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Human factors in road traffic
Hossein Naraghi
CE 590 Special Topics
Safety
February 2003
Time Spent: 9 hrs
Human in the road traffic system
Vital inputs to much of the road and traffic engineering system
•Human performance•Human capabilities•Behavioral characteristics
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
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
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
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
Human performance Three key aspects of human
performance
Information processing Visual characteristics Information needs
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
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
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
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
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
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
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
Reaction time Reaction time involves four elements
1. Perception See visual signal
2. Identification Identify signal
3. Emotion Take action in response to stimulus
4. Volition Execute the action
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
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
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
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
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
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
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
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
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
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
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