Human Factors in Driving Task: Safe Distance Maintenance

While people engage in driving as a daily task, it involves complex processes of information processing and imposes heavy cognitive demands due to its dynamic nature. The dynamism occurs due to the change in environmental conditions with which the driver must contend in the course of their activity, as well as the variations in their internal nature during specific task performances. One of the most seemingly simple tasks in driving is the maintenance of safe distance between one vehicle and that ahead of it. However, indications have been that drivers often fail in the execution of this task due to factors on the road or those emanating from their human nature (Mazureck & Hattem, 2006).

Drivers in many cities have to contend with matters of traffic jams, especially in the peak hours of the morning and in the evening. The traffic often results in many drivers exhibiting tailgating, a practice whereby the driver behind drives too close to the driver in front (Song & Wang, 2012). Global recommendations for safe distance driving are of about 3 seconds, whereby 30 meters is considered the equivalent of a second (Mazureck & Hattem, 2006). The rule changes depending on the road conditions, increasing to four seconds in cases when the road is unclear due to conditions like fog, and ten seconds for roads tainted by snow (Mazureck & Hattem, 2006). Regardless, drivers will often overlook these recommended distances and especially with pressure from traffic. These driving tendencies have multiple motivators, including the perceptions of lateness and the illusion that the safe distance space is a waste of time, distractions, or even the desire to demonstrate subtle aggression while driving on the road (Song & Wang, 2012).

This paper focuses on analyzing the human factors that potentially interfere with the maintenance of safe driving distances. Understanding these factors is integral to understanding the possible way in which rear-end collisions may be mitigated on the highways.

Collisions Associated with Safe Distance Driving

Sarkar et al (2000) classify following a vehicle with insufficient headway as a severe form of aggressive driving. According to the typical reaction time of a driver, the driver is advised to maintain a distance of more than two seconds between their vehicle and the vehicle ahead. Any performance of this task that is within less time than this, depending on the speed of the moving vehicle, is likely to result in a road collision (Sarkar, Martineau, Emami, Khatib, & Wallace, 2000). This is an especially considerable challenge to road safety, considering statistics that indicate approximately 18-20 percent of the global incidence in crashes involved multiple vehicles moving in the same direction, not at an intersection (Mazureck & Hattem, 2006).

The most common accident occurring from this type of driving task is rear-ended collisions. According to data from the National Center for Statistics and Analysis (NCSA, 2010), rear-end crashes accounted for more than 30% of the 5.9 million accidents in the US between 2006 and 2008. These accidents resulted in about 2200 fatalities and about 500,000 injuries annually (National Center for Statistics and Analysis, 2010). While tailgating is only accountable for about 70% of these automobile collisions, the factor was also recorded as bearing the most prominent fatalities compared to the other rear end collision factor- inattention. As such, safe driving distance is identified as a critical source of rear-end collisions for automobiles, especially in the USA. As the number of vehicles on the road continues increasing and the periods when the roads encounter heavy traffic increase, the following distance for vehicles on the road becomes a significant task to understand and consider in the design of highways.

Task Analysis- Keeping Safe Distance

Maintaining a safe distance between a vehicle and the vehicle ahead is a relatively uncomplicated task. Nevertheless, there are particular actions in which the driver must engage in order to successfully maintain its accomplishment.

Sub-Tasks

• 2-3 second rule: the driver is expected to maintain a consistent difference of between 2 and 3 seconds between them and the vehicle that precedes them. This aspect requires deliberate visual scanning, whereby the use of stationary landmarks is effective. In this case, the driver should mark the point at which the previous vehicle passes a landmark, and between then and the moment when they pass the same landmark there should be a difference of about 3 seconds (Knipling, et al., 1993). It may be necessary, however, to increase this time when the weather conditions are unfavorable to the degree of increasing the required stopping distance (Knipling, et al., 1993).
• Obstacles: the driver also requires to visually scan the road for potential obstacles both to their driving or the driving of the vehicle in front of them. Potential obstacles include possible pedestrians, debris, or upcoming intersections that may force the driver in front to slow down (Adell, Verhelyi, & Dalla Fontana, 2011). In these cases, a change in speed for the driver in front may compel an equal change in speed in order to maintain the safe driving distance. Distracted drivers may lack the capacity to effectively scan the environment, thereby failing to identify potential obstacles that could end up decreasing their distance (Adell, Verhelyi, & Dalla Fontana, 2011).
• Consideration for vehicular characteristics: the characteristics of the vehicle in front of the driver are also crucial for their determination of the safe distance to maintain. Vehicular characteristics could include heavy vehicles like lories, or different forms of automobiles like motorbikes on the same roads (Knipling, et al., 1993). For heavy vehicles, the stopping distance tends to be lengthier. At the same time, some of the lories are fitted with instant breaks, in which case these will often be indicated at the back. Vehicular characteristics may also involve faults in the vehicle ahead, such as break-lights that do not function. In the case of the latter, the driver must remain conscious enough to scan the break-lights of the vehicle ahead of the immediate one, which they will use as a basis for adjustment of speeds and the maintenance of the safe distances (Song & Wang, 2012).

