Human Factors in Driving Task: Safe
Distance Maintenance


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Table of Contents

1     Task Description. 3

2     Collisions Associated with Safe Distance Driving. 4

3     Task Analysis- Keeping Safe Distance. 5

3.1      Sub-Tasks. 5

3.2      Required Information. 6

4     Human Factors and Potential Errors. 8

4.1      Vision. 8

4.2      Cognition. 9

4.3      Motor function. 9

5     Countermeasures. 10

5.1      Advisory Signs. 10

5.2      Road Chevrons. 11

5.3      Intelligent Transport Systems. 11

6     Summary. 12

7     References. 14



Human Factors in Driving Task: Safe
Distance Maintenance

Task Description

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 closely 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) classifies 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 end 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.  


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,
Várhelyi, & Dalla Fontana, 2011). In these cases, a change in speed
for the driver in front may compel and 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, Várhelyi, & 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, Várhelyi, & 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

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 on 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
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 their compromising the safe distance (Fuller, 2005).   

Motor function

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 collision.

Advisory Signs

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 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

Road Chevrons

Another counter measure 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
travelling 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 on 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

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 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



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