TRAFFIC FLOW FUNDAMENTALS
Describing traffic is considered very difficult due to several factors that cause its irregularity or unpredictability. These factors could be attributed to several events which could happen on the road:
Accidents
Stalled vehicles
Lane changing or swerving
Parking maneuvers
Indiscriminate loading and unloading of PUV, etc.
Without these events, traffic flow could be expected to be fairly regular and predictable within a day or even within a week
Types of Flow
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Traffic flow is usually classified as either:
uninterrupted
flow occurring at long sections of road where vehicles are not required to stop by any cause external to the traffic stream
Interrupted
flow occurring at intersections or driveways where vehicles are required to stop by any cause outside the traffic stream – such as (STOP or YIELD), traffic signal lights, etc.
Major Traffic Variables
Uninterrupted flow can be described using any of the following traffic variables: A. Flow rate or volume B. Speed C. Density or concentration
Flow Rate or Volume
Flow rate is defined as the number of vehicles passing a point during a specified period of time. It is often referred to as volume when measured over an hour.
Flow rate (q) may be expressed as: q = N/T
Where: N = number of vehicles T = observation period
If T is set to one hour, q is called volume and will have a unit of vehicles per hour
Example: Suppose a 15-minute count of vehicles bound for Manila was conducted at a particular location on Quezon Avenue. A summary is shown in the table below TYPE
15-minute Count
Car / Van
420
Jeepney
300
Bus
16
Truck
28
Estimate the flow rate in vehicles per hour Solution: The total number of vehicles in 15 minutes is 764. This is expanded to one hour by multiplying by 4. Therefore, q = 3,056 vehicles per hour
Speed
Speed (u) is defined as rate of motion in distance per unit time. When describing traffic stream, two types of speed are used: A. Time Mean Speed B. Space Mean Speed
A. Time Mean Speed
Also called spot speed. It is simply the arithmetic mean of the speeds of vehicles passing a point within a given time interval. Strictly speaking, distance or length of road must be known in order to measure speed. However, with the use of radar, spot speed can be measured at a certain point on the road. The known distance is normally called trap length in spot speed studies. With the use of stopwatch, the time it takes for the vehicle to traverse this strap length is measured. Individual speed of vehicles is then computed by dividing this trap length by the measured time.
Knowing the individual speeds of n vehicles observed within time T, the time mean speed or spot speed of the traffic stream is computed.
Example: The speed of 25 cars were observed. 10 cars were noted to travel at 35 kph, 8 cars at 40 kph, 2 cars at 50 kph, and 5 cars at 45 kph. Assuming that each car was traveling at constant speed, determine the time mean speed.
Solution: u = [(10x35) + (8x40) + (2x50) + (5x45)] / 25 = 39.8 kph
It is common practice among traffic engineers to report “spot speed” for a given location. Spot speed is often used as basis for establishing speed limits.
B. Space Mean Speed Space mean speed is used to describe the rate of movement of a traffic stream within a given section of road. It is the speed based on the average travel time of vehicles in the stream within the section. It is also called the harmonic mean speed.
Example: Using the previous example, determine the space mean speed
Solution: u = 25 / [(10/35) + (8/40) + (2/50) + (5/45)] = 39.26 kph
Density
Density (k) is defined as the number of vehicles in a given length of road at an instant point in time.
k=n/L
Among these three variables, density proves to be the most difficult or expensive to observe. Aerial photography is the most commonly used method to get its exact values. It will be seen later that these three variables are actually related to each other and that density can be derived if volume and speed are known.
Other Traffic Variables
Time Headway (h)
Time headway is defined as the time interval between passage of consecutive vehicles at a specified point on the road with a unit of time per vehicle.
h=1/q
Example: During morning peak hour, the average headway of UP-Katipunan jeepneys is determined at 5 minutes. If the passenger demand during the same period is 240, determine whether there is a need to increase the number of jeepney units (or shorten the headway) for this route. Assume that passenger demand is evenly distributed within that period and the average load/occupancy is 14 passengers per jeepney.
Solution: q = 60 / 5 = 12 jeepneys per hour With an average load of 14 passengers per jeepney, the total number of passengers that can take a ride is 12 x 14 = 168 passengers. Since the demand is 240 passengers, it can be said that there is a need to increase the number of jeepney units during peak period
Spacing (s)
Spacing is the distance between two vehicles measured from the front bumper of a vehicle to that of another.
s=1/k Example: During heavy traffic congestion, it was observed that the average spacing of vehicles in queue in the innermost lane of EDSA is 6.5 m. Determine the jam density or density of stopped vehicles.
Solution: k = 1,000 / 6.5 = 153.85 vehicles / km
Time Occupancy
Another useful measure of traffic flow is time occupancy. It can only be measured, however, if a detector is installed at a specific point on the carriageway. It is defined as the total time a detector is occupied divided by the total time of observation.