Required Information

The performance of the specific sub-tasks by the driver towards accomplishing safe distance maintenance relies on the access to a given set of information. As with all driving tasks, the perception-reaction time combines with the maneuver time to define the sight distance (Song & Wang, 2012). The capacity of the driver to process information, therefore, determines their ability to complete the task or the specific sub-tasks efficiently.

• Meaning of road signs: Some of the critical information that the driver requires is the understanding of the meaning of road symbols. Drivers typically operate in an environment whereby symbols are the most common forms of communication. Under the assumption of adequate visual capacity, the remaining components are of the understanding of the symbols they encounter along the road (Adell, Verhelyi, & Dalla Fontana, 2011). For instance, pedestrian signs will alert the driver of the possible change in road driving conditions such as speeds, and the implications these changes have on the current safe distance requirements. The driver also gains the need to look out for pedestrians as obstacles that could increase the possibility of a rear-end collision where the safe distance is not maintained.
• Driver behavior: the driver on the highway also requires information on the behavior and condition of the other drivers on the road. Driver characteristics such as unnecessary aggression or distraction could compromise the capacity of the rest of the drivers for maintaining safe distances (Song & Wang, 2012). Information on the degree of driver attention of the vehicle ahead may compel the one behind to keep wider distances between them. At the same time, flashing taillights by the driver ahead may indicate a degree of irritation or indications they feel the one behind is too close. Consequently, the driver has the obligation to constantly assess this driver information, using it to monitor and negotiate consistently safe distances.
• Environmental or weather implications: additional information requires by the driver is on the current weather or the environment. Some weather conditions vary across regions, often changing abruptly, such as rain or hail or even sections of snowy roads. Driver information on the weather on the road is crucial, as sudden encounters of weather changes will always compel a change in the required safe distance between the vehicles. Constantly monitoring for slippery roads will also guide choices on when to vary the speed and the resulting implications on safe distance (Mazureck & Hattem, 2006).
• Current speeds: driving on multiple types of roads tends to have different characteristics, especially where the speeds are concerned. Drivers on highways may be driving at higher speeds than on smaller scale roads, which determines the perception of safe distances that drivers should possess (National Center for Statistics and Analysis, 2010). Understanding the current speed enables the driver to make mental computations of the total stopping distance and, consequently, the required safe distance between them (Knipling, et al., 1993). Indications have been that driving at high speeds requires more reaction times from the drivers even where the perception time may not change. For instance, while the average reaction time is only about 0.75 seconds, the reaction time at 80mPh is about 1.5 seconds (Knipling, et al., 1993). Awareness of the current speed of the vehicles on the road, therefore, is critical information to guide the determination of the safe distance between the vehicle and the one ahead.

Human Factors and Potential Errors

Driver actions may be subject to errors, which will often either have severe consequences or near-miss reports. According to the Indiana Tri-State Level study, indications were that human factors account for about 93% of road driving errors, with environmental and vehicular factors only influencing about 34% and 13% of these incidences respectively (Treat, Tumbas, & McDonald, 1979). Considering the factors in their individuality, without cause overlap, human factors account for about 57% of errors resulting from road accidents (Treat, Tumbas, & McDonald, 1979). Human factors are divided into cognition, vision, and motor function. Their expression determines the perception, decision-making, and reaction time that the driver exhibits.

Multiple aspects connected to vision may impair the capacity of the driver to maintain safe driving distance. Evidence indicates that errors in the distance maintenance emanate from problems emanating from the visual processing of dynamic information (Song & Wang, 2012). For instance, the capacity of the driver to assess central movement in depth- as in the judgment of vehicles slowing ahead- may compromise their ability to maintain safe distances. The visual acuity of the driver may also be compromised, making it difficult for them to read road signs while they are in motion (American Association of State Highway and Transportation Officials, 2010). The result is that the driver ends up changing speeds later than they should have, severely compromising the distance between the vehicles.

Another cause for errors in the performance of maintaining safe distance is the range of cognitive influences. Cognition, in this context, may vary to include inattention and divided attention, vigilance, and memory (American Association of State Highway and Transportation Officials, 2010). Divided attention implies the driver is monitoring multiple tasks at the same time, such as eating while also driving. While eating may be a trivial task, evidence has indicated that such task ranges tend to interfere with the driving activity by about 350 milliseconds (Levy, Pashler, & Boer, 2006). Consequently, where a driver is engaged in eating while driving, they may fail to slow down in time to maintain the safe distance when the vehicle in front of them breaks, even if not suddenly.

At the same time, vigilance and memory are critical cognitive factors that determine the occurrence of errors in maintaining safe distances. Where a driver has been on the road for some time, the attention they devote to the driving task is limited (Fuller, 2005). This driver may be drowsy and fail to notice when they come too close to the vehicle in front of them. Memory, long-term, and short-term, also acts as a basis for error. Drivers in familiar routes can anticipate features such as road maps, and where they fail to remember the breaking of vehicles in front of them may cause they're compromising the safe distance (Fuller, 2005).

