Human Factors Spring 2006

Human Factors for Safety

Human Factors
• Physical
• Sensory and perception
• Cognitive (decision making)
Human Factors Spring 2006

Importance of Human Factors

• Human factors are involved in 85% of
crashes
• Human factors are impeded by aging
processes - diseases

Elderly Involvement in Crashes

• Elderly drivers over-involved in fatal
accidents for mileage driven
• More elderly pedestrians per 100,000
population killed
• Older driver fatalities increased by 27%
from 1991 to 2001
Human Factors Spring 2006

Physical
• Reaction Time
– Movement Time after initiation of movement
• Changes in muscle mass and elasticity
• Joint strength- arthritis
• Strength
• Flexibility and ROM
• Head and Neck mobility
• Agility (new concept)

Strength
• Decrease in muscle tone
• But no research to show direct impact on
driving
– braking
Human Factors Spring 2006

Movement
• Older people often suffer from muscle and
joint stiffness
• Makes it difficult to turn head and body
• Consequently, motor component of
reaction time slows with increasing age

Agility
(World Health Organization classification)
• Dexterity in the use of limbs
• Impact on use of controls
• Size of controls
Human Factors Spring 2006

Flexibility and ROM

• Decline in over all flexibility with age
• Increase in osteoarthritis-joint problems
• Can impact:
– Entering and exiting vehicle
– Backing up, parking, turning-etc
– Lane Changing
• Decrease in Physical Fitness

Head and Neck Mobility

• Vehicle Operations
– Scanning, mirror use
– Problems with merging
– Problems with Blind Spots
– Skewed Intersections particularly a problem
K
Human Factors Spring 2006

Driver Fragility
• Fragility increases beginning at 60-64 yrs.
• By age 80, elderly more likely than 20 yr.
To die from injuries sustained in a crash:
– 4 times – male
– 3.1 times - female

Vision
• Primary external stimuli
• 90% of driver information is visual
• Required visual acuity
– Static and dynamic
• Major problem for elderly drivers
Human Factors Spring 2006

Vision
• Visual acuity standards must be met for
licensing
– 20:60 in Oregon
• Quality of vision deteriorates away from focal
point
– 3-5° cone, excellent vision
– 10° cone, clear vision
– 20° cone, satisfactory vision
• MUTCD standard for important control devices
– Beyond, increasingly blurred
Human Factors Spring 2006

Sensory Perceptual
• Changing Eye Physiology
– Requirements for increased lighting
– Due to decrease in pupillary diameter
– Optic media – more scattering of light
– Changes in lenses
Human Factors Spring 2006

Foveal Vision
• Most acute vision central angle of 30o
• Cones: color vision center of retina
• Rods: motion detection outer part of retina
(Peripheral vision up to about 170o )

30°

(actually 3-5°)
Human Factors Spring 2006

Terminology-Vision
• Acuity
– Static
– Dynamic
• Light Sensitivity
– Glare recovery

Age Related Changes in Lenses

• Increase absorption of “blue” light
• Increase in thickness of lens
• Problems of muscles that adjust shape of
lens
– Difficulty in accommodation
– Loss of visual acuity
– Increased use of bi-focals
Human Factors Spring 2006

Age Related Changes in Lenses

• Dynamic Visual Acuity
– Ability to see moving objects
– Scanning
– Most relevant to driving, but not measured

Visual Acuity
• Fewer receptor cells on retina with age
results in:
– Resolution of fine details is coarser
– Solved by greater size and contrast of
important details, and increased illumination
• Use of large print media
Human Factors Spring 2006

Visual Acuity
• Younger drivers – average 20 / 20
• Older drivers – average 20 / 60

Visual Acuity
• Near age 70, progressive loss in ability to
focus on near objects
– Due to loss in lens elasticity
• Speed eye changes focus at various
distances decreases
– Problem: viewing dashboard to road scene
• Drivers typically become more long-
sighted with age
Source: Corso, J.F. (1981) Aging Sensory
Systems and Perception. Praeger: New York.
Human Factors Spring 2006

Elderly Static Visual Acuity

• Visual Acuity
– Relatively constant to 50 yrs.
– Declines progressively faster with age

Source: Pitts, D.G., Aging and Human

Visual Function, 1982 Liss: N.Y.

Elderly Static Visual Acuity

• Normal physiological changes causes:
– Greater sensitivity to glare
– Reduction in contrast sensitivity
• Diseases that reduce acuity:
– Cataracts
– Macular Degeneration
– Glaucoma
Human Factors Spring 2006

Eye Movement
• Drivers shift their eyes to see full visual
field
• Eye movement time
– Eye shift, 0.15 – 0.33 sec
– Eye fix/focus, 0.1 – 0.3 sec
• Elderly takes longer to
– Shift eyes and focus
– Recover from glare

Dynamic Visual Acuity

• Acuteness of vision for an object with
angular movement or ability to see a
moving target detail
• Performance improves with illumination
• Unrelated to static acuity
Human Factors Spring 2006

Dynamic Visual Acuity

• Acuity of moving target:
– Decreases with increasing target velocity
– Improves with increased exposure time
– Is better when target is “foveal” on central
rather than peripheral
– Varies for drivers with the same static acuity

Source: Burg, A., Journal of Applied

Psychology, 50, 1966.

