This document discusses human factors that influence road safety. Younger drivers have difficulty judging speed/distance and underestimate risks, while older drivers have slower reaction times. Both groups can compensate through safer driving behaviors. The driving task involves navigation, guidance and control, which can overload human processing abilities. Signs and road design should reduce demands on drivers and match expectations. Visual abilities decline with age, so signals must be conspicuous. Traffic elements should maximize legibility, comprehensibility and credibility of information for all road users.

1. Human factors in road traffic
Hossein Naraghi
CE 590 Special Topics
Safety
February 2003
Time Spent: 9 hrs

2. Human in the road traffic system
 Vital inputs to much of the road and
traffic engineering system
•Human performance
•Human capabilities
•Behavioral characteristics

3. Human in the road traffic
system(Continued)
 Problems of younger drivers
 Difficulty in judging speed, distance and
reaction time
 Tend to concentrate on near objects
 Missing the important information,
because its relevance is not understood
 Having poor perception of how
hazardous a situation can become

4. Human in the road traffic
system(Continued)
 Fixating the eyes on an object for a
longer period
 Having difficulty in integrating
information
 Under-estimating the risk of accident
involvement
 Making less effective driving decisions

5. Human in the road traffic
system(Continued)
 Problems of older drivers
 Difficulties in rapid decision making
• At intersections
 Take time to absorb traffic control
information
 Difficulty at night
• Lower light level
• Headlight glare
 Readily fatigued

6. Human in the road traffic
system(Continued)
 Older drivers are not over-represented
in crashes
 Tend to make adjustments in behavior
• Speed
• Route of travel
• Avoid congested areas
• Seeking longer gaps in traffic
• Time of day driving

7. Human performance
 Three key aspects of human
performance
 Information processing
 Visual characteristics
 Information needs

8. Information processing
 Driving task
 Comprising three essential tasks
• Navigation
• Trip planning and route following
• Guidance
• Following the road
• Maintaining a safe path
• Control
• Steering
• speed

9. Information processing
(continued)
 Some problems arising from both capabilities
of drivers and interfaces between driver and
other components of the road traffic system
in sequence of driving task
 Inadequate or insufficient input available for that
task
 Difficulty to handle extreme inputs or uncommon
events
 Process inputs too slowly
 When become overloaded,drivers shed part of the
input demand to deal with the more important

10. Information processing
(continued)
 Human beings have essentially a
single channel mind
 They must divide attention while driving
and process information sequentially
• If the rate at which decisions need to be
made (rate of input) exceeds the driver’s
capability (maximum rate of output), the
resulting stress could cause an error which
may in turn lead to a crash

11. Information processing
(continued)
 The road traffic system should encourage the
driver to shed the information which is not
immediately relevant to the driving task
• Listening to the radio
• Looking at scenery
• Engaging in conversation
 There is a need to have a balance between
input and output based on the human
capabilities
• e.g. discard irrelevant tasks if a new task is interposed

12. Information processing
(continued)
 Drivers can be assisted to adjust their driving
performance
 Provide trend information
• Series of signs on an approach to a freeway ramp
• Provide advance warning
• Directional instruction
 Avoid sudden imposition of demand
• Speed limit signs not at intersection itself
 Limit the amount of information on signs
 Signs which requires a series of simple decisions rather
than a single complex decision
• Usefully controlled turns at traffic signals, rather than
requiring drivers to select gaps in oncoming traffic

13. Driver expectancy
 Three types of driver expectancy
 Continuation expectancy
• Events of the immediate past will continue
• Road markings
 Event expectancy
• Events which have not happened will not happen
• Disregard of railway crossings
• Disregard minor intersections
 Temporal expectancy
• In cyclic events, the longer a given state occurs, the
greater the likelihood that change will occur
• Traffic signals

14. Driver expectancy(continued)
 Traffic design should consider driver
expectancies
 Drivers tend to anticipate common events
 The more predictable, the less chance for
error
 Experiencing problems when surprised
 Drivers assume that they need to react to
standard situations
 Drivers experience problems in locations with
inconsistent design or operation

