Drivers ’reaction time research in the conditions in the real traffic

2.1 The need to test the reaction time

In manuals and training materials for forensic and traffic experts, data on drivers’ response times is one of the basic data. In many publications, their values are presented, often with significant differences. Meanwhile, as mentioned earlier, the final ruling on the possible guilt of the driver may largely depend on what values will be used in the analysis by the court expert. These differences are often due to different test methodologies (tool - test environment, test methodology, number and "composition" of the group of subjects, presentation of results). The assessment of response time in psychotechnical tests is one of a set of tests carried out in psychological laboratories for the general assessment of the test person’s ability to drive. These tests are characterized by the methodology developed for many years and the way of evaluating their results, e.g. [10]. In the case of reaction time, so-called reflexometers are used. The response time is assessed as the period from the appearance of a given light or sound stimulus to pressing the appropriate button on the desktop. In the case of data from experiments on the road or test track, most often these are the results of tests of the reaction to the so-called simple stimulus (a single light or sound signal), while the way the driver’s reaction is also simplified - it is supposed to act on one of the car control elements (service brake pedal, hand brake lever or steering wheel) [1, 2, 9, 15, 16].

The results of such studies are often published as a recommendation to experts, e.g. [1, 17]. In real road situations (apart from driving, e.g. in a column on a motorway, where the reaction occurs to the "stop" light of the preceding car) the driver reacts to complex stimuli. However, in the literature from 10-15 years ago, it is difficult to find data on reaction times in which both the stimulus and the driver’s response are complex (similarly to real-life accident situations). So far, studies on the response to complex stimuli have been carried out, but these have often been highly simplified situations. For example, in the studies [2, 9], stimulator lamps glued to the windscreen of a car were used to study the response to a complex stimulus. In recent years, studies on roads or tracks have increasingly relied on the implementation of contractual accident scenarios, which were considered representative. The authors in their earlier works [3, 4, 14] presented such tests. The "observation" type tests in real traffic conditions should also be mentioned. These tests most often consist in the analysis of recordings from cameras (e.g. monitoring) placed near the roads [11]. However, the result of the assessment (reaction time) here is strongly dependent on the arbitrary assumptions of the observer as to the time of the initial threat situation, and thus also the stimulus. The development of simulation techniques, increased efficiency of computers and systems to generate images has made it possible to use a virtual environment for testing drivers - driving simulators. They are increasingly used in driver research. Their use increases the independence from weather conditions and favours the increase in repeatability of research conditions and results [3, 10]. It also enables the implementation of practically any accident situation scenarios in a way that does not endanger safety.

Moreover, the authors’ experiences indicate a strong correlation of research results in the simulator and in the real car on the test track [3, 5]. Summarizing this brief review of the literature, it can be concluded that research is necessary to determine the response times not to a stimulus or simple stimulus system, but to some simulated accident hazard situation. The number of results available in the literature for this type of tests is small and includes selected special cases.

2.2 Stopping distance

The stopping distance is an important parameter affecting the safety of vehicle traffic and the legal regulations concerning permitted speeds in particular areas. Its components include the distance covered by the vehicle with the same speed during the driver’s reaction time and the distance covered by the vehicle from the brake pedal depression to the actual stop (braking distance) [8]. The total stopping time of the vehicle is the sum of the driver’s reaction time and the braking time, which includes [6]:

• brake application delay time t₀ - time between the foot position on the brake pedal and the occurrence of braking force;

• increasing deceleration time t_n - the time that elapses between the driver applying the brake pedal and the full braking force of the braking system;

• time of full braking t_h - occurs from the moment when the braking system reaches full braking force to complete stopping of the vehicle.

The course of the braking process in total stopping time shall, taking into account the parameters of brake pedal force, deceleration and speed. This graph shows the change of individual parameters during the total stopping time, dividing it into individual components of reaction and braking times.