Dynamic Visual Acuity

• Of visual factors, dynamic acuity relates
strongest to driving record
• Regular deterioration in dynamic visual
acuity with advancing age
• Dynamic visual acuity is strongly related to
accident involvement regardless of age

Source: Burg, A., Journal of Applied

Psychology, 50, 1966.
Human Factors Spring 2006

Glare
• Brightness in the field of vision that is
substantially greater than luminance eyes
are adapted
• Glare recovery
– 15 year old person – 2 sec
– 65 year old person – 9 sec

Light Sensitivity
• Contrast sensitivity function
– Better predictor than visual acuity of ability to
see
– Problems with small objects
– Need high levels of contrast
• Increased time to adjust to dark
– Problems with tunnels and shadows
• Ability to see contrast begins to diminish in
40s
Human Factors Spring 2006

Glare Sensitivity and Recovery

• Light to dark : ~6 sec to full visual acuity
• Dark to Light : ~3 sec to full visual acuity
– Due to increased lens opacity
– Problem for people with cataracts
– Problem with on-coming headlights
– Glare recovery time increases with age

Glare Sensitivity and Recovery

• Due to increase opacity of lens
• Problem for people with cataracts
• Problems with on coming headlights
• Glare recovery time increases with age
Human Factors Spring 2006

Contrast Sensitivity
• Ability of an individual to process contrast
information
• Provides driver’s ability to see patterns in
the environment
• In general, older adults have decreased
contrast sensitivity

Visual Acuity in Poor Light

• Minimum amount of light needed to see,
increases with age
• For every decade over 25, twice as much
brightness at night is needed
• By age 75, drivers need 32 times the
brightness of 25 yrs.
Human Factors Spring 2006

Dark Adapted Vision

• Minimum energy needed to elicit a
sensory response after a period in dark
• Threshold for dark adapted vision
increases with age (i.e., light sensitivity
decreases)
• Increases over 4% per year between 22
and 43 yrs

Night Time Myopia

• Perhaps most important problem
• Affects large proportion of drivers over 40
years
• Also especially a problem for short sighted
drivers of all ages
Human Factors Spring 2006

Night Time Myopia

• Many drivers with this problem avoid
driving at night
• Elderly drivers most reported difficulty at
night is:
1) Seeing the road
2) Glare
• Possible improvements:
– Fog stripes and more street lighting

Source: AA Foundation for Road Safety

Vision in Poor Light

• Viewing a target 20/200 in poor light of 0.20
cd/m2 resulted in:

Age group 18-25 60-64 65-69 70-74 75-79 80-87

% failing to
see 20/200 1.7 23.1 48.3 57.1 70 75
target

Source: Sturr, et.al., Human Factors, 32

Human Factors Spring 2006

Depth Perception
• Primarily provided by environmental cues
for driving
• Ability to judge depth decreases with age
• Requires two eyes
• Impact of dark environments

Peripheral Vision
• Useful field of View (UFOV)
– Measure of spatial area and usually much
smaller than visual field size
– Involves binocular vision
• Detection, localization and identification of
targets
Human Factors Spring 2006

Peripheral Vision

• Visual field shrinks as people age

• Little decrease in visual field before 55

• Shrinkage is dramatic after 65-70

Peripheral Vision
• Shape and movement detected outside
65-90° (from line of sight)
• Less peripheral vision as speed increases
– 37° at 40 mph
– 29° at 50 mph
– 20° at 60 mph
Human Factors Spring 2006

Aging Characteristics in UFOV

(Usable Field of Vision)
• Reduced speed of visual processing
• Reduced ability to divide attention
• Reduced ability to discriminate against a
background

Visual Localization
• Older drivers have difficulty in identifying a
target among distractions
• Changes of vision and cognition make it
difficult for elderly drivers to detect moving
or stationary objects in the periphery
Human Factors Spring 2006

Detection and Tracking of

Moving Objects
• Driver in 60’s and 70’s lag farther behind
targets
• Elderly less able to detect movement and
changes of vehicles in visual field
• Also difficulties of understanding behavior
of vehicles in front: stopping, slowing,
speeding up and reversing

Eye Movements Slow

• Eye muscles commonly atrophy with
increasing age
• Elderly less able to raise their eyes
• Much slower to make eye movement to
fixate object
Human Factors Spring 2006

Diseases
• Cataracts: Opacity of eye
• Glaucoma: high pressure within eyeball
– Prevents cataract surgery
• Macular degeneration
– Impaired central vision

