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Cal Poly Pomona CE 222 - Notes for Chapter 6 Traffic Characteristics

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CE222 Highway EngineeringNotes for Chapter 6 Traffic Characteristics1. Traffic Flow CharacteristicsTraffic on a roadway is considered as a flow just like water flowing in a pipeline or ariver. This assumption is not accurate but realistic in highway engineering. Anunderstanding of traffic flow characteristics is very important for highway design.Parameters measuring traffic flow:Traffic Speed, Travel Time, Volume, Density, and Capacity1) Traffic Speed and travel timeKeep in mind the speed and travel time here are related to traffic FLOW.Speed:Spot speed: Instantaneous speed of a vehicle MPH or KM/HMeasuring equipment: laser gunTime-mean speed: Equation 6-1 on page 132Space-mean speed: Equation 6-2 on page 132Relation between time-mean speed and space-mean speed:See Equation 6-3Spot speed is related to an individual vehicle in the traffic flow.Time-Mean speed and space-mean speed are measures obtainedfrom a sampling set of vehicles in the traffic flow.Overall speed A measure of traffic flow on a relatively long section of street orhighway between an origin and destination. Testing vehicle represents traffic flow, “floating” in the trafficflow, no passing over other vehicles and no being passed by othervehiclesTotal distance / total time including traffic delayRunning speedA measure similar to overall speed Total distance/ running time2) Traffic Volume and rate of FlowTraffic Volume = # of vehicles passing a point along a roadway or traffic lane perunit of timeADT – average daily traffic volume# of vehicles passing a point on a roadway during a period of 24consecutive hours averaged over a period of 365 days.DHV – design hourly volume It is a measurement to indicate the demand or intensity of the facility.30th-highest hourly volume of the design year, Why 30th? Rate of Flow: See Page 135. Very important conceptPeak Hour Factor: ratio of total hourly volume to the maximum rate of flowwithin a hour.3) Traffic Density or traffic concentrationAverage number of vehicles occupying a unit length of roadway at a given instant.Vehicles per mile4) Spacing and Headways Exam traffic flow at the level of individual vehiclesRequire knowledge of probabilityStudy Example 6-1 with Poisson distribution if students have time5) Speed-Volume-Density relationshipsThree charts are very important. Students should understand the relationship anddraw them without reading the book.2. Highway CapacityCapacity is a measure to describe the ability of accommodating vehiclesLOS concepts: level of services A, B, C, D, E, and FService flow rate conceptTable 6-2 shows measures of effectiveness for level of service definition,. This tableis important.Highway classification from the capacity calculation point of view:Uninterrupted flow and interrupted flowFactors affecting capacity, service flow rate and LOSIdeal conditions: Students MUST KNOW these conditionsSF = MSF * N* Fw*Fhv*FpStudents MUST understand this equation and its related


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Cal Poly Pomona CE 222 - Notes for Chapter 6 Traffic Characteristics

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