9.4 Traffic Engineering and Safety

9.4 Traffic Engineering and Safety

Introduction to Traffic Engineering and Safety

Traffic Engineering is a branch of civil engineering that applies engineering principles to the planning, design, operation, and management of roads and streets to achieve the safe, efficient, and convenient movement of people and goods. Road safety is an integral and critical component of this field, aiming to reduce the risk of death and injury by systematically addressing factors related to road users, vehicles, and the roadway environment. This section explores the fundamental elements that govern traffic behavior, the tools for managing it, and the strategies for making roads safer for all users.

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1. Impact of Human and Vehicular Characteristics on Traffic Planning

1.1 Human Factors (Road User Characteristics)

Human factors are the most variable and critical component in traffic systems. Understanding them is key to designing forgiving roads and effective controls.

1. Sensory Factors:

   • Vision: Critical for information gathering (90% of driving info). Includes visual acuity, peripheral vision, depth perception, glare recovery, and night vision.

   • Hearing: Important for detecting emergency sirens, horns, and train whistles.

2. Mental Factors:

   • Reaction Time: The time lag between perceiving a stimulus and initiating a response. Total reaction time (Perception + Intellection + Emotion + Volition) is typically 2.0 to 2.5 seconds for braking decisions.

   • Information Processing: Driver's ability to process multiple inputs, make decisions, and predict other users' actions. Affected by fatigue, alcohol, drugs, and distraction.

   • Attitude and Personality: Risk-taking vs. cautious behavior.

3. Physical Factors:

   • Age, fatigue, disability, and the influence of alcohol/drugs.

1.2 Vehicular Characteristics

Vehicle design and performance directly influence highway design and traffic operations.

1. Static Characteristics:

   • Dimensions: Overall length, width, height, wheelbase, overhang. Govern lane width, parking space design, clearance requirements, and curve design.

   • Weight: Gross Vehicle Weight (GVW), axle loads. Determine pavement structural design and bridge loading.

2. Dynamic Characteristics:

   • Power and Speed: Acceleration and maximum speed capabilities affect climbing lanes, sight distances, and intersection design.

   • Braking Performance: Deceleration rates govern stopping sight distance (SSD).

   • Turning Characteristics:

     • Turning Radius: Minimum radius required for a vehicle to turn (minimum for passenger car ~7m, for large truck ~12m). Governs intersection corner design.

     • Off-tracking: The path of the rear wheels cuts inside the path of the front wheels. Critical for designing horizontal curves and intersection channelization.

   • Lighting and Conspicuity: Ensure the vehicle is visible to others, especially at night and in poor weather.

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2. Traffic Operations and Regulations

1. Objective: To establish a legal and behavioral framework that ensures orderly and predictable movement, minimizing conflicts and maximizing safety.

2. Key Elements:

   • Traffic Laws (The Motor Vehicles and Transport Management Act in Nepal): Define rules for vehicle registration, driver licensing, speed limits, right-of-way, overtaking, and parking.

   • Rules of the Road: Keep right, obey traffic signals and signs, yield right-of-way, use of seat belts and helmets.

   • Lane Discipline: Proper use of lanes (e.g., slower traffic keep left, no overtaking from the left).

   • Right-of-Way Rules: Establish priority at uncontrolled intersections, roundabouts, and during merging (e.g., major road vs. minor road, first-to-arrive at 4-way stop).

   • Speed Limits: Statutory (maximum legal speed), advisory (recommended speed on curves), and design speed (engineering basis).

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3. Traffic Control Devices (TCDs)

TCDs are the primary means of communicating regulations, warnings, and guidance information to road users. They must fulfill five basic requirements: Fulfill a need, Command attention, Convey a clear meaning, Command respect, and Give adequate time for response.

3.1 Types of Traffic Control Devices

1. Traffic Signs (As per Nepal's 'Traffic Sign Manual'):

   • Regulatory Signs (Circular, red border): Convey mandatory instructions (Stop, Give Way, No Entry, Speed Limit). Violation is a legal offense.

