumed to be affected by the primary deceleration of the lead vehicle. If the lead vehicle decelerates more strongly, TTC at detection time is smaller resulting in a faster open-loop response. The closed-loop interval (BCMT) is affected by the number of movement correcti­ons. If the deceleration of the lead vehicle changes after the subject’s braking response has started, the speed and intensity of the braking response has to be changed, based on visual feedback. This means that a change in the level of deceleration after the braking response has started (secondary deceleration) is assumed to result in more movement corrections and thus affects the duration of the closed-loop phase. If short followers differ from long followers in both the open- and closed loop phases, then an interaction is expected between following group (short vs. long follo­wers) and primary deceleration on BIMT and between following group and secondary deceleration on BCMT, the maximum brake force exerted and the number of movement corrections. These hypotheses are tested in the present experiment.

 

8.2 Method

 

Subjects. Twenty-two subjects participated in the experiment. The subjects were selected from the TRC database by the following procedure. First a preselection was made on the basis of age and driving experience. Only subjects between 25 and 40 years of age with a minimum driving experience of 10000 km that were known not to be susceptible to simulator sickness were preselected from the database, resulting in 150 persons. These were send a small photo-preference test that measures preferred THW. This test consisted of 6 numbered photographs with scenes of a lead vehicle on a highway at different distances in front of the car. The preselected subjects were required to fill out on a form the number of the photograph that best matched the THW chosen by the subject while driving with a speed of 110 km/h on a highway and to return the form if they were intere­sted in participating in the experiment. This test procedure has been shown to result in a reliable estimate of preferred time-headway during car-following on the road (Heino et al., 1992). From the returned forms, 11 subjects with a small preferred THW and 11 subjects with a larger preferred THW were invited for participation in the experiment. Subjects with a preferred photo number of less than or equal to 3 were assigned to the group of ‘short followers’. Subjects who preferred photo number 5 or 6 were assigned to the group of ‘long followers’, see table 1. These two groups are referred to as ‘THWpref groups’.

 

Table 1. Relation between photo number and headway on the photo preference test and number of subjects. DHW=distance-headway in meters, THW=time-headway in seconds.

 

 

Photo number      DHW        THW         number of subjects

 

1                            6                0.20          0

2                            11              0.36          2

3                            25              0.81          9

4                            33              1.08          0

5                            45              1.47          4

6                            65              2.13          7

 

 

Apparatus. The experiment was performed in the driving simulator of the Traffic Research Centre (TRC). This fixed-based simulator consists of two inte­grated subsys­tems. The first subsystem is a conventional simulator composed of a car (a BMW 518) with a steering wheel, clutch, gear, accele­ra­tor, brake and indica­tors connected to a Silicon Graphics Skywriter 340VGXT compu­ter. A car model converts driver con­trol actions into a displacement in space. On a projection screen, placed in front, to the left and to the right of the subject, an image of the outside world from the perspective of the driver with a horizontal angle of 150 degrees is projected by three graphi­cal videopro­jectors, controlled by the graphics software of the simulator. Images are presented with a rate of 15 to 20 frames per second, resulting in a sug­gestion of smooth move­ment. The visual objects are buil­dings, roads, traffic signs, traffic lights and other vehicles. The sound of the engine, wind and tires is presented by means of a digital soundsam­pler recei­ving input from the simulator computer.

The second subsystem consists of a dynamic traffic simu­la­tion with interacting artificially intelligent cars. For experimental purpo­ses different traffic situa­tions can be simulated. The simula­tor is described in more detail elsewhere (Van Wolffe­laar & Van Winsum, 1992 and Van Winsum & Van Wolf­felaar, 1993).

 

Procedure. The circuit was made of two-lane roads with a lane-width of 3 m., and straight road sections alternated with left-turning curved road sections. All roads had delineation with broken center lines and closed edge lines. Before the experiment started subjects practiced braking several times by approaching a traffic light that turned on red after a certain time-to-intersection was exceeded. This required the subjects to come to a full stop. After this, preferred time-headway was measured by the following procedure. The subject was instructed to drive with a fixed speed of 100 km/h, while continuously being overtaken by other cars. One of these cars merged in front of the subject and adopted a time-headway of 2 s. The subject was asked to rate on a scale from 1 to 10 how well the present THW resembled the THW normally adopted by the subject in similar situations on the road. If THW was too small it was increased with 0.5 s. If THW was too large it was decreased by 0.5 s. After this the subject was again asked for a rating. This continued until a definite peak was found in the subject’s rated THW, i.e. until the preferred THW was found. After this the braking trials started. The subject was instructed to drive with a constant speed of 100 km/h, to stay in the right lane and to avoid a collision with a lead vehicle in case it braked. While driving, the subject was overtaken by another vehicle every 5 seconds on average. The lead vehicle merged in front of the lead vehicle and started to drive at a fixed THW of either 0.8 or 1.2 seconds (THW condition). After a stable THW was reached it either pulled up again or it braked from 100 to 60 km/h. Braking occurred on average once in every 5 minutes. The lead vehicle applied either a deceleration of 3 or 6 m/s² (initial deceleration). After the subject touched the brake in response to the lead vehicle, the deceleration of the lead vehicle changed either to 3 or 6 m/s² (secondary deceleration), resulting in the following deceleration patterns for both THW conditions (0.8 vs. 1.2 s.): 3 to 3 m/s², 3 to 6 m/s², 6 to 3 m/s² and 6 to 6 m/s². Thus, the driver was subjected to a total of 8 braking trails, that were counterbalan­ced.

