Fundamental Traffic Flow Characteristics - Speed, Flow and Density

The three fundamental traffic flow characteristics are speed (v), flow (q) and density (k). The characteristics are generally defined by traffic flow measurements, and time/space mean speed measurements

Three fundamental characteristics of traffic are


  • Flow (q)

  • Speed (v)

  • Density (K)

Traffic flow (q)


Traffic flow is the number of vehicles passing through a given point in a particular time (veh/hr).

Traffic flow measurements


We have seen traffic flow is measured in the unit of vehicles per hour. There are different ways to measure traffic flow.


  1. AADT (Annual average daily traffic): The number of vehicles travelling in particular points, averaged over 365 days.

  2. AAWT (Annual average weekday traffic): The number of vehicles travelling in particular points, averaged over weekdays.

  3. ADT(Average daily traffic): Traffic is averaged over particular days, instead of whole year traffic measurement.

  4. AWT(Average weekday traffic): It’s similar to ADT, but the total traffic is averaged over weekdays only.


Density


The number of vehicles occupying a stretch of road of unit length at a given instant is called traffic density of the stretch.


Speed


Speed of a vehicle is nothing but the distance covered by the vehicle in unit time. Some of the points to remember about speed are


  • Speed of the vehicle at a particular instant is known as spot speed (Instantaneous speed).

  • Based on this speed we design the radius of curve and geometric features of the road.

  • It helps the designer to decide the safe speed limit and the location of speed limit design.


Types of Speeds


  1. Running speed: Speed measured when the vehicle is in running condition

  2. Journey speed: Speed measured considering the delays of the vehicle during the travel


  1. 15th percentile speed: It is the speed, below which 15% of the vehicles will pass. It is the minimum speed with which the vehicles should pass on the road

  2. 85th percentile speed: It is the speed, below which 85% of the vehicles will pass. It is the upper speed limit, most of the vehicles on the road would travel with this speed

  3. 98th percentile speed: It is the speed, below which 98% of the vehicles will pass. It is used as the design speed


Design speed plays an important role in decding the geometric characteristics of the road, and stopping sight distance in particular. 

Time mean speed (TMS)


The average speed of the vehicles passing through a given point is called Time mean speed.


Space mean speed (SMS)


The average speed of the vehicles in a given section is known as Space mean speed.


  • If we know the instantaneous speeds of all the vehicles at one instance we could find space mean speed.

  • As it is difficult to measure the speed of all the vehicles in a single instant, we measure it at a particular section, from there find out space mean speed.

  • Space Mean speed is the harmonic mean of the instantaneous speeds.


Space mean speed is always lesser than or equal to the time mean speed. In space mean speed over a section, for slow-moving vehicles, the weightage will be given more.

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