Calibrating Lane-Change Urgency for Mandatory Maneuvers TSIS

Dr. Ehsan Amini

Introduction

Mandatory Lane Changes (MLC) occur when a vehicle must change lanes to reach a specific target (goal) lane to execute its next turn movement. This behavior is governed by a dynamic assessment of “urgency,” which scales driver aggressiveness and risk acceptance as the vehicle approaches the point where the maneuver must be completed. By calibrating lane-changing parameters, users can fine-tune when and how drivers become desperate to change lanes. Users can adjust these parameters in the Network → NETSIM → Calibration → Lane Changes (Driver Behavior) and Lane Changes tabs (Figure 1).

Figure 1: Urgency Threshold in NETSIM calibration parameters

Understanding the Urgency Factor (U)

The Urgency Factor (U) is an internal calculation that considers a driver’s natural aggressiveness, the number of lane changes remaining to reach the goal lane, the vehicle’s speed, and the distance to the object (such as a lane drop or turn pocket). As the distance to the maneuver point decreases, the Urgency Factor increases. This “urgency” [U] is computed as follows:

NLC: The number of lane changes required to reach the “goal” lane, for a mandatory lane change (for a discretionary lane change, only one lane change will be made so NLC = 1)
V_f: User-specified free-flow speed for this link
X: Current position of the vehicle
X₀: Position of the object that causes the lane change

Setting the Point of Escalation: Urgency Threshold

The Urgency Threshold (U_t) defines the point at which a driver’s risk acceptance begins to increase. As long as the Urgency Factor is below this threshold, the driver only accepts a base level of risk (the minimum deceleration). Once U exceeds U_t, the driver’s acceptable deceleration (risk) begins to escalate linearly (Figure 2).

Diagram of a stop-controlled intersection showing vehicle movements and priority ranking, where eastbound left-turn traffic yields to oversaturated westbound through traffic where an EBL driver yields to oversaturated WBT traffic.

Figure 2: Urgency Threshold VS Acceptable Deceleration (Risk)

If you observe that drivers are waiting until the very last second to attempt a required lane change, increase the Urgency Threshold. This forces urgency values to reach a higher level before drivers are willing to accept riskier maneuvers, effectively discouraging early risk-taking

Controlling Risk Acceptance: Mandatory Deceleration

The minimum deceleration for lane changing (D_min) sets the base level of risk a driver is willing to accept. A separate parameter defines how much higher the maximum acceptable deceleration can be for a mandatory lane change (D_max). As urgency increases, this additional deceleration is added to the minimum value, eventually reaching the maximum acceptable risk.

If vehicles in your simulation are frequently missing their turns or getting stuck at the end of a lane because they cannot find a large enough gap, increase this difference between minimum and maximum acceptable deceleration. Larger values allow drivers to accept much higher deceleration rates (riskier maneuvers) from followers in the target lane, increasing the probability that they will successfully force their way into the goal lane.

Adjusting the Contemplation Distance

CORSIM looks up to 12 links ahead to determine a vehicle’s goal lanes. The mean longitudinal distance over which drivers decide to perform one lane change defines how far in advance they begin planning each maneuver.

To encourage smoother traffic flow and more realistic early lane positioning, increase this distance. When this value is higher, drivers are more likely to seek lane-changing opportunities further upstream, reducing the need for aggressive, “urgent” maneuvers and preventing vehicles from having to slow to a stop to enter a turn lane.

For locations with directional access—such as entrance-only or exit-only configurations—only the relevant directional volume should be specified.

The Role of Driver Cooperation

A mandatory lane change is only successful if the driver is willing to take a risk and, in many cases, if the follower in the target lane is willing to cooperate.

If mandatory maneuvers are causing excessive “stop-and-go” conditions, consider increasing the percentage of drivers who cooperate with a lane-changer. Cooperative drivers will slow down or even stop in a queue to allow a lane-changer to merge, which significantly helps vehicles reach their goal lanes further upstream and reduces overall system delay.

Note: These parameters are specific to the NETSIM (urban street) model. While FRESIM (freeway) uses similar concepts of risk and cooperation, they are configured via different Record Type. For more information, please refer to CORSIM Reference Manual.

Calibrating Lane-Change Urgency for Mandatory Maneuvers in TSIS-CORSIM

Introduction

Understanding the Urgency Factor (U)

Setting the Point of Escalation: Urgency Threshold

Controlling Risk Acceptance: Mandatory Deceleration

Adjusting the Contemplation Distance

The Role of Driver Cooperation

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