Mod 1-1

TTE4274 – Module 1

Traffic Stream Components & Characteristics

  • Focus on understanding the different components that affect traffic, including road users, vehicles, roadways, controls, and the environment.

Outline

  • Traffic Stream Components

  • Dealing with Diversity

  • Driver Characteristics

    • Vision

    • Perception-Reaction Time (PRT)

  • Pedestrian Characteristics

    • Walking speeds

    • Gap acceptance

  • Vehicle Characteristics

    • Design vehicles

    • Braking and acceleration performance

  • Stopping Distance

    • Sight distance

    • Decision distance

    • Clearance intervals

Traffic Stream Components

  • Main Components:

    • Road Users

    • Vehicles

    • Roadways

    • Control Devices

    • General Environment

Components Affecting the Traffic System

  • Types of Components:

    • Vehicles

    • Drivers

    • Pedestrians

    • Roads

    • Control Devices

    • General Environment

Road Users

  • Categories:

    • Drivers

    • Pedestrians

    • Bicyclists

  • Complexity:

    • Deals with diversity among human participants in the traffic system

    • Active component that is unpredictable compared to other engineering fields

  • Characteristics:

    • Physiological: Typically measurable and quantifiable

    • Psychological: More difficult to measure and quantify

Examples of Psychological Factors

  • Key Aspects:

    • Desired speeds

    • Desired safety distances

    • Physical aspects include perception and reaction limits

    • Impacts of imperfect vehicle control

Driver Characteristics

  • Variables Affecting Drivers:

    • Age, gender, physical condition,

    • Disabilities, alcohol concern, mental capability, skills, and attitudes.

  • Demographic Insights:

    • Percent of driving population varies widely across age groups.

Dealing with Diversity

  • Percentiles:

    • Acknowledge variability and use the 85th percentile for speed limits to ensure safety for at least 85% of users.

    • Pedestrian speeds considered for safety in crossings include the 15th percentile which ensures the slower walkers are accommodated.

Visual Factors in Driving

  • Visual Acuity Elements:

    • Static: Ability to see fine details at rest (e.g., reading signs)

    • Dynamic: Ability to detect moving objects (e.g., other cars)

    • Depth Perception: How distance is judged

    • Key Visual Factors: Glare recovery, peripheral vision, and color differentiation.

Perception-Reaction Time (PRT)

  • Stages of Reaction:

    • Perception: Senses a situation arises

    • Identification: Recognizes and processes that it needs a reaction

    • Emotion: Decides on an appropriate action

    • Volition: Begins the physical action (e.g., foot on brakes)

  • Formula:

    • Reaction distance (db) affected by speed and time.

Pedestrian Characteristics

  • Walking Speed: Typical range 3.5 - 4 ft/s

    • Variability due to physical impairments.

  • Gap Acceptance Factors:

    • Influenced by vehicle speed, street width, traffic flow, and waiting time.

  • Pedestrian Facility Design:

    • Importance of understanding variability in walking speed for the design of pedestrian signals and crossing intervals.

Vehicle Characteristics

1. Static Factors

  • Dimensions: Width, length, overhang, wheelbase relevant to road design.

    • Impact on lane width and vertical clearance.

  • Weight: Influential on pavement design and roadway demand.

2. Dynamic Factors

  • High-Speed Turns:

    • Centrifugal forces at play and how they're counteracted by friction and roadway design (superelevation).

Acceleration and Braking Performance

  • Acceleration Capabilities:

    • Varies across vehicle types (cars vs. trucks) and affects roadway layouts and sight distances at intersections.

  • Braking Performance:

    • Impacted by tire condition, roadway surface, and initial vehicle speeds.

Stopping Sight Distance (SSD)

  • Calculation Essentials:

  • Must account for perception-reaction time along with vehicle speeds and roadway conditions to ensure safety at signalized intersections.

Recommended Reaction Times (AASHTO)

  • Examples of Standard Situations:

    • Normal stop at signals: 1 second

    • Stopping on highways: 2.5 seconds

    • Maneuver times for avoidance vary; significant for planning in urban environments.