 

Data collection and analysis. During the braking trials the following data were sampled with a frequency of 50 Hz.: speed of the simulator car and the lead vehicle in m/s, accelera­tor pedal position, brake pedal position and force excerted in Nm, accele­ration in m/s², time-to-collision (TTC) and bumper to bumper distance from the lead vehicle. At t0 the lead vehicle started to brake.

 

Figure 1. Time history of braking and dependent variables.

The moment the accelerator pedal position was less then 4% after t0 was registered as tacc, and RT was computed as tacc-t0. The moment after tacc at which the brake pedal force was more than 3 Nm, was registered as tbr (the moment the brake pedal was touched). BIMT (Brake Initiation Movement Time, or the open-loop ballistic response) was computed as tbr-tacc. The maximum brake force was detected on-line and the moment this was reached was registered as tmaxbr. BCMT (Brake Control Movement Time, or the closed-loop braking response) was computed as tmaxbr-tbr. The maximum brake force excerted, MAXBRFO, was stored as well. During the closed-loop phase a number of decelerations typically occur in the brake pedal signal. These decelera­tions reflect movement velocity correc­tions. The number of decelerati­ons in the brake pedal signal (NRCOR) was analyzed, together with the maximum deceleration (DECmax) of the simulator car, the minimum TTC (TTCmin) and the minimum distance to the lead vehicle (DISmin) during the braking maneuver. In previous studies, the TTC on the moment the driver initiates braking, that is, TTC on tacc (TTCtacc) has proven to be an important variable controlling subsequent phases in the braking process. For this reason TTCtacc was analyzed as well. The time-history of braking can be seen in figure 1.

The dependent variables were analyzed with repeated measures analysis of variance with THW, initial deceleration and secondary deceleration as within-subjects factors, and THW-group as a between-subjects factor.

 

8.3 Results

Characteristics of THWpref groups. Table 2 presents the results of analysis of variance and the averages of THW as measured in the simulator, age, number of years licensed and kilometrage per year as a function of THWpref groups.

Table 2. Statistical effects and averages of THW as measured in the simulator, age, number of years licensed and kilometrage per year as a function of THWpref groups.

Dependent variable                F(22,1)     short                  long

 

THW simulator                      34.24**      1.5             2.9

Age                                            0.03        31.6          31.3

Years licensed                         0.65        12.8          11.3

Annual kilometrage               5.30*       28084       13227

 

** = p<0.01; * = p< 0.05.

 

There was a strongly significant difference between THWpref groups on the preferred THW as measured in the simulator. This supports the validity of the simulator for measuring car-following behaviour. There were no signifi­cant differences in age or number of years licensed to drive a car between short and long followers, but short followers drove significantly more kilometers per year.

 

Effects of manipulations. Table 3 lists the main effects of the manipulations on the dependent variables. The averages of RT, BIMT, BCMT, MAXBRFO and NRCOR as a function of the manipulations are shown in table 4. RT was significantly affected by the factor THW (0.8 vs. 1.2 s): a smaller THW at which the lead vehicle started to brake resulted in a smaller RT. BIMT was both affected by THW and by initial decele­ration: a smaller THW and a larger initial deceleration both resulted in a smaller BIMT. These effects match the significant effects of THW and initial deceleration on TTCtacc.

 

Table 3. Main effects of manipulations on dependent variables. ** = p<0.01; * = p< 0.05,

dec-1 represents primary deceleration and dec-2 secondary deceleration.

