Engineer has to quantify how well the 'system' or 'facility is working'.
The facilities will usually assembled by specific qualitative and quantitative
index of flow characteristics termed as Level of Service (LOS), in this regard
engineer has to do following works.
- Assessing the existing condition
- Evaluating alternative improvements
- Quantifying associated cost and benefits
- Communicating results to both technical and non technical people
As far as operational performance of urban streets is considered we are
interested in determining arterial level of service which is discussed in
succeeding section.
Urban streets LOS is mainly based on average travel speed for the segment or
for the entire street under consideration.
The average travel speed is computed from the running times on the urban street
and the control delay of through movements at signalized intersections.
The control delay is the portion of the total delay for a vehicle approaching
and entering a signalized intersection.
Control delay includes the delays of initial deceleration, move-up time in the
queue, stops, and re-acceleration, these delays are also known as intersection
approach delays.
The LOS for urban streets is influenced both by the number of signals per
kilometer and by the intersection control delay.
Inappropriate signal timing, poor progression, and increasing traffic flow can
degrade the LOS substantially.
Streets with medium-to-high signal densities (i.e., more than one signal per
kilometer) are more susceptible to these factors, and poor LOS might be
observed even before significant problems occur.
On the other hand, longer urban street segments comprising heavily loaded
intersections can provide reasonably good LOS, although an individual
signalized intersection might be operating at a lower level.
The term through vehicle refers to all vehicles passing directly through a
street segment and not turning.
Considering all the above aspects, HCM provides a seven step methodology to
determine the level of service of an arterial which will be discussed in
following section.
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