2.3 Driver response time

The driver’s response time is critical to the braking process of the vehicle and therefore also has a significant impact on safety in critical situations. It can be defined as the time passing from the moment when a hazard occurs to the moment when the driver takes action to avoid it, by braking and/or turning the steering wheel [1]. When analysing the driver’s response time, when the hazard occurs, one should distinguish [6]:

• perception time t_r1 - the period between the moment the hazard appears in the driver’s field of vision and the moment the driver focuses his attention on it and recognizes it;

• basic psychological reaction time t_r2 - includes the time of analysing the situation by the brain and making a decision about the type of reaction (braking, bypassing an obstacle) and sending a signal by the nervous system about the start of the action to perform the manoeuvre (the beginning of foot movement in order to apply the brake pedal);

• t_r3 foot transfer time - the time of the motor reaction consisting in transferring the foot from the accelerator pedal to the brake pedal.

The sum of the perception times (t_r1) and the fundamental psychological reaction times (t_r2) shows the mental reaction time; the transfer time (t_r3) shows the motor reaction time. When considering the driver’s response time, the total psychomotor response time, being the total period from the onset of danger to the transmission of the foot from the accelerator pedal to the brake pedal (t_r = t_r1 + t_r2 + t_r3), should be taken into account [6].

The driver’s response time parameter at the moment of danger has a direct impact on the safety of the driver and other persons involved in the accident. According to police reports from the years 2005-2015 [19], about 80% of accidents are caused by the drivers. The response time is an individual parameter for each driver and depends on many factors such as experience, fatigue and stress. The value of this parameter has a direct impact on safety when an accident occurs - often the degree of risk to the health and life of accident participants depends on the speed of the driver’s reaction. This issue was described in detail by J. Unarski in the article "Driver’s reaction time - standards and reality". [13]. He discussed the study of reaction time on the measuring track with the use of three different stimuli. The results of the tests in the form of reaction time intervals are presented in Table 1. The reactions to [13] have been taken into account:

• a simple stimulus in the form of a lamp placed within the driver’s field of vision;

• complex stimulus, expected stimulus - drivers were informed that at some point during the tests there will be a signal;

• complex stimulus, unexpected - the drivers did not know that there would be a signal while driving.

Reaction time studies are justified in the field of accident reconstruction. In calculations carried out by car technology experts and forensic experts, the adopted response time value may determine the results of the analysis of the circumstances of the event. The performed analysis influences the court’s decision on the guilt or lack of guilt of the driver [3]. In the accident investigation, the braking time can be determined from known, constant and quantifiable parameters, while the psychomotor response time is an individual value for each driver, so the calculations should use the average value adopted on the basis of experience and research conducted by experts. An example of the formula used in the tests is the one presented in the book "Road accidents. Vademecum of the court expert". [12], showing in a simplified way the individual phases preceding the braking process of a passenger car (Figure 1).

Figure 1

Diagram of the driver’s reaction to the braking manoeuvre of the vehicle [12].

Table 2 summarizes the times corresponding to each of the stages shown in the diagram. The quoted concept of "basic time loss" corresponds to the sum of the times of individual phases of the braking process - from the moment of recognition of an object to the beginning of the significance of braking tracks. Adding the perception time to this parameter, the concept of "basic time loss with peripheral perception" was obtained.

2.4 Time delay build-up

The build-up time of the deceleration is one of the components of the stopping time and has a direct effect on the stopping distance. It can be defined as the time elapsing from the moment when the braking force is applied to the moment when the braking system reaches the value of deceleration expected by the driver or the full braking efficiency resulting from wheel lock-up [6]. The value of the rise time of the deceleration depends on the type of braking system. Examples of measured values for pneumatic and hydraulic systems, together with the name of the author of the tests, are presented in Table 3.

2.5 Full braking time

The full braking time is the last component of the stopping time, covering the period from the moment when the vehicle reaches the expected deceleration to the moment when the vehicle comes to a halt [6]. The distance covered by the vehicle during full braking is called the braking distance, the length of which depends on the length of the braking distance:

• the initial speed of the vehicle;

• the type of surface;

• the condition of the surface;

• the state of repair of the vehicle.