 

Dependent                     Independent       F(21,2)

 

RT                                  THW                  8.44 **

dec-1                 0.57

dec-2                 3.63

BIMT                             THW                  20.06 **

dec-1                 17.68 **

dec-2                 0.19

BCMT                            THW                  0.11

dec-1                 19.23 **

dec-2                 0.73

MAXBRFO                   THW                  26.83 **

dec-1                 41.68 **

dec-2                 71.64 **

NRCOR                         THW                  0.04

dec-1                 7.65 *

dec-2                 7.65 *

TTCmin                            THW                  14.89 **

dec-1                 77.23 **

dec-2                 35.13 **

DECmax                          THW                  17.43 **

dec-1                 65.31 **

dec-2                 57.20 **

DISmin                            THW                  81.40 **

dec-1                 80.23 **

dec-2                 9.02 **

TTCtacc                           THW                  15.69 **

dec-1                 160.42 **

dec-2                 0.86

 

Thus, if criticality, measured by TTCtacc, is higher the open-loop ballistic motor response is faster. The duration of the closed-loop phase, BCMT, was only significantly affected by initial deceleration, but not by THW or secondary decelera­tion: a larger initial deceleration resulted in a smaller BCMT. The maximum force, MAXBRFO, excerted on the brake pedal was signifi­cantly affected by all independent factors. Thus, a higher secondary deceleration, after the brake pedal was touched, resulted in a higher maximum brake force instead of a faster BCMT. The number of decelerations in the brake pedal signal (NRCOR) was both affected by initial and secondary deceleration. A larger initial and secondary deceleration both resulted in fewer decelerations in the brake pedal signal.

 

Table 4.  Averages as a function of the manipulated factors time-headway on which lead vehicle starts to decelerate, initial deceleration and secondary deceleration. RT, BIMT and BCMT in seconds, MAXBRFO in Nm. dec-1 represents primary deceleration and dec-2 secondary deceleration.

 

THW                                             0.8                                                                   1.2                           

dec_1                          3                                  6                                  3                                  6             

dec_2                 3                6                3                6                3                6                3               6      

 

RT                      0.73          0.79          0.74          0.88          0.91          1.05          0.91          0.83

BIMT                 0.63          0.73          0.49          0.50          0.98          0.89          0.66          0.58

BMCT                1.64          1.50          1.13          1.16          1.43          1.69          1.09          1.30

MAXBRFO       51.68        113.22      103.91      218.22      48.26        87.21        80.24        98.21

NRCOR             3.68          2.77          3.09          2.32          3.73          3.05          2.73          2.55

 

 

This indicates that this variable basically measures the necessity to move the pedal straight to the maximum without hesitation. Finally, a smaller initial THW resulted in a smaller minimum TTC, a larger deceleration and a smaller minimum distance to the lead vehicle. Similar effects were found for a larger initial deceleration by the lead vehicle and a larger secondary deceleration by the lead vehicle.

 

Effects of THWpref group. Table 5 lists the effects of THWpref group and interactions between THWpref group and the independent factors on the dependent variables.

 

Table 5. Main effects of THWpref and interactions on depen­dent variables. ** = p<0.01;

* = p< 0.05, dec-1 represents primary decele­ration and dec-2 secondary deceleration.

 

Dependent            Independent                F(21,2)

 

RT                         THWpref                     0.10

THWprefxTHW           0.08

THWprefxdec-1          0.60

THWprefxdec-2          0.25

BIMT                    THWpref                     0.36

THWprefxTHW           0.09

THWprefxdec-1          0.11

THWprefxdec-2          0.01

BCMT                   THWpref                     2.04

THWprefxTHW           0.13

THWprefxdec-1          0.30

THWprefxdec-2          0.67

MAXBRFO          THWpref                     0.76

THWprefxTHW           1.15

THWprefxdec-1          0.02

THWprefxdec-2          0.10

NRCOR                THWpref                     4.47 *

THWprefxTHW           0.45

THWprefxdec-1          8.18 **

THWprefxdec-2          4.03 *

 

 

There were no significant main effects of THWpref group on RT, BIMT, BCMT and MAXBR­FO. Also none of the interactions of THWpref with the independent factors reached significan­ce on any of these dependent variables. This means that these results do not support the hypotheses mentioned in the introduction. However, NRCOR, the number of movement corrections during the closed-loop phase, was significantly affected by THWpref group and revealed significant interactions of THWpref group with initial and secondary deceleration, see figure 2. The effects on NRCOR were as follows: only for the group of short followers was NRCOR affected by dec-1 (F(10,1)=20.65, p<0.001) and by dec-2 (F(10,1)­=12.08, <0.006). For the group of long followers, the effects of both dec-1 and dec-2 on NRCOR were not significant (F(10,1)=0.05, p<0.822 for dec-1 and F(10,1)=0.46, p<0.512 for dec-2).

 

These effects strongly indicate that long followers moved their foot directly to the maximum brake position, irrespecti­ve of the development of criticality in time, while short followers were more sensitive to the manipulations of initial and secondary deceleration on this measure.

Figure 2. NRCOR as a function of THWpref group, initial deceleration (dec-1)

and secondary deceleration (dec-2).

