Tidbit Tuesday
Tuesday, February 18, 2020 at 3:05:06 PM
Tidbit Tuesday #12

Let's Talk About Demand - Pt 3

Welcome back to our demand series!

We calculated unmet demand by approach in our last tidbit, so now it’s time to analyze it in traffic analysis software. The software needs to know what volumes arrived at the intersection in each 15 minute period, so whether that’s one overall number or you’re separating the volumes that went through the intersection (TMCs) with those that wanted to, but couldn’t (unmet demand), it’s still the same overall volume.

Since demand was captured for each approach, not per movement, it’s necessary to proportion the unmet demand to get the per movement numbers. Use the TMCs to determine the percentage of each movement per approach, then multiply the movement percentage with the unmet demand and add it to the corresponding movement (see below table for example). Count software typically calculates approach and intersection percentages/proportions, so those percentages can be pulled straight from the count report.

The example below shows a south approach of collected TMCs with all movements. The unmet demand is 40 vehicles for the entire approach, so multiply 40 with each proportional movement. The results are that 8 vehicles went left, 16 vehicles went through and 16 vehicles went right.

The software inputs for total demand are 645 left, 1417 through and 1376 right for this 15 minute period. Follow this format to enter the total demand for the rest of the approaches. Though current DOTD approved software asks for the total demand, DOTD still requires raw counts and unmet demand calculations in the appendix.

If you'd like to contact us with any comments, questions or tidbits of your own, please email TrafficEngineering@la.gov.

Thanks for reading!

Bonus tip: In HCS7, you can select a field and hit the F1 (Function 1) key to see an explanation of the input. The F1 key is usually linked to Help files in a lot of software. For laptop users, you may need to press the Function key with F1.