13.6 Unmet Demand

13.6.1 Overview

Unmet demand refers to the number of vehicles that were destined to travel through a network at a specific time period (e.g., signal phase, signal cycle, peak hour, etc.) but were unable due to capacity constraints. A visual representation of unmet demand is shown in .
The need to include unmet demand in an analysis is determined prior to model development, typically in the project scoping stage and data collection stage. The main components with unmet demand are:
  • Determining locations that may have unmet demand;
  • Estimating unmet demand; and
  • Integrating unmet demand into a microsimulation model
Each of these steps is discussed below.
Graphical Depiction of Unmet Demand
Figure 13-7: Graphical Depiction of Unmet Demand
Source: UDOT Traffic Analysis Guidelines

13.6.2 Unmet Demand Locations

Unmet demand is common during oversaturated conditions on network facilities. Intersections and freeway merge, diverge, and weaving segments sometimes experience unmet demand. During field visits, attention is given to locations that experience long queues, heavy congestion, and/or trip diversion, as these situations often indicate unmet demand exists. Unmet demand is estimated for facilities that experience the situations described above.

13.6.3 Estimating Unmet Demand

Unmet demand is measured in vehicles and is typically estimated by dividing the queue length by the average vehicle occupancy length. The unmet demand queue length is equal to the total queue length minus the served queue length. Average vehicle occupancy lengths typically range from 25 to 50 feet and vary based on local conditions.
Field measurement is a common method to estimate queue length. Real-time online congestion maps (e.g., Google maps, Bing maps, INRIX, etc.) may be used to understand average queue lengths. For intersections, queue length is collected at the start of red and averaged over the entire analysis period. For freeway facilities, queue length is collected and averaged over the entire analysis period. Unmet demand data may also be collected from Traffic Management Centers (TMCs). In circumstances where vehicles are diverting from their normal route due to congestion, a diversion analysis using a TDM or other methods may be used to estimate the number of diverting vehicles.
After unmet demand is estimated, the additional volumes are added to the microsimulation model. Appropriate measures are taken to confirm that all vehicles are allowed entry into the model (e.g., review error logs to see how many vehicles were denied entry). It is important to confirm all vehicles enter the model, because vehicles that are denied entry to the model are not tracked which may lead to inaccurate model results.

13.6.4 Integrating Unmet Demand into a Microsimulation Model

The following steps are recommended to be taken after unmet vehicle volumes have been added to the model. These steps may help troubleshoot denial of entry to the network.
  • View error logs to see where vehicles are being denied entry.
  • Verify denial of entry is not due to coding errors (e.g., incorrect lane assignments, signal timing errors).
  • Extend geographic limits (spatial) of the network to accommodate the maximum queue lengths.
  • Extend time limits (temporal) to accommodate the traffic demand. Typically, the analysis begins before the onset of congestion and ends after congestion dissipates.
  • If the previous steps do not resolve denial of entry, the model results are typically adjusted to account for unmet demand in the analysis outputs. Unmet demand delay is typically reported as a separate category to served demand delay.
The unmet demand may not be included in volume served calibration thresholds. For more information on calibration thresholds, see
Section 13.5
.