The theory and concepts module on this topic covered the LOS grades and
their corresponding traffic flow descriptions, and also presented a simple formula for the
calculation of the delay. This module discusses the longer and more complex process for
calculating the delay and LOS of an existing intersection. The excerpt below gives an
overview of the process used in operational analysis. Your text probably contains a
detailed description of this process, and you would be wise to look it over.
The following excerpt was taken from the 1994 edition of the Highway Capacity
Manual, published by the Transportation Research Board.
Operational analysis results in the determination of capacity and level of service for
each lane group as well as the level of service for the intersection as a whole. It
requires that detailed information be provided concerning geometric, traffic, and
signalization conditions at the intersection. These may be known for existing cases or
projected for future situations. Because the operational analysis of signalized
intersections is complex, it is divided into five distinct modules, as follows:
- Input Module: All required information upon which subsequent computations are based is
defined. The module includes all necessary data on intersection geometry, traffic volumes
and conditions, and signalization. It is used to provide a convenient summary for the
remainder of the analysis.
- Volume Adjustment Module: Demand volumes are generally stated in terms of vehicles per
hour for a peak hour. The volume adjustment module converts these to flow rates for a peak
15-min analysis period and accounts for the effects of lane distribution. The definition
of lane groups for analysis also takes place in this module.
- Saturation Flow Rate Module: The saturation flow rate is computed for each of the lane
groups established for analysis. The flow rate is based upon adjustment of an
"ideal" saturation flow rate to reflect a variety of prevailing conditions.
- Capacity Analysis Module: Volumes and saturation flow rates are manipulated to compute
the capacity and v/c ratios for each lane group and the critical v/c ratio for the
- LOS Module: Delay is estimated for each lane group established for analysis. Delay
measures are aggregated for approaches and for the intersection as a whole, and levels of
service are determined.