193. Lane Departure

The lane_departure_checker monitors whether the vehicle maintains its lane throughout the simulation and detects any deviation that indicates the vehicle has crossed a lane boundary. It continuously evaluates the vehicle's lateral position, orientation, and signaling behavior to determine when a lane departure occurs, ensuring that intentional maneuvers such as lane changes or turns are properly distinguished from unintended deviations.

When a lane departure is detected, the checker begins an interval during which it measures the vehicle's motion characteristics, including speed, acceleration, and distance from the lane boundary, and tracks how the event evolves over time. At the end of this interval, the checker classifies the outcome as return_to_lane, lane_change, timeout, or undefined_maneuver, depending on the vehicle's behavior after the initial lane departure.

193.0.1 Start condition

The interval starts when the vehicle begins to leave its lane and exceeds the leaving_lane_threshold. This threshold defines the distance from the crossed lane boundary to the vehicle's outermost edge, expressed as a percentage of the vehicle's width. Lane departure is based solely on the positions of the vehicle's front corners, so only the leading edge of the vehicle is considered when determining the start and end of the interval. The interval begins only if the vehicle's relative yaw angle exceeds the relative_yaw_threshold and the turn signal is inactive. Turn signal detection can be disabled during checker initialization by setting turn_signal_detection_enabled to false.

193.0.2 End condition

The interval can end under any of the following conditions:

Return to lane: The vehicle is considered to have re-entered its lane when the portion of its width overlapping the adjacent lane falls below the threshold defined by the re_entering_lane_threshold parameter. This condition ends the interval with a return_to_lane outcome.
Lane change: The vehicle is considered to have completed a lane change when the portion of its width remaining within the original lane falls below the threshold defined by the lane_change_threshold parameter. This condition ends the interval with a lane_change outcome.
Timeout: If the vehicle neither returns to its original lane nor completes a lane change within the period specified by the lane_departure_timeout_threshold parameter, the interval ends with a timeout outcome.
Undefined maneuver: If the vehicle performs an unexpected maneuver or the checker receives inconclusive data, the interval ends with an undefined_maneuver outcome.

193.1 Plot

The figure below illustrates the lane departure start condition.

The figure below illustrates the lane departure end condition.

193.2 Attributes

The following attributes define the behavior and characteristics of the evaluator.

Checker attribute	Description
Parent object	`vehicle.lane_departure`
Issue category	`sut` or `other`
Issue kind	`lane_departure`
Default severity	warning
Trigger condition	Based on `lane_departure` watcher
Operation modes	`lane_departure_checker`

193.3 Configuration parameters

The following parameters can be configured to customize the evaluator's behavior.

Parameter	Type	Unit	Description	Default Value	Range
`leaving_lane_threshold`	float		Percentage of the vehicle's width that has to leave the lane to trigger the check	0.15	[0.0, 1.0]
`re_entering_lane_threshold`	float		Percentage of the vehicle's width that must remain in the adjacent lane to be considered a return_to_lane	0.05	[0.0, 1.0]
`lane_change_threshold`	float		Percentage of the vehicle's width that must remain in the original lane before it is considered a lane_change	0.05	[0.0, 1.0]
`lane_departure_timeout_threshold`	time	s	Maximum allowed time for the vehicle to either complete a lane change or return to its original lane before the interval times out	30s	[0, 30]s
`relative_yaw_threshold`	angle	deg	Minimum relative yaw angle required for the vehicle to start a lane departure interval	0.2deg	[0.0, 90.0]deg
`turn_signal_detection_enabled`	bool		Enables or disables turn signal detection. When enabled, the interval will not start if the vehicle’s turn signal is active; when disabled, the interval starts regardless of turn signal state.	true

193.4 Metrics

The following metrics are recorded and tracked by the evaluator.

Metric	Type	Description	Unit
`min_speed`	speed	The vehicle's minimum speed during the interval	kph
`max_speed`	speed	The vehicle's maximum speed during the interval	kph
`min_lon_acceleration`	acceleration	The vehicle's minimum longitudinal acceleration during the interval	mpsps
`max_lon_acceleration`	acceleration	The vehicle's maximum longitudinal acceleration during the interval	mpsps
`interval_duration`	time	The duration of the interval	s
`vehicle_category`	enum (`sedan`, `van`, `bus`, `box_truck`, `semi_trailer_truck`, `full_trailer_truck`, `motorcycle`, `bicycle`)	The vehicle's category
`is_valid_lane_position_at_interval`	bool	Indicates if the vehicle has a valid lane position during the interval
`side`	enum (`left`, `right`)	The vehicle's movement direction during the interval
`end_condition`	enum (`undefined_maneuver`, `return_to_lane`, `lane_change`, `timeout`)	The vehicle's end condition for the interval
`max_distance_to_lane`	length	The vehicle's maximum distance to the lane during the interval (If the vehicle did not return to the lane or the vehicle does not have a valid lane position during the interval, the value is set to MAX_LENGTH)	m
`average_distance_to_lane`	length	The vehicle's average distance to the lane during the interval (If the vehicle did not return to the lane or the vehicle does not have a valid lane position during the interval, the value is set to MAX_LENGTH)	m

193.5 Log and Error Messages

Output Condition	Description	Default Severity	Action
After the interval ends, a warning is issued	Vehicle left its lane for `lane_departure_checker.data.interval_duration`. Outcome: `lane_departure_checker.data.end_condition`	Warning	Investigate the vehicle's behaviour.

