VISSIM

For protected-permissive left turn phasing, UDOT uses either a Flashing Yellow Arrow (FYA) or a five-section (a.k.a., “doghouse”) signal head. FYA signals are becoming more common, largely due to the flexibility that they provide, including the ability to switch between protected-only and protected-permissive based on time of day or traffic conditions (this can be determined through field observations or reviewing the signal controller logic). While both signal head types operate similarly in protected-permissive mode, there is a key difference that needs to be included in any traffic model. This difference is that the permissive left turn phase is tied to a different phase for each signal head type as shown in the table below. For five-section heads the permissive phase is tied to the adjacent through movement and for FYA the permissive phase is tied to the opposing through movement. 

This means that there are times during the signal cycle when a FYA signal head will allow a permissive left turn even though the light is red for the adjacent through lane(s). This occurs when the opposing left turn phase is either skipped or runs longer than that of the subject left turn. During this time the FYA provides additional time for permissive left turns, reducing the delay for the left turn movement. An illustration of how protected-permissive phasing is coded in Vissim for each signal type is shown in Figure 29 and Figure 30 below.

Figure 29 - Priority Rules are another tool in Vissim to help calibrate the traffic to behave as life-like conditions.

Input Volumes & Routing

For vehicle inputs, the consultant should include hourly flow rates in 15-minute intervals, except for the seeding period where a single time interval can be adjusted as appropriate. The consultant may need to vary the 15-minute input values according to the peak hour factor or based on the variation in the collected volume counts.

Static routes should typically extend through the length of the model. In general, it is not realistic to assume that vehicles make new decisions in short increments. If necessary, the consultant can use routing breaks only for locations when the signal spacing in conducive (e.g., signal spacing is close to a mile). The consultant can, at times, break routing at the freeway ramps, but both the freeway and arterial routing should be continuous. The consultant should consider separate routing decisions for heavy vehicles, if 1) their travel patterns do not match those of passenger vehicles or 2) if there are high occupancy vehicle (HOV) lanes in the model. The consultant should coordinate with UDOT Traffic Management Division if deviating from this routing methodology. 

The consultant should always input vehicle routing as hourly flows rather than as percentages, and the consultant should verify that pedestrians are included in the traffic counts and in the Vissim models.

Car Following Model Parameters (Freeways – Wiedemann 99) 

UDOT uses VISSIM to model complex traffic behavior. VISSIM’s fundamental logic is built on car following models with a series of parameters available for adjustment.  It is important to calibrate these models using the correct parameters to produce accurate flow rates.  To help identify the flow rates in a VISSIM model, download a VISSIM layx file from the UDOT website at https://udot.utah.gov/connect/business/design/traffic-modeling-guidelines. This file is set up to visualize the links based on the link flow rate.  

The Wiedemann 99 car following model should be used for all freeway facilities as included in the UDOT VISSIM template. Typically, calibration of the freeway only requires small adjustments to the CC1 and CC2 values. These adjustments should be used to replicate realistic flow rates, particularly in congested conditions. Field flow rates for Utah freeways are measured by radar and are publicly accessible online through PeMS. If the segment being modeled does not currently experience congestion, then similar freeway segments for other locations in the state should be reviewed. Adjustments to CC1 & CC2 require justification among field observations.  Note that UDOT requires additional documented data to support changes to all other CC values as situations that justify these changes are extremely rare. 

Table 1. UDOT Template Default Values for Wiedemann 99 Car Following Model 

CC1 controls the gap time distribution (in seconds), which is the gap that a following vehicle wants to maintain in addition to the standstill distance (CC0). CC1 is the dominant factor that controls capacity at high volumes. A smaller time gap would increase capacity while a larger time gap would decrease capacity.  

Desired safety distance = CC0 + CC1 × speed 

Freeway Flow Rates 

Below are several examples of the various maximum flow rates achieved in the field along locations with regular congestion.  This data was measured using side-fire radar. 

Speed vs flow diagrams based on these data are shown in Figures 1 to 6.  

The maximum capacity of a corridor is affected by a number of factors including: 

• Closely spaced interchanges which create a weave condition 

• Lots of heavy trucks which reduce maneuverability 

• More than 4 lanes in width 

• Lower speeds 

• Steep grades 

• Curves in the roadway 

• Narrow lanes 

• Barrier that is too close or steep drop offs 

The Department has found that, in Utah, the optimum capacity for a freeway is generally reached at a width of 4 lanes.  As the number of lanes increases, the number of lane changes required to utilize those lanes also increases.  As vehicles maneuver into new lanes, other vehicles slow to accommodate them.  This can artificially increase the density, meaning there is empty space on the freeway that could have otherwise been utilized, had vehicles not been crossing over so many lanes.  This reduces the available space for vehicles to occupy which ultimately reduces capacity.   

Other conditions may also affect capacity.  Field observations should be performed to identify these and model them appropriately. 