An important parameter in the braking process is the coefficient of adhesion. It is characterized by the properties of different types of tyres cooperating with surfaces of different types and condition. As the coefficient of adhesion decreases, the braking distance of the vehicle increases, so this parameter has a real impact on the safety of this manoeuvre. Average values used in the analysis of road events are presented in Table 4.

2.6 Factors worsening the reaction time

The driver is exposed to a number of distractions while driving. This can lead to an accident situation that endangers the driver, passengers and bystanders. Lack of focus on driving is associated with a loss of control over the perception of the road scene. Any additional activity can distract the driver, which, when a hazard occurs, can significantly increase the response time and lead to an accident. The most common distractions are:

• talking to the passengers;

• controlling the behaviour of transported child/animals;

• use of a mobile phone;

• operation of devices in the car (air conditioning, radio, navigation);

• stress;

• alcohol;

• malaise or ill-health;

• food or drink while driving;

• road surroundings;

• reverie;

• looking for something in your car or environment;

• stress from inexperience in driving.

Distracting the driver’s attention from the road scene delays the response time and increases the overall response time. A distraction can be considered as a distraction:

• a diversion of the pattern from the road,

• a picture of the hands on the steering wheel,

• stop consciously thinking about driving a vehicle [12].

Examples of differences in response times in different dangerous traffic situations when using a mobile phone and without one are presented in Table 5.

Averaging the values of the reaction time presented in Table 5, the average difference of 0.43 s was obtained. Taking into account the permissible speeds on Polish roads, it is easy to determine the distance a vehicle will travel before the reaction time of a driver talking on a mobile phone. These values are as follows:

• in the built-up area with a limit of 50 km/h - 5.98 m;

• in an un-built area with a limit of 90 km/h - 10.75 m;

• on the expressway with a limit of 120 km/h - 14.33 m.

As the speed increases, the distance travelled by the vehicle increases before the driver reacts to the emergency situation and, consequently, the total stopping distance of the vehicle increases. Any driver distraction can therefore have a significant negative impact on safety and reduce the chance of accident avoidance or mitigation.

2.7 Driver experience

One of the main factors influencing the driver’s behaviour while driving is his or her experience. It sets out all the skills and knowledge acquired between the time of obtaining the driving licence and the present. Together with the experience gained, the driver improves his or her judgmental skills, improves his or her driving technique and learns to respond to emergencies. In an accident, the driver’s experience can be crucial to the driver’s progress and the way and timing of his response will help to reduce the damage involved.

Drivers’ experiences cannot be clearly divided, as they are influenced by several factors. The main criteria taken into account are the duration of the driving licence and the number of kilometres driven. The difficulty in determining the distribution parameters lies in the lack of dependence between these criteria - both are individual for each driver.

When considering the experience of drivers, “young drivers” are the most frequently mentioned group. In Poland, they include persons aged 18-24. This is the most numerous age group of victims in the annual reports of the National Road Safety Council [18].

Drivers in this age group have many features that may have a negative impact on safety, which may disappear with the acquired experience. Such features include:

• overestimating one’s own skills;

• the desire to impress others;

• lack of driving skills;

• looking for excitement;

• inadequate/error risk assessment;

• no learned reflexes, too slow a response’

• lack of self-confidence and/or driving stress;

• fixation of incorrect behaviour caused by the lack of a person suggesting errors.

With the acquisition of experience by the driver, such characteristics may weaken or even disappear altogether, being replaced by positive characteristics. Drivers with longer experience already know their capabilities and feel more confident behind the wheel, and their driving technique improves as well. Experienced drivers are more knowledgeable and able to cope with difficult situations, and some activities are carried out with a reflex without having to think about how to react.

