Tidbit Tuesday
Tuesday, January 28, 2020 at 4:11:59 PM
Tidbit Tuesday #11

Let's Talk About Demand - Pt 2

Welcome back to our demand series!

Last Tidbit Tuesday we said that demand is captured throughout the peak period, but that doesn't help us determine what or where the unmet demand exists during the peak hour.

To capture unmet demand, we take turning movement counts (TMCs) for each approach and measure their respective approach queues at 15 minute marks throughout the period. That means having separate counters and queue observers since approach queues often spill back into other intersections or stretch out of sight.

The approach queue is not demand, nor is it unmet demand. Unmet demand is the change in the approach queues from each 15-minute period to the next. To determine true demand for each 15-minute period, subtract the previous approach queue (previous arrivals) from the TMC (departures), then add the current approach queue (arrivals).

Let’s recap with an example using the chart below:

Using the numbers for the 3rd period as an example, we perform the calculation of 193 - 25 + 64 = 232. This shows that 232 is the total 15-minute demand and 39 is the unmet demand since they arrived before 7:45 am, but weren’t able to go through the intersection until the 4th 15-minute period. This helps to illustrate the importance of capturing demand per approach since approaches usually have different demands throughout the hour.

Next time, we’ll talk about how to distribute the unmet demand.

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

Thanks for reading!