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Introduction to Traffic Engineering. BY: ALAMEREW M. Objectives of this chapter. Help students lay solid foundation of traffic engineering as a whole Help students get general knowledge of traffic engineering from both theory and practice

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Objectives of this chapter

  • Help students lay solid foundation of traffic engineering as a whole
  • Help students get general knowledge of traffic engineering from both theory and practice
  • Lead students to traffic/transportation professional world

1.1 Definition, scope and goal

Definition of Traffic Engineering --- It is the phase of transportation engineering that deals with the planning, geometric design and traffic operations of roads, streets and highways, their networks, terminals, abutting lands, and relationships with other modes of transportation


Transportation Engineering is defined as a discipline applying technology and scientific principles to the planning, functional design, operation, and management of facilities for all modes of transportation


Scope of Traffic Engineering --- surface (land) transportation; relationships and connection with other modes of transportation

  • Major modes of surface transportation --- automobile, bus, truck and bike

Goal of Traffic Engineering --- explore how to provide for the safe, rapid, comfortable, convenient, economical, and environmentally compatible movement of people and goods.

  • Safe --- public safety
  • Rapid --- time value and customer service
  • Comfortable/convenient --- level of service
  • Economical --- social cost
  • Environmental --- clean air and sustainability
  • Movement = mobility

Capacity and level of service analyses

The no. of vehicles on our highways increases every year

and transportation engineers are often faced with the challenge of designing modifications to existing facilities that will service the increased demand.


Capacity: is the maximum no. of vehicles that a given highway can accommodate.

  • LOS: is an operating condition under capacity.

Traffic engineers use capacity & LOS analysis to:

      • Determine the no. and width of lanes needed
      • Assess service levels and operational characteristics of existing facilities.
      • Identify traffic and roadway changes needed for new dev’t.
levels of service

Free-flow operation


Reasonably free flow

Ability to maneuver is only slightly restricted

Effects of minor incidents still easily absorbed

Levels of Service

LOS concepts

Highway capacity manual defines the LOS categories for freeways and multilane highways as follows:

From Highway Capacity Manual, 2000

levels of service1

Speeds at or near FFS

Freedom to maneuver is noticeably restricted

Queues may form behind any significant blockage.


Speeds decline slightly with increasing flows

Density increases more quickly

Freedom to maneuver is more noticeably limited

Minor incidents create queuing

Levels of Service

From Highway Capacity Manual, 2000

levels of service2

Operation near or at capacity

No usable gaps in the traffic stream

Operations extremely volatile

Any disruption causes queuing


Breakdown in flow

Queues form behind breakdown points

Demand > capacity

Levels of Service

From Highway Capacity Manual, 2000


LOS determination

Base condition:

* lane width * terrain

* lateral clearance * traffic condition stream

* access frequency * driver population characteristics


  • Measure Free-flow speed from the field-is the mean speed of traffic as measured when flow rates are low to moderate
  • Analysis flow rate –the highest volume in a 24-hr (the peak-hr volume) is used for V in traffic analysis computation.
  • Determine LOS