Errors in maintaining safe distance may also occur due to compromised motor skills. Common evidence indicates that motor skills become compromised with age, diminishing the simple reaction time for an individual (Dewar, Olson, & Alexander, 2007). Therefore, where an elderly driver is required to adjust their speed in response to an unexpected event in order to keep the safe distance, they will often fail to accomplish this task due to slower motor reflexes. The result will often be, even if only momentarily, the driver ending up tail-gating the vehicles in front or even causing a collision.

The errors occurring on the road regarding maintaining safe distance emanate from relying on the judgment of the drivers to determine the adequacy of the headway. However, there are specific engineering and road construction solutions that could avert or diminish the occurrence of these errors and the resulting incidents of the collision.

One of the possible countermeasures is the installation of advisory signs that either warn or advise against tailgating. The signs act as reminders of the proper distance to maintain, reducing the incidence of deviation among drivers. In a 1983 study at Ascot, Berkshire, the installation of an automatic warning sign helped reduce the incidence of drivers using the 1-second gap by about a third (Helliar-Symons, Wheeler, & Scott, 1984). The sign was automatically triggered when vehicles at the location had a less than the 0.7-second gap, but over time this gap was raised to between 1 second and 2 seconds (Hutchinson, 2008).

Alternative interventions have also included a mechanical and permanent sign, which reminds the drivers to avoid tailgating. This sign as applied in Tennessee, Memphis, involved a hand-held indication advising against tailgating. It yielded an increase in compliance by 13 percent (Hutchinson, 2008). Therefore, drivers that would otherwise be distracted, such as one that is eating while driving, will be forced to consciously perceive the expected modification in the safe distance.

Another countermeasure applicable in the situation is the presence of dots or chevrons on the road surface. These chevrons are installed at regular intervals, reflecting the ideal distance between vehicles at average speeds on the given road. Under the assumption of vehicles traveling at 60 miles per hour, and dots spaced about 80 feet apart, it will be possible for drivers to generate mental patterns for maintaining adequate headway. In this instance, the requirement would be that while each driver is on a given chevron, they can see two more between them to imply 180 feet apart. Consequently, considering the speed, the headway would be about 1.8 seconds (Hutchinson, 2008).

Studies have supported the use of the dots or chevrons as a countermeasure to road errors emanating from safe distance challenges. The trial of the same in the UK and French motorways successfully managed to improve the understanding of drivers on the correct headway that vehicles should maintain (Hutchinson, 2008). On the other hand, there have been disputes regarding the challenges that chevrons could impose. In some instances, they may act as distractions to the drivers. In other cases, their permanency hinders the flexibility that should accompany changes in speed and the resulting variation in headway (Song & Wang, 2012). Regardless, where these challenges are overlooked, the dots or chevrons do facilitate proper measurement of the headway for vehicles.

Intelligent Transport Systems

Hutchison (2008) proposes the use of intelligent transport systems as approaches to countering the errors of tailgating by drivers that are unaware. These systems could include advanced collision warning systems as well as cruise control to act as cost-effective measures against rear-end collisions. Several trials have been implemented, such as the Following Distance Warning system, but these have yet to be introduced onto the road (Hutchinson, 2008). Such systems are installed on the vehicle, allowing the driver behind to receive warnings when they drive too close to the vehicle in front.

However, the transfer of such systems to the actual application may prove difficult. For instance, the drivers may be irritated by the multiple warnings especially where their behavior is considered normal. Distances they may have perceived as safe may be highlighted as tailgating, which could influence the cognitive processes of other drivers on the road (Song & Wang, 2012). At the same time, the drivers of the vehicle behind may lack the knowledge to properly interpret warnings by the vehicle that is fitted with the system. However, as it is still an idea in progress, these shortcomings may be overcome by future developments.

Drivers have to maintain safe distances between them and vehicles in front. However, relying on the judgment of the drivers often results in tailgating, with the influence of traffic conditions causing compromised distances. Tailgating has been the basis for many rear-end collisions, a serious outcome considering the magnitude of these collisions both in the USA and globally. Consequently, understanding the events and information necessary to maintain proper headway is critical for all drivers. These forms of information involve details on vehicle speeds, the weather, and the conditions of vehicles on the road as well as those of other drivers.

Regardless, the capacity for human factors to induce errors in the maintenance of safe distances as a driving task is high. Indications are that the vision of the driver, their cognitive state, and their motor skills are critical determinants of efficiency. As such, distractions such as eating while driving will cause distance reductions and poor vision may result in limited judgment on the proper distance to maintain. Particular engineering countermeasures emerge in the form of advisory signs, the use of measurement chevrons, and intelligent driving systems. Advisory signs may be automated to alert specific drivers of compromised headway, or manual to act as reminders for drivers in traffic. On the other hand, chevrons allow measuring distance objectively by the individual drivers. The presence of intelligent driving systems will have the vehicle ahead issue warning signs when the driver behind comes too close.

Consequently, engineering modifications will ensure that the maintenance of safe distance is assisted by elements on the road. This aspect will eliminate the absolute dependence on the judgment of the driver to execute the safe distance between them and the next vehicle while on the motorway.