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

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