   • Warning Signs (Triangular, red border): Alert to potentially hazardous conditions ahead (Sharp Curve, School Ahead, Pedestrian Crossing).

   • Informatory/Guide Signs (Rectangular): Provide directional and facility information (Destination, Distance, Hospital, Parking).

2. Traffic Markings:

   • Applied on the road surface using paint or thermoplastic.

   • Types: Longitudinal lines (center lines, lane lines, edge lines), Transverse lines (stop lines, pedestrian crossings), Word/symbol markings (arrows, "STOP"), and Object markers.

   • Colors: White (general), Yellow (no-overtaking zones, parking restrictions).

3. Traffic Signals:

   • Electronic devices that assign right-of-way to different traffic movements using colored lights (Red, Yellow/Amber, Green).

   • Functions: Reduce certain types of accidents (right-angle), improve capacity at busy intersections, and provide orderly movement.

4. Traffic Islands and Delineators:

   • Islands: Raised or painted areas used for channelization, pedestrian refuge, and traffic separation.

   • Delineators: Posts with reflectors along the roadside or median to indicate the alignment, especially at night and on curves.

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4. Traffic Studies

Traffic studies (surveys) collect data to diagnose problems, plan improvements, and design facilities.

4.1 Volume Study

• Objective: To measure the number of vehicles (and/or pedestrians) passing a point on a roadway during a specified time period.

• Metrics:

  • Average Daily Traffic (ADT): Total volume during a period divided by the number of days.

  • Annual Average Daily Traffic (AADT): ADT averaged over a full year.

  • Peak Hour Volume (PHV): The highest volume observed in a single hour.

  • Design Hourly Volume (DHV): The volume used for design, typically the 30th highest hourly volume of the year.

• Methods: Manual counts, automatic counters (pneumatic tubes, inductive loops), video recording.

4.2 Speed Study

• Objective: To determine the speed characteristics of a traffic stream under prevailing conditions.

• Types:

  • Spot Speed: Instantaneous speed of a vehicle at a specific location. Used for: geometric design, setting speed limits, accident analysis.

  • Running Speed: Average speed over a length of roadway, excluding stopped delays.

  • Journey/Travel Time Speed: Average speed over a route including all stops and delays.

• Presentation: Speed data is presented as a distribution. Key parameters are:

  • Average Speed (Mean): Arithmetic mean.

  • Median Speed: 50th percentile speed.

  • Pace: The 10 mph (or km/h) range containing the highest number of observations.

  • 85th Percentile Speed: The speed at or below which 85% of vehicles travel. Often used as a basis for setting rational speed limits.

• Methods: Radar gun, stopwatch (for short known distance), moving observer method.

4.3 Origin-Destination (O&D) Study

• Objective: To determine where trips begin (origin) and end (destination), their purpose, and the routes taken.

• Uses: Planning new roads, bypasses, public transit routes, and evaluating traffic impact of new developments.

• Methods: Roadside interview, license plate matching, home interview surveys.

4.4 Traffic Capacity and Level of Service (LOS)

• Traffic Capacity: The maximum hourly rate at which persons or vehicles can reasonably be expected to traverse a point or uniform section of a lane or roadway under prevailing conditions.

• Level of Service (LOS): A qualitative measure describing operational conditions within a traffic stream (A to F, with A being best). Based on factors like speed, travel time, maneuverability, comfort, and safety.

• Factors Affecting Capacity: Lane width, lateral clearance, heavy vehicles, driver population, intersections, and terrain.

4.5 Traffic Flow Characteristics

• Describes the relationship between the primary flow variables:

  • Flow (q): Volume, vehicles per hour (vph).

  • Speed (u): Space mean speed, km/h.

  • Density (k): Vehicles per kilometer (vpk).

• Fundamental Relationship: $q = u \times k$

• Macroscopic Flow Models:

  • Greenshields Model: Assumes a linear speed-density relationship.

  • Critical values: Jam Density (kj), Free Flow Speed (uf), Maximum Flow (qmax) which occurs at optimum density and speed.

