Geometry > Capacity Modifiers > Confidence Level

Confidence Level

HCM does not use Confidence Level.

Roundabout entries with identical geometry and flows do not give identical capacity but have between site "error" with some having higher and some lower capacity curves.

Models predict the mean value such capacity distributions.

For any situation the precise capacity is not known. All that is known is that it is somewhere on the capacity distribution. As the geometric capacity model knows the between site "error" capacity distribution it can produce the capacity for any point on the distribution including the mean capacity by using the Confidence Level (CL).

If the CL is set to 50% the mean capacity is estimated. Higher CL gives a more pessimistic capacity estimate. For example for an 85% CL capacity there is a probability of 0.85 that the capacity will not be less than the estimate.

The CL is very useful for testing designs to assess the risk of large queues and delays. A design that has acceptable queues and delays at 50% CL may be fine at 85% also or may have one leg where the queues and delays have greatly increased. This informs the designer that there is a risk with this leg while the others are robust. This encourages some minor redesign to achieve acceptable queues and delays at 85% CL resulting in a far more robust design with greatly reduced risk of failure.

When the CL is reset to 50% the queues and delays will appear little different than the original. If comparison is to be made with other models or any type of evaluation the CL should be set at 50%.

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