Traffic Flow Fundamentals.ppt

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

•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. n

Σ ui

i= 1 ut = _________ n

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.

us = Example:

n _________ n 1 __ i= 1 ui

Σ

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.

RELATIONSHIP OF FLOW, SPEED, AND DENSITY

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• Dimensional analysis will show that flow rate (veh/hr) is simply the product of density (veh/km) and space mean speed (km/hr) or q = k x us

OBSERVED RELATION

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Emperical Relation Free flow speed (uf) – speed corresponding to zero density

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• Speed-Density Relation

As Speed decreases, density increases (v.v.) Their relationship is linear

Jam density (kj) – density corresponding to zero speed

Example

Data on density and speed were obtained from a fourlane, two way rural highway (in one direction only):

Density, veh/km

Speed, kph

75

45

15

85

142

10

100

30

Determine the relation between density and speed

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VolumeDensity Relation

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VolumeSpeed Relation

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VolumeSpeed Relation

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CAPACITY AND LEVEL OF SERVICE

Capacity – defined as the maximum hourly rate at which persons or vehicles can reasonably be expected to traverse a point or uniform section of lane or roadway during a given time period under prevailing roadway, traffic, and control conditions

Level of Service (LOS) – is a qualitative description of how a certain facility is performing

Traffic engineers rely on capacity and level of service analyses to determine the width and number of lanes when planning for new facilities or when expanding existing facilities that are already experiencing congestion problems. 3/28/2019

CAPACITY AND LEVEL OF SERVICE

The Philippine Highway Planning Manual (PHPM) developed by the Planning service of the DPWH provides a methodology to carry out the process of such analysis.

The LOS concept uses qualitative measures that characterize operational conditions within a traffic stream and perception of these conditions by motorists and passengers.

Six levels of service are defined for each facility (A-F). A is the best operating condition and F the worst. In the PHPM method, LOS are defined based on the computed volume and capacity ratio and the space mean speed of the traffic flow 3/28/2019

Level of Service

Description

A

Free flow, with low volumes and high speeds. Drivers are virtually unaffected by the presence of others

B

Comfort and convenience is somewhat less than at LOS A. Zone of stable flow with operating speeds beginning to be restricted somewhat by traffic conditions

C

Still in zone of stable flow, but speed and maneuverability are most closely controlled by higher volumes

D

Approaches unstable flow. Speed and freedom to maneuver are severely restricted, and the driver experiences a generally poor level of comfort and convenience

E

Flow is unstable, and there may be stoppages of momentary condition. Freedom to maneuver within the traffic stream is extremely restricted

F

Forced or breakdown flow. It is the point at which arrival flow causes the queue to form 3/28/2019

Volume-capacity ratio and LOS Level of Service

Volume-capacity ratio

A

Less than 0.2

B

0.21 - 0.5

C

0.51 – 0.7

D

0.71 – 0.85

E

0.86 – 1.0

F

Greater than 1.0

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Example:

From the previous example, if the traffic volume in the same direction where capacity was estimated was 2,050 vehicles per hour at a particular period, determine the level of service of the higway (in that direction)

Solution: qmax = 3,547.82 vehicles per hour

v/c = 2,050 / 3,547.82 = 0.578

From the table its LOS is C

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