11.3.6 Additional Operational Analysis

Many tools are available to perform traffic operational analysis. Depending on the type of analysis and level of detail necessary, different tools are needed. below summarizes the overall tool selection process based on interchange type and analysis level of detail.
Appendix L, Section 1 – Interchange Analysis Checklist
 can also be used to guide the operational analysis.
Traffic Operational Analysis Tool Selection Flow Chart for Interchanges
Figure 11-15: Traffic Operational Analysis Tool Selection Flow Chart for Interchanges
The most common tools used for traffic operational analysis for interchanges are HCS, Synchro, and Vissim. A summary of the capabilities of the tools, how the tools can be used, and their limitations is below.

11.3.6.1 HCS

HCS is an HCM-based (mesoscopic) tool that allows for evaluation of several types of interchanges. HCS covers the following interchange types:
  • Diamond (stop control, signals, or roundabouts);
  • Partial cloverleaf A and B;
  • DDI; and
  • SPUI (Signals or roundabout)
HCS outputs include traffic operations MOEs (e.g., LOS, delay, queue lengths) and ETT.
HCS is one of the only tools that directly reports ETT. It is important to note that HCS cannot be used to evaluate oversaturated conditions
, particularly when there is queue spillback affecting upstream and downstream operations. Other limitations of HCS include analyzing impacts of interchange operations on the arterial street, providing travel time for upstream and downstream links, and interchanges with two-way stop control junctions or combination of signalized and unsignalized junctions. It is recommended to use microsimulation (microscopic) tools in these scenarios.

11.3.6.2 Synchro

Synchro is a deterministic tool that has capabilities for performing HCM-based (mesoscopic) analysis. Synchro evaluates all interchange types mentioned in
Section 11.2.1
. Synchro analyzes signalized and unsignalized intersection delay for HCM-based (mesoscopic) interchange types (see HCS discussion above). However, Synchro is limited in evaluating and properly reporting results for interchanges with complicated geometry. Results from single roundabout, double roundabout, displaced left turn, and contraflow left turn interchanges may need to be scrutinized further or validated by another tool to confirm the results are reasonable. Similar to HCS, Synchro is limited in evaluating oversaturated interchanges with queue spillbacks that might affect upstream and downstream operations. Unlike HCS, Synchro does not directly report ETT.

11.3.6.3 Vissim

Vissim is a Microsimulation (microscopic) tool and can be used based on the project scope and complexity. Vissim evaluates all interchange types mentioned in
Section 11.2.1
. Because it is a Microsimulation (microscopic) software, Vissim evaluates oversaturated conditions well when compared to deterministic or HCM-based (mesoscopic) tools. However, Vissim analyses are typically data-intensive and demand more effort. Vissim does not capture real-world rerouting due to extreme congestion. Similar to other analysis tools, Vissim does not directly output ETT, but travel time routes can be set up in the network to account for ETT.