occurs when flow is periodically interrupted by external means, primarily traffic control
devices such as stop and yield signs and traffic signals. Under interrupted flow conditions,
traffic control devices play a primary role in defining the traffic flow, while vehicle – vehicle
interactions and vehicle-roadway interactions play only a secondary role. For instance, traffic
signals allow designated movements to occur only part of the time. In addition, because of
the repeated stopping and restarting of traffic steam on such facilities, flows occur in
platoons.
occurs when vehicle traversing a length of roadway are not required to stop by any cause of
external to the traffic stream, such as traffic control devices. Uninterrupted flow is regulated
by vehicle-vehicle interactions on one side and by the interactions between vehicles and the
roadway environment and geometry on the other side.
– Volume is simply the number of vehicles that pass a given point on the roadway
or a given lane or direction of a highway in a specified period of time. The unit of volume is simply vehicles,
although it is often expressed as annual, daily, hourly peak and off-peak
Daily Volume. Daily volumes are frequently used as the basis for highway planning, for general trend
observations, as well as for traffic volume projections.
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Average Annual Daily Traffic (AADT)
Average Annual Weekday Traffic (AAWT)
Average Daily Traffic (ADT)
Average Weekday Traffic (AWT)
Hourly Volumes. Hourly Volume are designed to reflect variation of traffic over the different time period
of a day. They are also used to identify a single hour or period of the highest volume in a day occurring the
morning and evening commute, that is, RUSH HOURS. The single hour of the day corresponding to the highest
hourly volume is referred to us PEAK HOUR.
–
– 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). Most of the time, each vehicle will have a speed that is
somewhat different from the speed of the vehicles around it. In quantifying traffic streams, the average speed of
the traffic is the significant variable. There are two types of mean speeds: time mean speed and space mean
speed.
TIME MEAN SPEED - is the arithmetic mean of the speeds of vehicles passing a point on a highway
during an interval of time.
SPACE MEAN SPEED- is the harmonic mean of the speeds of vehicles passing a point on a highway during
an interval of time. It is obtained by dividing the total distance traveled by two or more vehicles on a section of
highway by the total time required by these vehicles to travel that distance
– 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. High densities indicate that individual vehicles are very close to each other, while
low densities imply greater distances between vehicles.
FLOW, SPEED, DENSITY RELATIONSHIP
The general equation relating flow, density, and space mean speed is given as:
FLOW, SPEED, DENSITY RELATIONSHIP
FLOW, SPEED, DENSITY RELATIONSHIP
SAMPLE PROBLEM
The following data were taken on five vehicles travelling a 1.5km portion of the NLEX
VEHICLES
TIME (MINUTES)
1
1.2
2
1.0
3
1.4
4
1.3
5
1.1
a.) Determine the space mean speed in kph
b.) Determine the time mean speed in kph
From the following data of a freeway surveillance, there are five vehicles under observation and the following distances
are the distance each vehicle had travelled when observed every 2 seconds.
VEHICLES
Distance (m)
1
24.4
2
25.8
3
24.7
4
26.9
5
22.9
a.) Determine the space mean speed in kph