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The level of service can be derived from a road under different operating
characteristics and traffic volumes.
The factors affecting level of service (LOS) can be listed as follows:
- Speed and travel time
- Traffic interruptions/restrictions
- Freedom to travel with desired speed
- Driver comfort and convenience
- Operating cost.
Highway Capacity Manual(HCM) used travel speed and volume by capacity ratio
(v/c ratio) to distinguish between various levels of service.
The value of v/c ratio can vary between 0 and 1.
Depending upon the travel speed and v/c ratio, HCM has defined six levels of
service, level A to level F based on a graph between operating speed and v/c
ratio as shown in the figure 1.
Figure 1:
Level of service A to F
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Level of service A represents the zone of free flow.
Here the traffic volume will be less, traffic will be experiencing free flow
also.
The drivers will be having the complete freedom to choose their desired speed.
Even at maximum density, for this LOS the average spacing between vehicles is
167 m.
Lane changes within the traffic stream, as well as merging and diverging
movements, are made relatively easy.
The effect of minor incidents and point breakdowns are easily aborted at this
level.
Level of service B represents zone of reasonably free flow.
Free flow speeds are still maintained at this level of service.
The drivers freedom to choose their desired speed is only slightly restricted.
The lowest average spacing between vehicles is about 100 m.
The effects of small incidents and point breakdowns are still easily contained.
At level of service C, the presence of other vehicles begins to restrict the
maneuverability within the traffic stream.
Average speeds remain at or near the free flow speed level, but significant
increase in driver vigilance is required at this level.
Minimum average spacing between the vehicles is in the range of 67 m.
Queues may be expected to form behind any significant blockage.
At level of service D, the average speeds begin to decline with increasing
flows.
Freedom to maneuver within the traffic stream is noticeably restricted.
At this level, density deteriorates more quickly with flow.
The spacing between the vehicles is about 50 m.
As the traffic stream has little space to absorb disruptions, minor incidents
can lead to queuing of vehicles.
Level of service E define operation at capacity.
At this level, the stream reaches it's maximum density limit.
There will be no usable gaps in the stream and even slight disruptions will
cause a breakdown, with queues forming rapidly behind the disruption.
Maneuvering within the traffic stream becomes extremely difficult.
Level of service F describes conditions in a queue that has formed behind a
point of breakdown or disruption.
As vehicles shuffle through the queue, there may be periods when they move
quickly, and others when they are stopped completely.
Thus this level of service is used to describe the point of breakdown as well,
eventhough operations downstream of such a breakdown may appear good.
Level of service F represents the region of forced flow, having low speed, and
complete breakdown of the system .
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