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Road User Characteristics in Traffic Engineering

road user characteristics

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Road users play an important role in studying traffic engineering. They take up various roles like drivers, pedestrians, cycles etc and assess the conditions in their own unique way. The physical, mental and psychological factors are the main road user characteristics that govern the safe operation of the vehicle on the road.

Described below are the important road user characteristics studied in traffic engineering.

Road User Characteristics

The important road user characteristics are:

  1. Physical Characteristics
  2. Mental Characteristics
  3. Psychological Characteristics
  4. Environmental Characteristics

1. Physical Characteristics

Vision, hearing, strength and the reaction of the road user are the important physical characteristics that are studied in traffic engineering.


The field of vision includes acute or clear vision cone, fairly clear vision cone and peripheral vision cone as shown in the figure below.

Fig.1. Field of Vision
  1. Acute or clear vision cone: This vision is around 3 to 10 degrees around the line of sight.
  2. Fairly clear vision cone: This vision is at 10 to 12 degrees around the line of sight. A vision in this field can identify the color and shape.
  3. Peripheral Vision: This field of vision extends up to 90 degrees to the right and left of the centerline of the pupil, as shown in figure-1. It also extends above and below the line of sight by 60 and 70 degrees respectively. A peripheral vision does not show stationary objects. It only detects the movement of objects within this field.


Hearing characteristics is more important for pedestrians and cycles compared to drivers.


This is a less important characteristic compared to others. The lack of strength makes it difficult to park heavy vehicles.

2. Mental Characteristics

The important mental characteristics of the road user include skill, intelligence, experience, knowledge and literacy. Proper knowledge of the vehicle characteristics, driving practices, rules of roads and traffic behavior is necessary for safe traffic operations.

3. Psychological Factors

Psychological factors affect the reaction of the road user to traffic situations. These reactions include anger, fear, superstition, impatience, anxiety, etc. This affects traffic performance.

4. Environmental Factors

Atmospheric conditions, traffic facilities, the traffic stream characteristics are the various environmental factors that govern the behavior of the road. The traffic stream can be heavy traffic or mixed traffic which may limit the ability to overtake. Hence, the behavior of the road user changes with the traffic streams.

Also Read: Vehicle Traffic Volume Counting

Driver Characteristics

The two main driver characteristics are visual activity and reaction process. Other characteristics include hearing, personality, the psychology of the drivers.

Visual Activity

In visual characteristics, the field of vision explained before is of primary consideration. From figure-1, with the increase in speed, the peripheral vision field narrows down as 100 degrees at 20mph and to 40 degrees at 60mph.

The field of vision decides some of the traffic engineering practices and functions like:

  1. Traffic signs are placed within 10 degrees range with the expected line of sight of the driver.
  2. The size and dimension of the traffic signs and boards are adjusted to this distance.
  3. The driver's attention must be taken for an event happening within the field of vision.
  4. Estimation of speed is also performed depending on the field of vision.

Important visual deficits like peripheral vision deficits, depth perception deficits, color blindness, cataracts, glaucoma etc also come under visual driver characteristics.

In case of green or red color blindness, blue and yellow pigments are added to green and red lights respectively in traffic signals to overcome this difficulty.

Reaction Process - Perception Reaction Time (PRT)

In the reaction process of a driver, the important characteristic is the perception-reaction time (PRT). This is the time taken by the driver for detection, identification, decision, and respond.

Detection: This phase is about being aware of an object when it has entered the driver's field of vision.

Identification: This phase involves acquiring sufficient information about the object or the new condition encountered to bring a proper response.

Decision: After identifying the condition, the situation is analyzed so that a proper decision is made.

Response: After deciding, a proper response is implemented by the driver.

PRT is also called as PIEV ( Perception Interpretation Evaluation and Volition). The four main factors that affect PRT are age, fatigue, the extent of the complexity of reactions and the presence of alcohol or drugs.

As per AASHTO,

  1. PRT for braking reactions on highways = 2.5 seconds
  2. PRT for reaction time to traffic signal = 1 Sec

Reaction distance is a critical impact of PRT. Reaction distance (d)Β is defined as how far the vehicle moves between the time he decides to apply the brake to the time he actually applies the brake. It is given by the formula:

d = 0.278 St

Where, the reaction distance is given by 'd' in meters, reaction time is given by 't', 'S' is the initial speed of the vehicle in km/h.

Also Read: Sight Distance in Highway Engineering

Pedestrian Characteristics in Traffic Engineering

Improper interaction between the vehicle and pedestrian forms the most critical safety factor in highway and streets. Generally, the walking speed of a pedestrian is 1 to 1.2m/s for 85%.

Gap acceptance is the distance between the pedestrian and the vehicle approaching at a time when the pedestrian starts to cross the road. The gap acceptance is taken normally as 35m. The speed of the vehicle approaching, the width of the street, waiting time, etc are the factors that govern the gap acceptance.

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