Comments we often receive regarding our use of PTV Vistro in comparison to Synchro/SimTraffic are in regard to micro-simulation. Specifically, Synchro/SimTraffic is purchased as one software package while VISSIM, which is the SimTraffic equivalent for Synchro, is a separate software from PTV. In these situations, we need to step back and first determine whether a micro-simulation is necessary at all.
In general, the traffic analysis software we use is either deterministic or stochastic. These are big, technical words, so let’s break them down.
Deterministic software uses formulas for capacity analyses with the best example being the Highway Capacity Manual (HCM). Basically, specific inputs like geometry and volume will lead to one specific answer. The formulas are based upon years of research of actual driving conditions providing a high-degree of confidence in the results. The methodology is also cost-effective and generally easy to use. Vistro, Synchro Studio, and TEAPAC are all examples of this type of software.
In contrast, stochastic software is a micro-simulation. Geometry and volume are still inputs along with information regarding vehicle and driver behavior theories. The software simulates the coded transportation network, measuring the performance of individual vehicles as they move through the system. These vehicles can be influenced by other vehicles, pedestrians and bicycles, roadway grades and curves, and many other factors. Since individual vehicles will react differently to these inputs, the results of two simulations will not necessarily match. In other words, multiple answers to one given set of conditions. The advantages of micro-simulations are the power of visually showing operations along with the showing the impacts of surrounding congestion and system-wide results. VISSIM, SimTraffic, CORSIM, and Paramics are all examples of this type of software.
Which one is best? Neither type can truly make this claim. Instead each has strengths and weaknesses. The better question is which methodology should you use in your situation. To answer that question, we consider the following:
- If the approving agency requires it
- If unique geometry or special situations exist that falls outside of the HCM formulas (such as three-lane roundabouts)
- If progression issues from over-capacity intersections exist
- If the study intersections include the mixing of traffic control types that will have significant influence over each other (like a signal providing gaps for a nearby side-street stop intersection)
- If system-wide measures of effectiveness are needed
- If visualization is needed for public involvement or other meetings
Practically speaking, engineering judgment should always be used on deciding which software or methodology is appropriate for your situation. We believe deterministic software based on the HCM, like Vistro, can be and should be used more. In fact, we complete 85 to 90 percent of our projects without micro-simulation. Given its advantages in other areas, detailed in our other articles here, starting with Vistro is a no-brainer in our view. When your studies get larger and more complicated, then you can add VISSIM to provide micro-simulation.
Are you interested in learning more about how we use Vistro for our traffic impact studies? Download our free case study: How Vitro Improved our Efficiency by 10%.
In this case study we:
– Compare PTV Vistro and Trafficware Synchro Studio
– Discuss the similarities and differences in scenario management and automated graphics
– Share our findings using both software programs on two traffic impact studies
When deciding when micro-simulation analysis is needed, I wouldn’t assume it is needed when our studies get “larger and more complicated”. When you say ‘larger’, I interpret that to mean the spatial extent of a model. Micro-simulation is more often effective for analyzing small physical areas. Meso-simulation, macro-simulation, and sketch planning models are often more effective for analyzing large areas. I don’t think micro-simulation is usually a practical way to analyze large areas. I agree with the “more complicated” part of your answer.
I believe that when the scale is large and the detailed outputs (like what single vehicles do) are not important, we can use Visum.
Otherwise we should consider the Vistro as the first option between Vistro and Vissim. It is because Vistro needs lesser effort (note that it doesn’t produce detailed outputs). If the conditions on using Vistro are not met, we need to go for Vissim; for example when the geometric design is not standard based on the HCM, when the methodology is not applicable based on the statement of the HCM (such as roundabouts with more than 2 circulating lanes for HCM2010).
If we need to use Vissim but don’t need detailed outputs, we can use Meso simulation of Vissim. Even we can use the Meso simulation on the parts of the network with lower importance only and simulate the important points Microscopically.
In addition to the limitation on the use of the methodologies of the HCM, there are two use cases of Vissim and also Visum, that Vistro can’t handle:
1) Dynamic demand and/or supply (Vistro only accounts for Peak Hour Factor)
2) Oversaturated network (when the queue of one intersection goes back to the upstream intersection and blocks some of its movements)