Freeway Lane Merging

Freeway Lane Merging

This methodology was developed by Michael Seely, P.E., Senior Traffic Engineer with Horrocks, and has been formally adopted by UDOT as our new standard of practice.

This technical procedures memorandum provides guidance methodology for creating tapered lanes and lane drop segments for Vissim microsimulation models. This procedure is valid for models created using Vissim 2025 or later. This updated document supersedes any previous version.

The introduction of tapered lane connections in Vissim 2025 provides a new way to direct and control the flow of traffic in Vissim models. When connecting two links with different number of lanes, the default Vissim behavior is to use a tapered connector. See Figure 1 below for examples of this connection behavior.

In all cases, there must be a valid routing decision from one link to the other. A vehicle that does not have a valid route/decision assigned to it will exit the link or connector without making a lane change to stay on the Vissim network.

Notes to Figure 1:

1.  A lane drop from two lanes to one lane. If there is a valid routing decision crossing the connector, the outside lane will merge into the single receiving lane.

2.  Vehicles in the outside lane must make a lane change prior to leaving the origin link, and then a second lane change to reach the destination link.

3.  A taper connector can drop a maximum of one right-side lane and one left-side lane.

4.  A taper connector can add a maximum of one right-side lane and one left-side lane.

5.  Lane assignments between links may be changed on the connector; i.e., lane 2 of the origin link can connect to lane 1 of the destination link.

6.  Be aware that non-sensical connections can be made. Be aware of the alignment of the striping showing on the connector, as this will indicate the lanes of traffic being enforced.

Connector Properties

Emergency Stop and Lane Change Distance properties of connectors continue to be used for controlling vehicle movements with this new configuration methodology. However, the values for Emergency Stop and Lane Change Distance are modified in how they are applied in the Vissim network. These distances are now measured from a reference point located 30% of the length of the connector. See Figure 2 for a schematic showing the reference point location.

Users should be aware of this reference point calculation when dealing with very long connectors, as the expected lane change distance point may be significantly offset when compared to previous Vissim versions. If older Vissim model networks are updated from versions prior to Vissim 2025, extra review should be given to lane merging sections to ensure that appropriate values are used for the Emergency Stop and Lane Change Distance variables.

Lane Reduction on Thru Roadways

In coding a lane drop segment, field observations are critical to determining which links and connectors should be assigned to the lane drop behavior types. At a minimum, the taper lane section, the transition link, and the final connector should be assigned the Lane Drop driving behavior. Depending on the level of congestion observed in the field, it may be appropriate to assign the Lane Drop behavior type to a link upstream of the taper section. See Figure 3 for the location of this upstream transition section. The specific length of this section is also a primary model calibration parameter and should be adjusted based on field observations of when merging actions occur. As a starting point, the upstream link should be at least as long as the taper section.

Driving Behavior

For this behavior to be appropriately modeled, some adjustments to the link driving behaviors must be made in the Driving Behavior settings.

W-74 Parameters

For W-74 driving behaviors (typically arterial roadways), the following should be changed:

• Average Standstill Distance should be at least 8.0 feet. This is a primary calibration parameter.

• Lane Change parameters:

  • • Safety Distance Reduction Factor should be reduced to 0.35. This is a secondary calibration parameter.

    • Cooperative Lane Change is enabled and is a secondary calibration value. Horrocks default value matches the Vissim default of 6.7 mph for speed difference, and 10.0 seconds for maximum collision time.

• Desired position at free flow is set to middle of lane, and Observe Adjacent Lanes is enabled. These are mandatory for lane drops.

W-99 Parameters

For W-99 driving behaviors (typically freeway roadways), the following additional settings should be changed:

• Number of Interaction Objects should be changed to 3. This is a mandatory value.

• Car Following parameters:

  • • CC-0 is a primary calibration parameter. This value should be changed to 6.0 feet.

    • CC-1 is a primary calibration parameter for lane drops. It is generally greater than the upstream link. Horrocks default value is 0.2 seconds greater than the upstream section.

    • CC-4 and CC-5 are secondary calibration parameters. These parameters allow for more variance in following speeds, which allows for smoother gaps to open in the taper section. Horrocks default is -0.60 and 0.60, respectively. Consult with the Traffic Operations Practice Lead if you feel these should be modified.

• Lane Change parameters:

  • • Necessary Lane Change, Trailing Vehicle Max Deceleration is set to -21 ft/s² 

    • Necessary Lane Change, Trailing Vehicle Accepted Deceleration is set to -8 ft/s² 

    • Min. Clearance (front/rear) is set to 4.99 ft.

    • Safety Distance Reduction Factor is set to 0.35.

    • Maximum deceleration for cooperative braking is set to -29.49 ft/s² 

• Desired position at free flow is set to middle of lane, and Observe Adjacent Lanes is enabled. These are mandatory for lane drops.

All of these default values are included in the Arterial Lane Drop and Freeway Lane Drop link behaviors in the Horrocks Vissim Base File.

Taper Connectors at Intersections

Taper connectors are also valid for use in turning movements. See Figure 4.

Note that in (8) that the added lane from the right turn sweeps out to lane 2 on the destination link. In (9) the added lane is to the inside. The specific connection in a model will depend on the behavior observed in the field, as well as other location conditions.

Figure 5 shows a case where a downstream driveway is situated very close to a turning movement. Previous to Vissim 2025, it would have been necessary to create a second connector to facilitate the movement shown by the yellow arrow. By using a tapered connector, the vehicle traveling will naturally use the available connector space to arrive at the desired destination lane.

When using turning movement connectors in this fashion (either right or left turns), the driving behavior for these connectors should be adjusted. In either W-74 or W-99 algorithms, turn on the Observe Adjacent Lanes option and adjust the Minimum Lateral Distance values to a sufficiently high value to prevent vehicles from using both lanes of the connector at the same time. A value of 12’ is recommended for both values.