Figure 1 shows a low flow rate location that breaks down around 1,500 vehicles per hour per lane (vphpl). Figures 2 and 3 show medium flow rate locations that break down around 1,700 vphpl. Figures 4-6 show high flow rate locations that break down around 1,900 vphpl. The figures are provided as examples to help determine appropriate flow rates. 

Figure 1. Northbound I-17 between Park Ln and Shepard Ln  

Figure 2. Southbound I-15 between I-215 and 9000 South  

Figure 3. Southbound I-15 between 500 East and Pleasant Grove Blvd  

Figure 4. Northbound I-15 between 600 North and I-215  

Figure 5. Northbound I-15 between 9000 South and I-215  

Figure 6. Southbound I-15 between 600 South and Pleasant Grove Blvd  

To demonstrate the correlation between flow rates and CC1 values, test model runs were performed for a basic 70 mph freeway segment with 8% heavy vehicles using a range of CC1 values. The results of this analysis are shown in Figure 7, which is provided for use as a reference. 

Figure 7. VISSIM Freeway Vehicle Flow Rates  

CC1 and CC2 Model Parameters 

The recommended starting value for CC1 is 1.3 seconds in the Wiedemann 99 Car Following Model.  Note that this is a starting value, not an absolute value. It is critical that accurate freeway flow rates which represent the operations in the field are reflected in the model. It should be noted that although the CC1 value is a time distribution in Vissim, the default is to have no variance in the time (i.e., the standard deviation should be 0). The CC1 can then be adjusted as needed to calibrate the model to a capacity more in line with those observed in the project study area but should be justified with data to support the change (e.g., PeMS peak flow rate). Abrupt changes in the CC1 from one link to another should also be avoided so that artificial congestion is not created at link boundaries. 

CC2 controls the following distance oscillation, which is the maximum additional distance beyond the desired safety distance that is accepted by the following vehicle before intentionally moving closer. The default value of 13.12 feet (4.0 meters) results in stable following behavior. The default value should almost always be used and only adjusted in exceptional cases after discussion with UDOT.  

Situations may arise where additional factors may need to be adjusted. The consultant should discuss the changes with UDOT and fully document the modifications and reasons why the changes were needed. 

CALIBRATION & VALIDATION

Number of Runs

A minimum of 10 runs is recommended to determine the MOEs for all Vissim analysis. If outliers are excluded from these runs, then the consultant should use additional runs. More than 10 runs may be required to reach the calibration target, and the consultant could determine this using the following GEH equation:

Traffic Operations Measures of Effectiveness (MOEs)

UDOT requires VISSIM to be used to analyze traffic operations for freeway interchanges and innovative intersections. UDOT is currently accepting VISSIM versions 2020 up through the latest edition.  If this creates unnecessary hardship, older versions of VISSIM may be approved by UDOT Operations on a case by case basis.

The following Measures of Effectiveness (MOEs) have been formally adopted into UDOT’s Traffic Analysis Guidelines.  Note when providing MOEs in an IACR or OSA, please remember to only list MOE’s in one place to avoid accidental inconsistencies.  If its a primary interchange, the MOEs should go in the report.  If its for an intersection of less importance, it can be listed in the appendix.

Freeway Travel Times

Travel Times are to be used for freeway operations only.  Probe Traffic Data is available statewide online through Clearguide.  Travel times are not an appropriate measure for signalized movements as they are highly susceptible to the amount of time allocated to conflicting movements and the cycle lengths chosen for the signal.  The amount of time a conflicting movement is entitled to is highly dependent on the location and is therefore too arbitrary and context sensitive to be an objective measurement.

Freeway Level of Service

The link density, as measured in the VISSIM link evaluations, should be used to determine the congestion level on basic freeway segments and weaving areas. The measured density on mainline links must be less than 35 vehicles per mile to be acceptable. The table below details the LOS thresholds.

Freeway Level of Service Density Table

Note that the density and delay thresholds for both of these LOS tables have been informed by HCM. The HCM defines LOS thresholds using passenger cars per mile per lane (pc/mi/ln); however, freeway operations should be evaluated using VISSIM microsimulation, which explicitly models individual vehicle interactions, including heavy-vehicle effects, car-following behavior, and lane-changing dynamics. Because these effects are directly represented in the simulation, densities are to be evaluated in vehicles per mile per lane (veh/mi/ln) without converting to passenger-car equivalents.

Intersection LOS

Level of Service for individual movements and for overall intersections is to be applied using the tables below.  Intersection delay should be measured using the Vissim node evaluations. Generally LOS for individual movements is preferred over aggregating to the approach.  However, an approach-level-aggregation may be used if problematic movements represent an excessively small number of vehicles.  Delays at signalized intersections must be 55 seconds or fewer per vehicle to be considered acceptable. At unsignalized intersections, delay must be less than 35 seconds per vehicle. 

Intersection and Interchange Level of Service Delay Table

Vehicle Template

Base File V2025