Positioning and Mandatory Distances

One of the best ways to calibrate the lane utilization in SimTraffic is using the Positioning and Mandatory Distances. This post will give you a good idea on when and how to use these settings to calibrate your model.

Traffic flow in SimTraffic is based on a series lane changing and car following algorithms. There are ten different driver types to provide various driver behaviors, including both aggressive and passive drivers. As vehicles are added to the network, they have a pre-determined path and already know several of their upcoming movements. This pre-determined path and the driver type will influence each driver’s lane change behaviors.

The decision on when a vehicle change lanes is based on two key parameters. The first is the Mandatory Distance and is defined as the distance from an intersection that a vehicle must have changed lanes to complete a required movement. If the vehicle has not changed lanes by this point, the vehicle will stop and wait for a gap. The second parameter is the Positioning Distance and is defined as the distance from the location of the Mandatory Distances that a vehicle will attempt to make a lane change to avoid being forced to complete a mandatory lane change. The figure below highlights the relationship of these parameters.

Both of these parameters can be adjusted at the local link or global network level.

Refinements at the local level can be made by pressing the Synchro Simulation Settings icon. The figure on the bottom-left highlights the default calculated distances (see red square) upon selecting a typical intersection. Adjustments to these values will only affect vehicles changing lanes due a movement at this intersection. It is important to note that this is not the final distance used in SimTraffic. Another set of factors are included in SimTraffic’s Drivers Parameters screen has an impact to all roadway links within the model. Notice in the lower-right figure that each driver type has a percentage adjustment value (see blue square). These percentages are multiplied by the distances listed in the Simulation Settings screen to arrive at the actual distance used while traveling through a model. Adjustments to these percentages affect all vehicles within the SimTraffic model.

In order to better understand these parameters, let’s examine how vehicles within SimTraffic react to a lane drop just downstream of Intersection 3.

Notice the Mandatory Distance for EB vehicles is 100 ft., while the Positing Distance for EB vehicles is 1,980 ft. Using the default values in SimTraffic’s Drivers Parameters for Driver Type 1, we can calculate the positioning distance to be 1,980 x 150% = 2,970 ft. This means that vehicles will begin making lane changes over 0.5 miles from the lane drop. With this in mind, the queuing shown below seems reasonable. However, this queuing may not match what might be observed in the field.

Lane Utilization with Default Parameters

In this example, our goal is to somewhat balance lane utilization. The Positioning Distance at Node 6 can be reduced. In the image below, a value of 200 ft. is used, resulting in more realistic queuing. The change allows even a Driver Type 1 to wait until 300 feet from the lane drop to being making lane choices.

Improved Lane Utilization by Adjusting Positioning Distance

By adjusting either the Mandatory and/or Positioning Distances, calibrating SimTraffic can often be completed rather quickly. Calibration of these parameters can be conducted at either the local link or global network level.