Perception-Reaction Time
PIEV Process – Reaction to external stimuli

• Perception – time to see

• Intellection – time to understand
• Emotion – time to decide
• Volition – time to execute
Human Factors Spring 2006

Perception-Reaction Time
• Range from 0.5 – 5.0 sec for a simple
event
• Minimum 0.5 PRT for emergency event
• Added conflicts and complexity increase
PRT, depending on amount of data

Control Perception-Reaction Time

• PRT for traffic signal control is assumed =
1.0 sec
• Recent research shows
– Alerted PRT for control ~ 0.9 sec
– Unalerted PRT for control ~ 1.3 sec
Human Factors Spring 2006

Design Perception-Reaction Time

• Assumed to be 2.5 sec
• Based on 85% PRT
• Recent research shows
– 95% of PRT = 2.5 sec
– 85% of PRT = 2.0 sec

Decision Perception-Reaction Time

• PRT for complex conditions with
numerous conflicts
– AASHTO decision PRTs range from 3 – 14.5
sec
– Each additional conflict adds increment of
time, ~ 1.5 sec
Human Factors Spring 2006

Elderly Driver’s PRT

PRT times for elderly drivers are slower
Average Elderly

Stopping Sight Distance

2.5s 4.95-5.95s
(Design = 2.5s)

Traffic Control
0.82 0.98-1.61s
(Control = 1.0s)

Crossing Road from

2.99s 3.31-4.56s
Stopped Position

Left-Turn thru Traffic

2.24s 2.56-3.81s
(Design = 2.0s)

Attention
• Problems with attention switching abilities
problems
• Dementia
– Problems getting lost
– Following directions
– Stopping for directions
– Failure to yield
Human Factors Spring 2006

Complex Situations
• Elderly drivers take about 1.5 sec to scan
each item

Prescribed Drug Effect

• Reliable evidence that prescribed drugs
increase risk of crashes, especially for
elderly drivers
• For elderly, many drugs have higher active
level of drugs for longer
• Central nervous system effects increase
with age
Human Factors Spring 2006

Cognitive Functions
• Important factors in aging cognition
– Variability of performance increases with age
– Speed of processing information decreases
with age
– Therefore, amount of processing is reduced

Difficulty Judging Gaps

• Older drivers tend to accept a gap to cross
based on spacing distances, regardless of
speed
• Older drivers’ poor distance/speed
perception and judgment may account for
right-of-way accidents, such as improper
left-turns
Human Factors Spring 2006

Stability Sensations
• Drivers react to
– Curves
• Limited by tangential accelerations or friction
– Rough roads
• Rough surface texture, RPMs for rumble strips
– Cross slope / superelevation
• Reduced or negative superelevation increases
side friction and accelerations

Modifying Factors
• Fatigue
– Reduces alertness and slows reactions
• Alcohol and Drugs, affects
– Alertness
– Judgment
– Self control
– Physical coordination
• Weather
– Gives unexpected vehicle/pavement response
– Limits visibility
Human Factors Spring 2006

Conditional Response
• Driver responses are dictated by habit,
standardized design, and control
• Drivers attempt to maintain speeds
Human Factors Spring 2006

Drivers Scan and Sift Data

• Driver scans, samples and selects from
available information to guide driving
• Complex situations and numerous
conflicts;
– Increase the workload
– May not give adequate time to select and
judge conditions
– Are difficult for elderly drivers

Divided Attention
• Brake reaction time slowed 0.5 sec. for
young drivers, for using cell phone
• Older drivers not tested. Likely elderly are
already using their compensatory capacity
to cope with other declines
Human Factors Spring 2006

ITS
• In vehicle devices:
– Navigation
– Audio
– Cell phones
• European Trends
• Japan

Primary Concept
• Driver must sift through all information and
determine relative importance
– Control – driver/vehicle interaction most important
– Guidance – path selection, next in importance
– Navigation – route selection, least important
• Elderly drivers have difficulty assessing and
shifting importance
Human Factors Spring 2006

Expectancy
• A driver is led to expect a given
design/control condition due to the
information presented
– Formal information – roadway alignment,
centerline stripes, regulatory signs, etc.
– Informal information – brushlines, tree lines,
fences, roadside barriers, etc.

Example of Expectancy
Curvature in one direction

Next curve, opposite direction; NOT

Human Factors Spring 2006

Example: Intersection Lanes

Should Line Up
Human Factors Spring 2006

Example: Driver Assumes Two

Lanes are Presented

24’ 12’
14’

Future Implications
• Presently, 85% crashes involve human
factors
• Elderly drivers are over-involved in fatal
crashes and pedestrian fatalities
• Older segment of population is growing
twice the rate of the rest of the population
• Traffic volumes and travel are increasing,
adding complexity and conflicts to present
day conditions
Human Factors Spring 2006

Road Signs
• Difference between Text and Symbols
– Better performance for distance
– Interpretation of symbols may be a problem
• Cultural Context
•