Traffic Engineering Fundamentals: Flow, Speed, and Safety
Traffic Engineering Basics
Traffic Flow Definitions
Volume/Flow: The total count of vehicles passing a specific point on a road within a given timeframe. This can be measured annually, daily, hourly, or in shorter intervals, typically expressed in vehicles per hour [v/h] or vehicles per day [v/d].
Volume represents the actual number of vehicles observed or predicted to pass a point during a set period.
Road Capacity: The maximum traffic volume a roadway can handle using all available lanes, usually measured in vehicles per hour or vehicles per day.
Density: The number of vehicles occupying a specific length of road, generally expressed as vehicles per kilometer (vehicles/km). It reflects how closely vehicles are packed, influencing maneuverability and driving comfort.
AADT (Average Annual Daily Traffic): The average 24-hour traffic volume at a location over a full year, calculated by dividing the total yearly vehicle count by 365.
ADT (Average Daily Traffic): The traffic volume counted on a road over a period longer than one day but less than a year, divided by the number of days in that period.
Speed Types and Percentiles
Vehicle speeds vary across time and space. Several speed types are used to represent these variations:
- Spot Speed: The instantaneous speed of a vehicle at a specific location. Used in road geometry design and accident analysis. Measured using pressure contact tubes or radar.
- Running Speed: The average speed maintained while a vehicle is moving, calculated by dividing the course length by the time the vehicle was in motion. It excludes delays.
- Journey Speed: The effective speed of a vehicle on a journey, calculated by dividing the distance by the total travel time, including delays and stops.
- Time Mean Speed: The average speed of all vehicles passing a point on a road over a specific time.
- Space Mean Speed: The average speed of all vehicles occupying a section of a road over a specific time.
Traffic Measurement Methods
- Manual Counting: Data collected using hand tally and manual counters.
- Direct Method: Data counted directly using hand tally counters.
- Indirect Method: Data collected via video recording and analyzed later.
- Contact System Based: Uses pneumatic, mechanical, magnetic, or radar methods.
- Contactless System Based: Uses electrical/optical, ultrasound/infrared radar, microwave, or CCTV/video image processing.
Factors Affecting Vehicle Counting Accuracy
Several factors influence traffic counting accuracy:
- Weather conditions
- Purpose of the traffic count
- Method of traffic counting
- Location of counting sites
- Traffic flow level
- Road type
- Traffic composition
Level of Service (LOS)
Level of Service (LOS): Qualitative measures describing operational conditions within a traffic stream and how they are perceived by drivers and passengers.
Factors Affecting Road Capacity and LOS
Factors influencing road capacity and level of service include:
- Roadway Factors
- Traffic Factors
- Speed and Travel Time
- Freedom to Travel at Desired Speed
- Driving Comfort and Convenience
- Operating Cost
LOS Classification for Roads
- LOS A: Free-flow
- LOS B: Reasonably free-flow
- LOS C: Stable operation
- LOS D: Borderline unstable
- LOS E: Extremely unstable
- LOS F: Breakdown
LOS Classification for Intersections
LOS for intersections is classified based on control type and delay:
| LOS | Delay (sec/veh, signalized) | Delay (sec/veh, unsignalized) |
|---|---|---|
| A | ≤ 10 | ≤ 10 |
| B | 10-20 | 10-15 |
| C | 20-35 | 15-25 |
| D | 35-55 | 25-35 |
| E | 55-80 | 35-50 |
| F | > 80 | > 50 |
Critical Traffic Intensity
In telecommunications and traffic, intensity measures the average degree of resource use over a period, often a busy hour. It’s measured in traffic units (erlangs) and relates to the cumulative rate of a facility’s use.
Factors Influencing Traffic Safety
Several factors impact traffic safety:
- Weather conditions
- Personal characteristics
- Topographic conditions
- Rescue services
- Traffic supervision
- Car park availability
- Traffic infrastructure
- Traffic intensity
Grade-Separated and At-Grade Intersections
Grade Separation: A method of aligning junctions at different heights using underpasses or overpasses to avoid traffic disruption.
At-Grade Intersection: Two or more roads joining or crossing at the same level.
Types of Interchanges
- Four-way interchanges
- Three-way interchanges
- Two-way interchanges
- Service interchanges
Intersection vs. Interchange
An intersection is where roads cross at the same level. An interchange is a grade-separated intersection with ramps connecting different levels.
Geometry Elements of a Road Intersection
- Three-leg (T)
- Four-leg
- Multi-leg
- Roundabout
Suggested Angle for Intersection Roads
Minor road intersections with major roads should ideally be close to 90 degrees. Angles of 60 degrees are acceptable, providing most of the benefits of right-angle intersections.
Classification of Roundabouts
- Mini
- Small
- Medium
- Large
Advantages of Roundabouts
- High level of road safety
- Reduction in dangerous traffic events
- Elimination or reduction of head-on collisions, left-turn collisions, right-side impacts, and pedestrian accidents
Roundabout vs. Intersection with Central Island
Roundabouts offer minimal delays at lower traffic volumes, require little maintenance, and typically have lower crash severity than cross intersections.
Characteristics of a “Safety Intersection”
- Provide good visibility
- Be understandable
- Major interchanges
Reasons for Intersection Channelization
- Accommodate pedestrians, handicapped individuals, and bicyclists
- Reduce vehicular delay and fuel consumption
- Enhance safety
Comparing Traffic Safety Between Countries
Traffic safety is compared using fatality rates per accident. The Road Safety Performance Index (PIN) Program compares country road safety performance and shows the percentage change in road deaths.
Road Traffic Death Rate in Poland
The estimated road traffic death rate in Poland is 10 per 100 accidents.
Safest Country in the EU
Sweden is considered the safest country in the EU, with an estimated road traffic death rate of 2 per 100 accidents.
Traffic Calming
Traffic calming involves physical measures to reduce the negative impacts of motor vehicle use, alter driver behavior, and improve conditions for non-motorized street users.
Examples of Traffic Calming Measures
- Street closure
- Semi or diagonal diverters
- Road humps, bumps, and tables
- Cushions
- Roundabouts
- Curb extensions