4.6 Parking Study

• Objective: To inventory parking facilities, measure demand and duration, and identify problems (cruising for parking, illegal parking).

• Metrics: Accumulation, volume, duration, turnover.

• Parking Types: On-street (parallel, angle), Off-street (surface lots, multi-storey garages).

4.7 Accident Study (Crash Investigation)

• Objective: To identify causes, high-risk locations (black spots), and develop countermeasures.

• Data Collected: Location, time, type (collision diagram), vehicles involved, road/environment conditions, driver behavior.

• Analysis: Statistical analysis to find rates (accidents per million vehicle-km) and patterns. Goal is proactive prevention, not just blame assignment.

4.8 Traffic Flow Study

• A comprehensive study analyzing the movement and interaction of vehicles in a network, often using simulation software (like VISSIM, Synchro) to model complex scenarios and test improvement strategies.

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5. Road Intersections

Intersections are critical points where traffic conflicts occur. Their design aims to minimize these conflicts.

5.1 Types of Intersections

1. At-Grade Intersections: All merging, diverging, and crossing movements occur at the same level.

   • Uncontrolled: No traffic signs or signals. Relies on right-of-way rules (rare except in very low volume areas).

   • Priority Controlled (Stop/Yield Controlled): Minor road traffic must stop or yield.

   • Signalized: Controlled by traffic signals.

2. Grade-Separated Intersections (Interchanges): Traffic streams cross at different levels using bridges/underpasses. Eliminates crossing conflicts. Types: Diamond, Cloverleaf, Trumpet, Directional.

5.2 Configurations and Design Elements

1. Basic Configurations: 3-leg (T or Y), 4-leg, Multi-leg, Roundabouts.

2. Design Considerations:

   • Alignment: Intersecting roads should meet at or near 90°.

   • Sight Distance: Adequate sight triangles must be clear of obstructions. Minimum sight triangle distance is based on SSD from both approaches.

   • Channelization: The use of islands, markings, and curbs to guide traffic into definite paths. Functions: Separates conflicts, controls angle of conflict, reduces excessive paved area, protects pedestrians, locates traffic control devices.

   • Auxiliary Lanes: Left-turn lanes and right-turn lanes are provided to separate turning traffic from through traffic, improving capacity and safety.

   • Superelevation and Cross Slope: Proper drainage must be maintained.

5.3 Roundabouts

• Definition: A circular intersection where traffic travels counterclockwise around a central island, and entering traffic must yield to circulating traffic.

• Advantages: Reduces severe (right-angle) crashes, lowers speed, improves traffic flow for certain volume conditions, reduces delay and fuel consumption compared to signalized stops.

• Key Design Features: Deflection of entry path, yield control at entry, inscribed circle diameter, central island, splitter islands, and truck apron.

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6. Traffic Signals

Traffic signals are time-sharing devices that allocate right-of-way to different traffic movements.

6.1 Types of Signal Control

1. Fixed-Time (Pre-timed) Signals: Cycle length, phase sequence, and interval timings are fixed based on historical traffic data. Simple but inflexible.

2. Traffic-Actuated Signals: Timings adjust in real-time based on vehicle demand detected by sensors (inductive loops, cameras). More efficient for variable traffic.

3. Coordinated Signal Systems: Multiple signals along an arterial road are linked and timed to provide a "green wave" for platoons of vehicles, reducing stops and delays.

6.2 Key Timing Parameters

• Cycle Length (C): The total time for one complete sequence of signal indications (all phases), typically 60-120 seconds.

• Phase: A part of the cycle allocated to a specific movement or set of movements receiving the right-of-way.

• Interval: A period during which signal indications do not change.

  • Green Time (G): Right-of-way interval.

  • Yellow/Amber Time (Y): Clearance interval, typically 3-5 seconds. $Y = t + \frac{v}{2a}$ (where t is reaction time, v is approach speed, a is deceleration).