Assuming that the driver’s response time decreases with experience, the impact of age-related psychophysical changes should be taken into account. Such changes may include, but are not limited to, poor perception or delayed reflexes, which may result in longer response times for older drivers. This leads to a situation where one of the parameters shortens the response time while the other one lengthens it. This means that up to a certain age these values may be tolerable [4].

3.1 Measuring system

In the research carried out, a time measurement station was used. It is constructed in such a way that it is possible to conduct tests according to various measurement scenarios and to install them in any vehicle in such a way that it is possible to conduct tests.

The measurement system consists of three main components:

M9S Dual Cam dual channel video recorder (Figure 2) - two lenses recording synchronized images of equal resolution and frequency; the length of the cable connecting both lenses and the power cable, amounting to 6 mand 3.5 m respectively, allow for almost any location of the lenses in the vehicle; the main technical parameters:

Figure 2

Lens set with the necessary cables [20].

• resolution: 1920 × 1080;

• the frequency of reprimands: 30fps;

• angle of view: front 170 degrees, rear 140 degrees;

• compound Stimulus Indicator (Figure 3) - two green and red LEDs, mounted on a flexible cable, which allows to be mounted anywhere in the car - together or separately,

• call buttons (Figure 4) - separate for each colour, mounted on a long and flexible cable, this allows the control person to occupy any position in the vehicle.

Figure 3

Compound stimulus siren [18].

Figure 4

Light signal call control buttons [18].

3.2 Testing location

The place chosen for the research was a car park around the Arena Lublin stadium, located between Stadionowa and Lubelski Lipca ’80 Streets. A large space with a good surface allowed the research to be carried out at speeds comparable to real city driving. The lack of other traffic participants was conducive to the safety of conducting the research and free riding on the parking space and performing any manoeuvres related to the maintenance of traffic.

3.3 Respondents’ group

15 drivers of different ages and sexes, marked with letters A to O, took part in the research. The number of kilometres driven was not taken into account, but it was assumed that the persons surveyed were active drivers. The shortest time of possession of certificates was 2 years for driver A and the longest - 11 years for driver O. The universal design of the measurement station allows to install them in any vehicle.

3.4 Methodology for conducting research

The test method closest to the test method quoted above is the one on roads or test tracks, but with greater emphasis on the method of reaction time measurement itself. The research was carried out on the basis of free driving within the research area and performing any manoeuvres. The route was not imposed, so the driver focused on driving, instead of reconstructing the route. The test site allowed the driver to drive at a speed corresponding to urban traffic of approx. 40 km/h. While driving, the person sitting in the passenger seat at any time triggered one of the light signals in red (Figure 5) or green (Figure 6).

Figure 5

Red light signal [18].

Figure 6

Green light signal [18].

The task of the tested driver was to react to the red signal by shifting his foot from the accelerator pedal to the brake pedal, braking the vehicle (not to a complete stop) and then continuing driving. In order to make it more difficult, a composite signal in the form of an additional green light (stimulus) was introduced, on which the driver was not supposed to react. The measurement ride lasted about 5 minutes, during which the cameras recorded at least 10 correctly performed measurements, i.e. those in which the driver held his foot on the accelerator pedal when the red light was on, then he put it on and pressed the brake pedal. Samples that did not record the full braking sequence were not taken into account.

3.5 Methodology for analysing the results

Analysis of measurement materials recorded during the research was conducted using Lightworks version 12.6.0. It is a freeware program for editing video files. It was used to open two synchronized video files from both lenses at the same time (Figure 7). The next step was to find on the material the moment when the red light turned on and play it back frame by frame. An important stage was to count the number of frames passing from the signal to the first move ment of the foot and then to the position of the foot on the brake pedal. Knowing the camera recording frequency of 30 fps, the next step was to divide the number of frames by 30. This way the perception time of t_r1 and the time of transfer of t_r3 were obtained. The sum of the results obtained is the total response time of the driver. Calculations were made for each sample and the results were rounded to 0.001 s.

Figure 7

Example of a screenshot from a sample analysis program with the selection of relevant areas [18].