Sub-problem 1c: Special Considerations for the Analysis

There are many conditions that may affect the analysis of these types of facilities. Examples of such conditions include:

• Significant queues occurring on a two-lane highway (violates assumption that the flow is uninterrupted).

• Backup from one intersection affecting the movement of traffic from another intersection.

• Overflow of a storage bay that inhibits the movement of through traffic.

• Short sections of a longer facility that have a different cross section than the main facility.

• Control features that are not covered by the HCM procedures (e.g., three-way stop at a four–legged intersection.

• Conditions that would suggest the use of traffic models that are more complex than the HCM to accommodate phenomena that are not envisioned by the HCM (e.g., signal preemption, platoon propagation and dispersion, advanced traffic control strategies, etc.).

It is important therefore to examine all points in a facility to identify any abnormal conditions that could affect the procedures or the results.

A specific condition could require a modification in the way a procedure is implemented to achieve more valid results. Another approach is to make a simplifying assumption in order to proceed with the analysis, followed by some post processing of the results for reasonableness. In some cases, especially those in which the demand volumes exceed the capacity, it could be preferable to eliminate the condition by, for example, assuming an improvement that increases the capacity. The objective of this sub-problem is simply to identify such conditions and not to prescribe the measures by which they should be accommodated. The accommodation of abnormal conditions will be dealt with under the site-specific sub-problems to be presented later.

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