PTV Vistro: Consistent Comparisons
Overriding Final Saturation Flow Rates
Saturation flow rate calculation overrides enable consistent comparisons between intersections for long-range planning.?Apply custom sat. flow rates manually or globally based on lane configurations.
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Overriding HCM values: When and Why?
You may be asking the question, “why would I override a standard HCM calculation?” PTV Vistro’s HCM saturation flow calculation plays a very important role in traffic engineering. For typical engineering-based HCM deterministic analyses, results are used for intersection designs, signal optimizations, and pattern timings.?However, the HCM analyses require a lot of design and field information for the saturation flow calculation.?For example, these details include measures to calculate the various adjustment factors listed below:
In transportation planning programs with fast-paced intersection screening, many of these measurements are unknown.?Luckily, PTV Vistro includes an option to override the saturation flow calculation.?This greatly simplifies the HCM analyses and enables you to obtain results very quickly with little required input.
Examples of agencies overriding saturation flow calculations
Overriding the calculated saturation flow rate provides a uniform look at similar intersection types and geometry, resulting in a powerful screening method for agencies.?For example, Santa Clara VTA (VTA) region in Silicon Valley, California uses PTV Vistro’s global setting for the Final Saturation Flow Rate Method.?VTA compares over 250+ intersections in their congestion management and monitoring program (CMP).?For VTA, this simplified method ensures consistency across the region removing any modeling inaccuracies. Additionally, VTA plans to work with its member agencies to expand the CMP Vistro network to over 2,000 intersections.?
To utilize saturation flow rate overrides for cities or regions, agencies should determine standard rates at various intersections through sample studies.?Below are VTA’s published rates.?PTV Vistro loads these values as default overrides under Edit->Global Settings. Learn more about VTA’s CMP and Guidelines here.
How to manually override saturation flow rates?
Overriding a signalized intersection’s saturation flow rate calculation in PTV Vistro is easy.?
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Tip: Don’t forget about PTV Vistro’s multi-edit feature!?To apply the same factor across an approach or for the entire intersection, left-click the mouse, drag across the grid, release the click, and enter a value (or activate a checkbox) for all grid cells selected.
How to globally apply saturation flow rates?
PTV Vistro applies global overrides to the saturation flow rate calculation in a few clicks. This process utilizes the preset rates for each lane configuration listed in the Global Settings for all intersections.
With the Global rates selected, the Final Saturation Flow Rate field in the Traffic Control workflow inserts the default values for each lane group. For very special cases, the local method above can still be used to override these preset rates.
Tip: When updating global settings to your local conditions, save a copy of PTV Vistro and make it your local agency seed file.
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