Congestion Index Calculator

Enter the volume of traffic (V) and the capacity of the road (C) into the calculator to determine the Congestion Index (defined here as the volume-to-capacity ratio expressed as a percentage: CI = (V/C) × 100). This calculator can also solve for any one of the three variables (V, C, or CI) when the other two are known.

Congestion Index

Travel Time Index

Enter any 2 values to calculate the missing variable

Volume of Traffic (vehicles per hour)

Capacity of the Road (vehicles per hour)

Congestion Index (V/C%)

Preset Capacity Helper (rough estimate, optional)

Road Type

Through Lanes (count in peak direction for freeway; total lanes for two-lane)

Terrain

Heavy Vehicles (%)

Free-flow speed ≥ 70 mph?

Peak Hour Factor (PHF, typical; used to convert peak 15-min flow to a peak-hour volume)

Estimated peak-hour capacity from presets (rough): —

Related Calculators

Congestion Index Formula

The following formula is used to calculate the Congestion Index.

CI = (V / C) * 100

Variables:

CI is the Congestion Index (%) (the volume-to-capacity ratio expressed as a percent)
V is the volume of traffic (vehicles per hour)
C is the capacity of the road (vehicles per hour)

To calculate the Congestion Index, divide the volume of traffic by the capacity of the road. Multiply the result by 100 to convert it to a percentage. This represents the percentage of the stated road capacity being used by traffic (i.e., V/C%). Values above 100% indicate demand exceeds the stated capacity (oversaturated conditions). Note: some organizations use the term “congestion index” for different, travel-time-based measures; this page’s CI is strictly a capacity-utilization measure.

What is a Congestion Index?

On this page, a Congestion Index is the volume-to-capacity ratio (V/C) expressed as a percentage. It is a simple indicator of how close a road segment (typically for a specific direction and time period) is operating relative to its stated capacity.

How to Calculate Congestion Index?

The following steps outline how to calculate the Congestion Index.

First, determine the volume of traffic (V) in vehicles per hour.
Next, determine the capacity of the road (C) in vehicles per hour.
Next, use the formula CI = (V / C) * 100 to calculate the Congestion Index.
Finally, interpret the result (for this definition, 100% means demand equals the stated capacity; values above 100% indicate oversaturation).
After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem :

Use the following variables as an example problem to test your knowledge.

volume of traffic (V) = 1800 vehicles per hour

capacity of the road (C) = 2000 vehicles per hour

CI = (V / C) × 100 = (1800 / 2000) × 100 = 90%. This means the road is operating at about 90% of its stated capacity.