  • All-Red Time (AR): A brief interval where all approaches show red, allowing vehicles in the intersection to clear. Optional but improves safety.

• Lost Time (L): Time per cycle not used by any movement (start-up lost time + clearance lost time).

6.3 Design Principles

• Webster's Method: An optimal cycle time $C_0$ to minimize average delay. $C_0 = \frac{1.5L + 5}{1 - Y}$ where $L$ is total lost time per cycle, and $Y$ is the sum of critical lane volumes to saturation flow ratios for all phases.

• Saturation Flow (s): The maximum hourly rate at which vehicles can pass through a signalized intersection under prevailing conditions, expressed in vehicles per hour of green (vphg).

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7. Factors Influencing Night Visibility

Driving at night is more dangerous due to reduced visibility. Understanding these factors is key to designing for safety.

1. Driver's Visual Capacity:

   • Reduced Visual Acuity: Sharpness of vision decreases.

   • Loss of Color and Contrast Perception: Difficulty distinguishing objects.

   • Glare: Disabling (blinds temporarily) and discomfort glare from opposing headlights or poorly designed streetlights.

   • Poor Depth Perception and Peripheral Vision.

2. Vehicle Lighting System:

   • Headlights (Beams): Low beam (for meeting traffic) and high beam (for open road). Proper aim is crucial.

   • Tail Lights, Brake Lights, and Turn Signals: For vehicle conspicuity.

3. Roadway Lighting:

   • Improves visibility, reduces glare, enhances the visual guidance system, and improves security.

   • Design considerations: Illumination level, uniformity, glare control, and pole placement.

4. Retro-reflectivity of Traffic Control Devices:

   • Signs, markings, and delineators must be made with retro-reflective materials to return light back to the driver's eyes. Regular maintenance is required as reflectivity degrades over time.

5. Geometric Design Implications:

   • Sight Distance on Vertical Curves: Sag curves must be designed for headlight sight distance, which is typically less than daytime SSD. The headlight beam (assumed 0.75m high) illuminates a distance ahead; the curve length must ensure this distance ≥ SSD.

   • Delineation and edge marking become critically important.

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8. Road Safety Measures

Road safety is a multi-disciplinary approach encapsulated in the Safe System philosophy, which acknowledges that humans make mistakes and the system should be forgiving.

8.1 Engineering Measures

1. Road Design and Improvement:

   • Black Spot Improvement: Systematic identification and treatment of high-accident locations (e.g., improving sight distance, adding turning lanes, installing barriers).

   • Road Safety Audits (RSA): A formal, independent examination of a future or existing road project to identify potential safety issues.

   • Self-Explaining Roads: Design that naturally guides drivers to appropriate behavior (e.g., narrower lanes in residential areas encourage lower speeds).

2. Traffic Control Devices: Proper application of signs, markings, and signals as described in Section 3.

3. Provision of Safety Hardware:

   • Guardrails and Crash Barriers: Prevent vehicles from leaving the road or crossing medians.

   • Impact Attenuators (Crash Cushions): At fixed hazards like bridge piers.

   • Safe Roadside Clear Zones: An unobstructed, traversable area beyond the edge of the travel lane for errant vehicle recovery.

8.2 Vehicle Safety Measures

• Active Safety: Helps prevent accidents (ABS, Electronic Stability Control, Lane Departure Warning).

• Passive Safety: Protects occupants during a crash (seat belts, airbags, crumple zones).

8.3 Education and Enforcement

• Public Awareness Campaigns: On speeding, drunk driving, helmet and seatbelt use.

• Driver Training and Licensing.

• Strict and Consistent Enforcement of traffic laws by traffic police.

8.4 Emergency Response

• Quick and efficient post-crash care to minimize severity of injuries (Golden Hour concept).

Conclusion: Traffic engineering and safety is a dynamic field that blends human science with hard engineering. Its ultimate goal is to create a road transport system where human error does not lead to death or serious injury. This is achieved through intelligent planning, forgiving design, clear communication via control devices, and a holistic safe system approach involving all stakeholders.