15. Reaction time
 Reaction time involves four elements
1. Perception
 See visual signal
1. Identification
 Identify signal
1. Emotion
 Take action in response to stimulus
1. Volition
 Execute the action

16. Reaction time(continued)
 Ways to reduce the average and variance
of reaction time
 Encourage familiarity
 Minimize number of alternatives
 Provide positive information
 Provide prior warning
 Provide clear sight distance
 Use symbolic signs

17. Reaction time(continued)
 Implications of hysteretic effect for traffic
design
 The ability to process information may be
lower on the departure side of an intersection
than the approach side
• Higher pedestrian crash rates on the downstream
side of intersections can be explained
 Pedestrian crossings and bus stops should not
be placed immediately downstream of an
uncontrolled intersection

18. Visual characteristics
 Visual field
 Eye and head movement
• Maximum possible rate of about 4 fixation per
second
• 2 fixation per second usual max rate for a busy
driver
• 1-1.5 fixation per second for normal driving
 Illumination
• Human visual system range of illumination
• From 0.75x10^-6cd/m^2 to 10 ^5cd/m^2
• A range from darkest to brightest varying by a factor
of 10^11

19. Visual characteristics(continued)
 Visual disabilities
• About 2.5 percent of adult male population
has color impaired vision
• Can not discriminate red, yellow and green
• Blurred vision
• Visual sensitivity declines with age
• Detection threshold of elderly drivers is
about double that of younger drivers

20. Visual characteristics(continued)
 About 90% information used by driver is visual
 Visual field is quite narrow (-3 to 10 degrees)
 Sign and signals within 10 to 12 degrees of the
line of sight can be seen and understood
 Objects can be detected in peripheral vision to 90
degrees left and right at rest
 At speed of 20 mph and 60 mph, the visual field
decreases to 100 and 40 degrees respectively
compared to 180 degrees at rest

21. Visual characteristics(continued)
 Important findings relevant to design of
traffic signals on top
 Signal lanterns should be located in a
standard fashion, with red on top, yellow in
the middle and green at the bottom
 The intensity of traffic signals, and the actual
colors used need to be closely specified
 Theses consideration also affect the sign of
traffic signs and the letters on them

22. Information needs of road users
 The key needs of road users in relation to
traffic control information are:
 Conspicuity
• Signal must be seen
 Legibility
• It’s message must be readable
 Comprehensibility
• Message must be understood
 Creditability
• Message must perceived to be true

23. Information needs of road users
(continued)
 Conspicuity is affected by several factors
 Size (larger more conspicuous)
 Brightness (brighter more conspicuous)
 Boldness (larger letters more conspicuous)
 Edge sharpness (a line around edge of a sign)
 Contrast (high contrast, especially in brightness)
 Visual simplicity (simple background more conspicuous
 Eccentricity
• A signal is unlikely to be detected if it is more than 6-7
degrees from the line of sight

24. Information needs of road users
(continued)
 Implications of conspicuity factors that
affect traffic engineering and road safety
practice
 Influence on the size, color, layout and
location of traffic signs
 Legislation for control of roadside advertising
 Reflector signs and pavement markings
 Illumination of signs (especially direction sign)
 Roadwork signing and work site protection
 Promotion of safety yellow raincoats for
pedestrians and brightly colored for road
maintenance crews

25. Information needs of road users
(continued)
 Sign legibility
 A sign is legible if it has enough detail and
sufficient visibility to allow its message to be
interpreted
• Increasing the size will increase the legibility
distance and give driver more opportunity to
observe and understand the sign
 Sign comprehensibility
• Driver must perceive the importance of the
signal

26. Information needs of road users
(continued)
 Sign credibility
 Drivers believe that a signal is both true and
refers to them
 Traffic engineers can aid credibility of signs
• Ensure that the sign is credible in its context
• Ensure that sign selection, color and shape conform
with national standard
• Avoid the unnecessary use of signs
• Avoid unnecessary restrictive signs
• Important messages should adequately displayed
• Speed limit repeater signs
• Advance direction signing should be consistent and
prominent