 

 

8.4 Discussion and conclusions

 

Based on a number of previous experiments it was tested whether short followers differ from long followers in both the open-loop and closed-loop phases of the braking process. This was tested by manipulating these phases. The open-loop phase was manipulated with two levels of initial deceleration of the lead vehicle. After the brake was touched by the subject, the deceleration of the lead vehicle changed. This secondary deceleration manipulated the closed-loop phase of the braking response. The hypotheses were:

1) there is an interaction between following group and primary deceleration on the duration of the open-loop phase (BIMT), defined as the interval between the moment the foot is released from the accelerator pedal and the moment the foot touches the brake pedal. This would support the idea that short followers differ from long followers in the open-loop phase.

2) there is an interaction between following group and secondary decelera­tion on the maxim­um brake force excerted, the number of movement corrections during the closed-loop phase and the duration of the closed-loop phase (BCMT), defined as the interval between the moment the foot touches the brake pedal and the moment the maximum brake force is excerted. This would support the idea that short followers differ from long followers in the closed-loop phase.

In general, these hypotheses were not supported. There was no signifi­cant interaction between following group and any of the independent factors, initial THW, primary and secondary deceleration, on RT, BIMT, BCMT and the maximum brake force. However, the interaction between following group and initial deceleration on the number of movement corrections was significant as was the interaction between following group and secondary deceleration on this variable. The number of movement corrections (NRCOR) during the closed-loop phase were conceived as an expression of uncertainty induced by a change in deceleration after the braking response was initia­ted. Although NRCOR was affected by secondary deceleration, it was also affected by primary deceleration. The pattern of effects suggests that NRCOR expresses the necessity to move the pedal straight to the maximum without hesitation. The results showed that only the group of short followers was sensitive to the effects of initial and secondary decelera­tion on NRCOR, while the long followers moved their foot to the maximum with the same number of movement corrections independent of primary and secondary deceleration. In a previous study it was found that NRCOR strongly determines the duration of the closed-loop phase. From this perspective, it would be expected that NRCOR and BCMT are affected by following group and the independent factors in a similar way. However, as was already apparent, there were no significant effects of following group on BCMT. Closer inspection of the data revealed that only in the trials where the secondary deceleration was high, the correlations between NRCOR and BCMT were significant, see table 6.

 

Table 6. Correlation between NRCOR and BCMT, depending on THWpref

group and secondary deceleration (dec-2).

 

dec-2

  3              6         

short                      0.30          0.68 **

long                       0.30                   0.59 **

 

 

This suggests that a causal relation between NRCOR and BCMT only exists if criticality is high enough.

 

The lack of support for the hypotheses may have been caused by specific task related factors. The subjects generally described the task as boring, mainly because of the long task duration and the low event-rate. There were only 8 braking trials over a duration of 45 minutes. This may have resulted in a vigilance task with two separate effects. On the one hand, the braking trials may have generated startle reactions, resulting in fast responses irrespective of the manipulations. On the other hand some responses may have been slow because of state-related factors. This would have resulted in a high variance in the data that was not caused by the manipulations of THW, initial and secondary deceleration. Figure 4 illustrates the distribution of RT as a function of the THW manipulation. It can be seen that the distributions are skewed on the right side, especially for the THW=1.2 condition, sugge­sting low-vigilance effects, although there are two distinctive peaks in the distributions.

 

Figure 4. Distributions of RT as a function of the THW manipulation.

 

Figure 5 and 6 show the distributions of BIMT as a function of initial deceleration for THW=0.8 and THW=1.2 respectively. These figures show that the distributions of BIMT are skewed on the right side and that the effects of initial deceleration are mainly caused by ‘outliers’ on the right side of the distributi­on. Especially for the THW=0.8 condition, the primary peak of the low deceleration condition occurs before the primary peak of the high deceleration condition, which obviously is not in the expected direction and opposite to the effects of the analyses of variance, which are based on the means. The distributions of BIMT in the THW=0.8 condition are visualized separately for the short followers and long followers in figure 7.

 

It can be seen that for the short followers there are two distinctive peaks as a function of initial deceleration in the expected direction, while for the long followers the primary peaks overlap. Moreover, it can be seen, that the primary peak in the BIMT of long followers occurs before the peaks of the short followers. This suggests that BIMT of long followers has suffered more from startle reactions resulting in BIMTs that were fast and not tuned to differences in initial deceleration, while the BIMTs of short followers were more sensitive to initial deceleration.

Thus, the distributions of the data and task-induced startle responses and low-vigilance effects may have given results that failed to support the hypotheses. This will be tested in the next experiment, with multiple measurements per manipulation, a higher event-rate and shorter task duration in order to prevent undesirable state-related effects.

Figure 5. Distribution of BIMT as a function of initial deceleration for the THW=0.8 conditi­on.

Figure 6. Distribution of BIMT as a function of initial deceleration for the THW=1.2 conditi­on.

Figure 7. Distribution of BIMT as a function of initial deceleration for the

THW=0.8 conditi­on, for short and long followers.

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