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A term closely related to capacity and often confused with it is service
volume.
When capacity gives a quantitative measure of traffic, level of service or LOS
tries to give a qualitative measure.
A service volume is the maximum number of vehicles, passengers, or the like,
which can be accommodated by a given facility or system under given conditions
at a given level of service.
For a given road or facility, capacity could be constant.
But actual flow will be different for different days and different times in a
day itself.
The intention of LOS is to relate the traffic service quality to a given flow
rate of traffic.
It is a term that designates a range of operating conditions on a particular
type of facility.
Highway capacity manual (HCM) developed by the transportation research board of
USA provides some procedure to determine level of service.
It divides the quality of traffic into six levels ranging form level A to level
F.
Level A represents the best quality of traffic where the driver has the freedom
to drive with free flow speed and level F represents the worst quality of
traffic.
Level of service is defined based on the measure of effectiveness or (MOE).
Typically three parameters are used under this and they are speed and travel
time, density, and delay.
One of the important measures of service quality is the amount of time spent in
travel.
Therefore, speed and travel time are considered to be more effective in
defining LOS of a facility.
Density gives the proximity of other vehicles in the stream.
Since it affects the ability of drivers to maneuver in the traffic stream,
it is also used to describe LOS.
Delay is a term that describes excess or unexpected time spent in travel.
Many specific delay measures are defined and used as MOE's in the highway
capacity manual.
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