Ch.6 Traffic Safety & Human Factors

CVEN 307

Agency that reported 3,893 fatalities in Texas during 2020

Texas Department of Transportation

Primary factors affecting highway safety

Vehicle safety standards, highway design, driver-related factors, vehicle safety devices, and traffic control devices

Ways to reduce accident or crash occurrence

Minimize driver error, improve driver training/testing, remove unsafe drivers, proper design, clear signage, adequate sight distance, and controlled intersections

Why has the term 'accident' been replaced with 'crash'?

To assign responsibility to the driver at fault and highlight the role of negligence

Number of conflict points in a standard 4-legged intersection

32 (8 diverging, 16 crossing, 8 merging)

Examples of forgiving roadside design features

Guardrails, barriers, impact attenuators, breakaway sign posts, and clear gore areas

Vehicle features that improve crash survivability

Energy-absorbing bumpers, padded dashboards, seat belts, airbags, and ABS

Examples of safety programs

Vehicle inspection, speed limits, drinking age enforcement, and DWI programs

Design aspects that influence safety

Vertical/horizontal alignment, intersections, sight distance, roadside and barrier design

Purpose of crash data collection and analysis

Identify high accident locations, determine causes, develop countermeasures, and assess improvements

Typical accident (crash) data collected

Type, number, severity, trends, and contributing factors

Types of accident (crash) rates

Population-based and exposure-based rates

Formula to identify high accident locations

Cs = x̄ + Zσ (using Z=1.645 for 95% confidence)

Example of identifying a high accident location

If Cs = 11.5 accidents per million entering vehicles, any site above that is unusually high

Most common reason for vehicle crashes

Driver errors

Three components of the driving task

Control, guidance, and navigation

What is driver expectancy?

The anticipation of roadway conditions; violations increase errors and reaction times

Definition of PIEV

Answer: Perception, Identification, Emotion, Volition – components of reaction time

Typical reaction times

Braking 0.6–1.7 sec (sometimes >2.5 sec); 2.5 sec used in design

Typical visual field values

Peripheral vision 120–160°, clear vision 10°, best vision 3°

Effect of speed on visual acuity

As speed increases, the cone of clear vision narrows

Relation between situation demand and driver error

Errors increase with higher demand and speed

Design recommendation to avoid driver fatigue

Avoid long, flat tangents; use slight curvature following natural terrain