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

Traffic Flow Characteristics and Theory

Primary Elements of Traffic Flow a. Flow Rate = q (vehicle per hour, vph) b. Density = k (vehicle per mile, vpm) c. Speed = u (mile per hour, mph) 2 Dr. Randa Oqab Mujalli

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 of 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 u t speeds of vehicles passing a point on a highway during an interval of time. • The time mean speed is found by:

Speed • Space mean speed ( ) : is the harmonic mean of the u s 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

Space mean speed Types: 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) 9 Dr. Randa Oqab Mujalli

Headway - Time headway (h): the difference between the time the front of 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 Time headway 10 Dr. Randa Oqab Mujalli

- 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 Space headway 11 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 of the vehicles in the traffic stream decreases - Free flow speed occurs at low density, point (approach) to zero) (assume one vehicle on the road) 17 Dr. Randa Oqab Mujalli

2. Flow-Density Relationship At zero density, the flow is zero (no vehicles on the highway) capacity q max Flow Increase in density results in reduction in flow , it reaches zero when the density is maximum (jam density, k j ) Density is maximum (Jam density, k j ), there is no flow of vehicles (q = 0) Jam density, kj As the density increase the flow increases 0 k m k j Density 18 Dr. Randa Oqab Mujalli

3. Flow-Space Mean Speed Relationship U f Space mean speed Uncongested Flow capacity Congested Flow 0 q max Flow 19 Dr. Randa Oqab Mujalli

Flow, Speed, Density Relationships Dr. Randa Oqab Mujalli 20