Appendix D. North Central Texas Council of Governments Ranking Model

This appendix is largely abstracted from the reference.

The NCTCOG ranking model is based on the existing traffic conditions. The variables used in the model and their weights are discussed in this section.

Variables

Total delay

Delay is the most frequently used measure of effectiveness for signalized intersections. Delay can be quantified in many different ways: stopped time delay, approach delay, travel time delay and time-in-queue delay (McShane and Roess, 1998). Travel time delay is used in this research. Travel time delay of an individual vehicle is the difference between the measured travel time and the travel time at the desired speed. Measured travel time is taken as an average of travel time in both directions of travel. The desired speed is taken as the posted speed. In this model, delay is used on an aggregate basis, and it is calculated below:

DPV = delay/vehicle/intersection = (measured travel time - desired travel time)/ (number of intersections)

Total delay/ intersection = DPV x ADT

Where ADT is the average daily traffic.

Number of Stops

The number of stops is taken as the average of the number of stops counted in both directions of travel along the corridor. To get the aggregate value, this average value per intersection is multiplied by the ADT.

Number of stops per intersection = (Number of stops/number of intersections) x ADT

System type

There are three types of existing systems. A value of one indicates that all intersections are part of an existing interconnected system with communications. A value of two indicates that some but not all intersections are part of an existing interconnected system with communications. A value of three indicates that there is no system (currently an isolated operation).

Weightings

The weighting for each factor is allocated by an expert group. The weightings are presented in Table 1.

Variable	Weighting
Total Delay (DELAY)	50
# of stops (STOPS)	30
System type (SYSTEMTYPE)	20

Calculation of Rank Order

Using the weightings applied by the NCTCOG, the following equation is developed.

Total Score(s) = (Delay /Max(Delay))×50 + (Stops/Max(Stops))×30 + System_Type×20

Where System_Type = 1.0 for type 1 (all signals interconnected)
0.5 for type 2 (some signals interconnected)
0 for type 3 (all signals isolated)

Quantitative variables DELAY and STOPS are normalized by dividing by the maximum value from all of the candidate corridors, which precludes any single variable dominating the total score because of its magnitude relative to the other variables. After normalization, each variable is expressed on a zero-to-one scale and the weights are an expression of the relative importance of each criterion.. The maximum value of a variable in the given data is used for normalization.

REFERENCE

Pulipati, S.B., "Regional Prioritization of Corridors for Traffic Signal Retiming", University of Texas Arlington.

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