Traffic Flow Characteristics and Theory Primary Elements of Traffic - - PowerPoint PPT Presentation

Traffic Flow

Characteristics and Theory

a. Flow Rate = q (vehicle per hour, vph) b. Density = k (vehicle per mile, vpm) c. Speed = u (mile per hour, mph)

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• Dr. Randa Oqab Mujalli

Primary Elements of Traffic Flow

Flow

• Flow (q): is the equivalent hourly rate at which

vehicles pass a point on a highway during a time period less than 1 hour.

• It can be determined by:

Density

• Density (k): sometimes referred to as concentration,

is the number of vehicles traveling over a unit length

• f highway at an instant in time.
• The unit length is usually 1 mile (mi) thereby making

vehicles per mile (veh/mi) the unit of density.

Speed

• Speed (u): is the distance traveled by a vehicle during

a unit of time.

• It can be expressed in miles per hour (mi/h),

kilometers per hour (km/h), or feet per second (ft/sec).

Speed

• Time mean speed ( ): is the arithmetic mean of the

speeds of vehicles passing a point on a highway during an interval of time.

• The time mean speed is found by:

t

u

Speed

• Space mean speed ( ): is the harmonic mean of the

speeds of vehicles passing a point on a highway during an interval of time.

• This is the speed that is involved in flow-density

relationships.

• The space mean speed is found by

s

u

Average running speed = another type of space mean speed, Only the time while the vehicle in motion is considered (stopping time is excluded) Average travel speed= average of speeds of a group of vehicles at long section (stopping + movement)

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• Dr. Randa Oqab Mujalli

Space mean speed Types:

Headway

• Time headway (h): the difference between the time the front
• f a vehicle arrives at a point on the highway and the front of

the next vehicle arrives at that same point (sec)

• Time headway distribution helps to measure platooning in

the traffic stream and can be used to obtain delay measures due to various traffic control

• Dr. Randa Oqab Mujalli

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Time headway

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Space headway

• Space headway is the distance between the

front bumpers of successive vehicles at any given instant, expressed in feet.

• It has a great effect on the drivers choice of

speed

• Dr. Randa Oqab Mujalli

FLOW-DENSITY RELATIONSHIPS

• Flow= density × space mean speed (q=k*Us)
• Flow = 1/time headway (q = 1/ha)
• Density = 1/ space headway (k = 1/da)
• space mean speed= space headway / average time

headway (Us= da/ha)

• 1. Speed-Density Relationships
• The relationship is approx

linear with –ve slope

• As density increase, the speed
• f the vehicles in the traffic

stream decreases

• Free flow speed occurs at low

density, point (approach) to zero) (assume one vehicle on the road)

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• Dr. Randa Oqab Mujalli

• 2. Flow-Density Relationship

qmax Flow Density kj km

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• Dr. Randa Oqab Mujalli

Jam density, kj capacity At zero density, the flow is zero (no vehicles on the highway) As the density increase the flow increases Increase in density results in reduction in flow , it reaches zero when the density is maximum (jam density, kj) Density is maximum (Jam density, kj), there is no flow of vehicles (q = 0)

• 3. Flow-Space Mean Speed Relationship

Flow Space mean speed

qmax Uf

Congested Flow Uncongested Flow

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• Dr. Randa Oqab Mujalli

capacity

Flow, Speed, Density Relationships

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• Dr. Randa Oqab Mujalli