193.6 Usage

193.6.1 How to instantiate

The lane_departure_checker can be instantiated for any vehicle actor.

checker lane_departure_checker is lane_departure(vehicle: actor,
        leaving_lane_threshold: ftlx_global_eval_config.ego_lane_departure_checker_config.leaving_lane_threshold,
        re_entering_lane_threshold: ftlx_global_eval_config.ego_lane_departure_checker_config.re_entering_lane_threshold,
        lane_change_threshold: ftlx_global_eval_config.ego_lane_departure_checker_config.lane_change_threshold,
        lane_departure_timeout_threshold: ftlx_global_eval_config.ego_lane_departure_checker_config.lane_departure_timeout_threshold,
        relative_yaw_threshold: ftlx_global_eval_config.ego_lane_departure_checker_config.relative_yaw_threshold,
        turn_signal_detection_enabled: ftlx_global_eval_config.ego_lane_departure_checker_config.turn_signal_detection_enabled,
        log_level: ftlx_global_eval_config.ego_lane_departure_checker_config.log_level)

193.6.2 Set parameters at instantiation time

Pass parameters in the keep(...) binding at instantiation time, either from ftlx_global_eval_config or as explicit values.

checker lane_departure_checker is lane_departure(vehicle: sut.car):
        keep(leaving_lane_threshold == ftlx_global_eval_config.ego_lane_departure_checker_config.leaving_lane_threshold)
        keep(re_entering_lane_threshold == 0.1)
        keep(lane_change_threshold == 0.1)
        keep(lane_departure_timeout_threshold == 1s)
        keep(relative_yaw_threshold == 0.1)
        keep(turn_signal_detection_enabled == true)
        keep(log_level == ftlx_global_eval_config.ego_lane_departure_checker_config.log_level)

193.6.3 Define and use a custom parameter set

Define a lane_departure_checker_config struct to customize the checker's parameters:

const custom_lane_departure_checker_config: lane_departure_checker_config = new
set custom_lane_departure_checker_config.leaving_lane_threshold = 0.1
set custom_lane_departure_checker_config.re_entering_lane_threshold = 0.1
set custom_lane_departure_checker_config.lane_change_threshold = 0.1
set custom_lane_departure_checker_config.lane_departure_timeout_threshold = 1s
set custom_lane_departure_checker_config.relative_yaw_threshold = 0.1
set custom_lane_departure_checker_config.turn_signal_detection_enabled = true
set custom_lane_departure_checker_config.log_level = trace_level

Then pass the struct value to the checker at instantiation:

checker custom_lane_departure_checker is lane_departure(vehicle: sut.car,
        leaving_lane_threshold: custom_lane_departure_checker_config.leaving_lane_threshold,
        re_entering_lane_threshold: custom_lane_departure_checker_config.re_entering_lane_threshold,
        lane_change_threshold: custom_lane_departure_checker_config.lane_change_threshold,
        lane_departure_timeout_threshold: custom_lane_departure_checker_config.lane_departure_timeout_threshold,
        relative_yaw_threshold: custom_lane_departure_checker_config.relative_yaw_threshold,
        turn_signal_detection_enabled: custom_lane_departure_checker_config.turn_signal_detection_enabled,
        log_level: custom_lane_departure_checker_config.log_level)

193.6.4 Disabling the checker

To disable the checker, set the enabled flag to false in the lane_departure_checker_config struct:

const lane_departure_checker_config: lane_departure_checker_config = new
set lane_departure_checker_config.enabled = false

193.7 Additional information

All parameters are required, but you don't need to customize all of them. For any parameter you don't customize with set, the default value defined in lane_departure_checker_config is used.

Limitations

The checker does not detect a lane departure if the vehicle drives toward an off-road area.
The checker relies solely on data from the vehicle's front corners.
The checker may fail to start the interval when the vehicle is in overlapping lanes (i.e., when lane geometry is shared), such as at a junction. In these cases, the checker cannot identify the departure lane, so the start condition is not triggered.
The interval starts only when both vehicle.state and vehicle.prev_state are available.
The start condition is evaluated only when the current lane has a valid driving lane index and the vehicle occupies that lane according to get_occupied_driving_lanes().
undefined_maneuver can be reported when the vehicle crosses more than one lane during the interval or when the checker cannot confirm a lane-change or return-to-lane condition.

Note

Negative values for the start or end condition thresholds are not recommended. They should only be used intentionally and with caution, as they may lead to unreliable or meaningless results, particularly for wide vehicles and narrow lanes. It is recommended to limit both the start and end condition thresholds to a maximum of 50% (0.5) of the vehicle's width.