04 CE 122 Traffic Characteristics_addendum

8/13/2019 04 CE 122 Traffic Characteristics_addendum

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School of CE-ENSEMapua Institute of Technology (MIT)


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traffic streams/flows are made up of individual drivers andvehicles, interacting in unique ways with each other and withelements of the road and the general environment

a given traffic flow through streets and highways will vary byboth location and time: non-uniform behavior due to varyingdriver behavior

Describing traffic is considered difficult due to several factorsthat cause its irregularity or unpredictability: accidents, stalledvehicles, lane changing/swerving parking maneouvers,indescriminate loading and unloading of public utility vehicles


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Types of Flow Uninterrupted flow (flow occurring at long sections of road where vehicles are

not required to stop by any cause external to the traffic stream

Interrupted flow ( flow occuring at intersections or driveways where vehiclesare required to stop by any cause outside traffic stream such as traffic sign,traffic signal light)

Traffic Flow ParametersMacroscopic traffic flow parameterscharacterize traffic stream as a whole

Traffic flow or volume

Speed

DensityMicroscopic traffic flow parameterscharacterize behavior of individualvehicles in the traffic stream with respect to each other

Headway

Spacing


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Traffic Flow /Volume defined as the number of vehicles that pass a point ona highway, or a given lane or direction of a highway, during a specified timeinterval

Unit: vehicles per unit time (ex. vehicles/hour or veh/hr or vph)

Common interval: day

q = N / Twhere q: flow or volume ; N: number of vehicles

T: time ; if T is set to 1 hour, q is called volume, will have unit ofvehicles per hour

Type 15-minute count

Car/van 420

Jeepney 300

Bus 16

Truck 28

Ex 1: Suppose a 15-minute count of vehicles bound for Manila was conductedat a particular location in Quezon Ave. Summary is shown below. Estimatethe flow rate in vehicles per hour.


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Annual Average Daily Traffic AADT) = average 24-hr traffic volume at a givenlocation over a full 365-day year

AADT = (total number of vehicles passing the site in a year)/365

Average Annual Weekday Traffic AAWT) = average 24-hr traffic volume occurringon weekdays over a full yearAAWT = (total weekday traffic volume)/260

Average Daily Traffic ADT) = average 24-hr traffic volume at a given location forsome period of time less than a year

= ADT may be measured for 6 months, a season, a month, a week or as little as 2days

Average Weekday Traffic AWT) = average 24-hr traffic volume occurring onweekdays for some period of time less than 1 year, such as for a month or aseason

if AADT > AAWT and ADT > AWT, this indicates weekend traffic is heavy andfacility serves primarily for recreational traffic

- volume varies throughout the day


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Peak-hoursingle hour of the day that has the highest hourly volumePeak-hour volume Directional volume flow directions are separated

Used as basis of highway design and many types of operational analysis

Highways must be designed to adequately serve the peak-hour traffic volume inthe peak direction of flow (both directions of the road facility)

Peak-hourly volumes are estimated from daily volume projectionsDDHV = AADT K Dwhere DDHV = directional design hour volume (vph)

AADT = average annual daily traffic (vpd)

K = proportion of daily traffic occurring during the peakhour, expressed as a decimal

D = proportion of peak-hour traffic traveling in the peakdirection, expressed as a decimal

Hourly Volumes


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-Rate of motion, in distance per unit time

u or s = d /t) 3.6where : U or S: speed of vehicle, in kilometer per hour

d: distance, in meters

t: time, vehicles has passed in seconds

-Two types of Speed: Time Mean Speed & Space Mean Speed

Time Mean Speed also called spot speed)- arithmetic mean of the speeds of vehicle passing a point within a

given interval of time.

um= ui) / n ; where um: time mean speed, ui: speed, n: no ofvehicles

Ex 2: The speed of 25 cars were observed. 10 cars were noted to travel at35kph, 8 cars at 40kph, 2 cars at 50kph, and 5 cars at 45kph. Assumingthat each car was travelling at constant speed, determine the time meanspeed.


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Space Mean Speed also called harmonic mean speed)- used to describe the rate of movement of a traffic stream within a

given section of road.

- speed based on average travel time of vehicles in the stream within

the section.

- If n vehicles are observed at an instant of time t, the space meanspeed is computed as follows:

- Us= n / (1/ui) ; where us: space mean speed, ui: speed, n: no of

vehicles

Ex 3 (Same as Ex.2): The speed of 25 cars were observed. 10 cars were notedto travel at 35kph, 8 cars at 40kph, 2 cars at 50kph, and 5 cars at 45kph.Assuming that each car was travelling at constant speed, determine thespace mean speed.


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-Travel time= total time to traverse a given highway segment;includes stopped delays

-Running time= total time during which the vehicle is in motion while

traversing a given highway segment- does not include stopped delays

-Average Travel Speed= distance of a given segment of roaddivided by average travel time on the given segment

-Average Running Speed = distance of a given segment of roaddivided by average running on the given segment


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Operating Speed = maximum safe speed at which a vehicle canbe conducted in a given traffic stream, w/o exceeding theroads design speed

Difficult to measure; it requires a test car driven through the trafficstream consistent with the definition

Percentile Speed =speed below which the stated percent of

vehicles in the traffic stream travel

85th percentile speed= often used as the maximum speed limits

Example: if 60 kph is the 85th-percentile speed = 85% of the vehicles inthe traffic stream travel at or below 60 kph


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-No of vehicles in a given length of road at an instant point in time.

-If n vehicles are found within section L, density k is computed as

K = n / L ; where k: density, n: no of vehicles, L: length

-among three variables, density proves to be most difficult or expensive

to observe. Aerial photography is the most commonly used method toget its exact values.


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Time Headway- defined as the time interval between passage of consecutive

vehicles at a specific point on the road with a unit of time per vehicle

- inverse of flow rate or volume

h= 1 / q where: h: ave. headway, q : flow rate or volume

Spacing- distance between two vehicles measured from the front bumper

of a vehicle to that of another

- inverse of density

sp = 1 / k where sp: ave. spacing, k: density


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Speed-Density relationship

u = uo[1-(k/kj)]k near zero (0):U=Uo=free-flow speedUo= 0: k =kj=jam density (high density

condition)

0 kj

EDIES MODEL [HOMBURGER (1982)]

Uo

Veh/km

Volume-Density relationship

q

qm

km k

u FREE-FLOW SPEED SEGMENT 1

uoMAX FREE

volumeum SEGMENT 2 VOLUME

CAPACITY

qm qFLOW

SPEEDUs

Speed-Volume relationship


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04 CE 122 Traffic Characteristics_addendum

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