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 Signal Timing Design: Professional Practice

 
Computing Delay and LOS, Operational Analysis Outline

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

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:

  1. 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.
  2. 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.
  3. 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.
  4. 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 intersection.
  5. 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.