A review of the methods currently available for use in developing intersection turning movementsindicates that many of the methods can be categorized as “intersection balancing” methods. Thedegree of accuracy that can be obtained from “intersection balancing” methods depends on themagnitude of incremental change in land use and travel patterns expected to occur between thebase year and future design year conditions.

These balancing techniques are used to adjust existing counts as well as model generated volumes.The balancing techniques are also used for corridor development. The assignment of future turnpaths is estimated, and often the departure and arrival volumes between intersections on the samelink need to be balanced. The algorithms used for the balancing may not be capable of achieving thedesired convergence criteria. Existing counts need to be balanced because the turning movementsoccurring at some driveways may not be included in traffic counts. The driveways which may notbe counted are often commercial strip centers, gas stations, and other curb cuts which influencethe traffic at intersections.

The roadway network coded in the model generally includes all important roadways. However, somecollectors and local roads that are not coded may be the key roadways serving the specific projectinfluence area. To account for the missing roadways and missing driveway information, balancingtechniques are used to estimate turning movement traffic volumes.

Most algorithms that have been developed to date are somewhat interrelated and involve theapplication of an iterative procedure that balances future year turning movements based onexisting turning movement counts, approach volumes and/or turn proportions. Spreadsheetsare usually utilized for the efficient implementation of “intersection balancing” methods. Thesebalancing methods can be used for peak hour volumes required by traffic operations engineers,future traffic movements for traffic forecasting engineers, or any other application which requiresbalanced intersection movements.

The following sections of this chapter present an overview of each of the primary methodologies used by FDOT, including the input data required and the relative ease of application. The pertinent methods included in the NCHRP Report 765 are also discussed. The estimation of future turning movement volumes requires the collection of existing year turning movement counts. The time period, location, and duration of the turning movement counts depend on the travel characteristics of the study area. Roadways serving commercial uses, shopping centers, and schools may peak during the midday period or during the weekends. Turning movement counts outside the typical AM Peak and PM Peak periods, such as Mid-Day Peak for either weekdays or weekends, should be collected to capture the peak traffic for the study area. When collecting the turning movement counts, it should be noted that most turning movement counts at signalized intersections are performed by counting vehicles as they pass through the intersection, ignoring the unmet demand. This produces unrealistic data in oversaturated conditions, which does not represent the true demand at the intersection. Performing capacity analyses using data collected this way can severely underestimate the delay and back-of-queue results and yield inaccurate levels of service. For congested signals, arrival demand (not departure flows) must be used for the capacity analysis to accurately match field conditions.

In some cases, if existing turning movement counts are available and no major changes in land use patterns are expected, a growth factor method can be used to develop future turning movement volumes. However, approval from the District Planning Office or Project Manager is required before applying the simple methodology.