Traffic Engineering
What is Traffic Engineering?
The number of vehicles passing a point or section of a roadway during a specified period, usually expressed in vehicles per hour (veh/hr).
Key formula / rule: Fundamental Equation of Traffic Flow
Key points
- Define and differentiate between traffic flow, speed, and density.
- Explain the fundamental relationship between these three variables.
- Describe the typical speed-flow and flow-density curves.
- Understand the concepts of capacity and Level of Service.
Common exam trap
Confusing average speed with space mean speed or time mean speed.
Definitions
- Term
Traffic Flow (q)
- Meaning
The number of vehicles passing a point or section of a roadway during a specified period, usually expressed in vehicles per hour (veh/hr).
- Term
Traffic Speed (v)
- Meaning
The rate of motion of traffic, typically expressed as average speed in kilometers per hour (km/hr). Space Mean Speed (vs) is generally used in traffic flow analysis.
- Term
Traffic Density (k)
- Meaning
The number of vehicles occupying a given length of roadway at a particular instant, usually expressed in vehicles per kilometer (veh/km).
- Term
Capacity
- Meaning
The maximum sustained flow rate that can be achieved under prevailing roadway, traffic, and control conditions.
- Term
Level of Service (LOS)
- Meaning
A qualitative measure describing the operational conditions within a traffic stream, based on factors like speed, travel time, freedom to maneuver, traffic interruptions, and comfort.
Learning objectives
Define and differentiate between traffic flow, speed, and density.
Explain the fundamental relationship between these three variables.
Describe the typical speed-flow and flow-density curves.
Understand the concepts of capacity and Level of Service.
Formulae
- Name
Fundamental Equation of Traffic Flow
- Note
Where q is flow rate (vehicles/hour), v is average speed (km/hour), and k is density (vehicles/km). Ensure consistent units.
- Expression
q = v * k
- Name
Time Mean Speed (vt)
- Note
Average speed of vehicles passing a point over a period of time.
- Expression
vt = (Σvi) / N
- Name
Space Mean Speed (vs)
- Note
Average speed of vehicles over a given length of roadway. Also, vs = q / k. This is the speed used in the fundamental equation q = v * k.
- Expression
vs = L / (Σ(ti))
Prerequisites
Basic understanding of kinematics and units.
Familiarity with basic statistical concepts (average).
Common mistakes
Confusing average speed with space mean speed or time mean speed.
Incorrectly applying the q = v * k equation without considering the correct units or type of speed.
Assuming a linear relationship between speed, flow, and density across all traffic conditions.
Overlooking the impact of external factors like road geometry, traffic composition, and driver behavior on flow characteristics.
Keywords
Traffic Flow
Speed
Density
Capacity
Level of Service
Fundamental Equation
Traffic Stream
Practice preview
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